A lot of annotations a bit everywhere.

src/main/java/net/minestom/server/Viewable.java

@@ -3,6 +3,7 @@ package net.minestom.server;
 import net.minestom.server.entity.Player;
 import net.minestom.server.network.PacketWriterUtils;
 import net.minestom.server.network.packet.server.ServerPacket;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.HashSet;
 import java.util.Set;
@@ -18,7 +19,7 @@ public interface Viewable {
      * @param player the viewer to add
      * @return true if the player has been added, false otherwise (could be because he is already a viewer)
      */
-    boolean addViewer(Player player);
+    boolean addViewer(@NotNull Player player);
 
     /**
      * Removes a viewer.
@@ -26,13 +27,14 @@ public interface Viewable {
      * @param player the viewer to remove
      * @return true if the player has been removed, false otherwise (could be because he was not a viewer)
      */
-    boolean removeViewer(Player player);
+    boolean removeViewer(@NotNull Player player);
 
     /**
      * Gets all the viewers of this viewable element.
      *
      * @return A Set containing all the element's viewers
      */
+    @NotNull
     Set<Player> getViewers();
 
     /**
@@ -41,7 +43,7 @@ public interface Viewable {
      * @param player the player to check
      * @return true if {@code player} is a viewer, false otherwise
      */
-    default boolean isViewer(Player player) {
+    default boolean isViewer(@NotNull Player player) {
         return getViewers().contains(player);
     }
 
@@ -53,7 +55,7 @@ public interface Viewable {
      *
      * @param packet the packet to send to all viewers
      */
-    default void sendPacketToViewers(ServerPacket packet) {
+    default void sendPacketToViewers(@NotNull ServerPacket packet) {
         PacketWriterUtils.writeAndSend(getViewers(), packet);
     }
 
@@ -65,7 +67,7 @@ public interface Viewable {
      *
      * @param packets the packets to send
      */
-    default void sendPacketsToViewers(ServerPacket... packets) {
+    default void sendPacketsToViewers(@NotNull ServerPacket... packets) {
         for (ServerPacket packet : packets) {
             PacketWriterUtils.writeAndSend(getViewers(), packet);
         }
@@ -78,7 +80,7 @@ public interface Viewable {
      *
      * @param packet the packet to send
      */
-    default void sendPacketToViewersAndSelf(ServerPacket packet) {
+    default void sendPacketToViewersAndSelf(@NotNull ServerPacket packet) {
         if (this instanceof Player) {
             if (getViewers().isEmpty()) {
                 ((Player) this).getPlayerConnection().sendPacket(packet);
@@ -97,7 +99,7 @@ public interface Viewable {
      *
      * @param packet the packet to send
      */
-    private void UNSAFE_sendPacketToViewersAndSelf(ServerPacket packet) {
+    private void UNSAFE_sendPacketToViewersAndSelf(@NotNull ServerPacket packet) {
         Set<Player> recipients = new HashSet<>(getViewers());
         recipients.add((Player) this);
         PacketWriterUtils.writeAndSend(recipients, packet);

src/main/java/net/minestom/server/advancements/AdvancementTab.java

@@ -8,6 +8,7 @@ import net.minestom.server.network.player.PlayerConnection;
 import net.minestom.server.utils.PacketUtils;
 import net.minestom.server.utils.advancement.AdvancementUtils;
 import net.minestom.server.utils.validate.Check;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.*;
 
@@ -134,7 +135,7 @@ public class AdvancementTab implements Viewable {
     }
 
     @Override
-    public synchronized boolean addViewer(Player player) {
+    public synchronized boolean addViewer(@NotNull Player player) {
         final boolean result = viewers.add(player);
         if (!result) {
             return false;
@@ -151,7 +152,7 @@ public class AdvancementTab implements Viewable {
     }
 
     @Override
-    public synchronized boolean removeViewer(Player player) {
+    public synchronized boolean removeViewer(@NotNull Player player) {
         if (!isViewer(player)) {
             return false;
         }
@@ -166,6 +167,7 @@ public class AdvancementTab implements Viewable {
         return viewers.remove(player);
     }
 
+    @NotNull
     @Override
     public Set<Player> getViewers() {
         return viewers;

src/main/java/net/minestom/server/bossbar/BossBar.java

@@ -6,6 +6,7 @@ import net.minestom.server.entity.Player;
 import net.minestom.server.network.packet.server.play.BossBarPacket;
 import net.minestom.server.utils.MathUtils;
 import net.minestom.server.utils.validate.Check;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.*;
 import java.util.concurrent.CopyOnWriteArraySet;
@@ -38,7 +39,7 @@ public class BossBar implements Viewable {
      * @param color    the boss bar color
      * @param division the boss bar division
      */
-    public BossBar(ColoredText title, BarColor color, BarDivision division) {
+    public BossBar(ColoredText title, @NotNull BarColor color, @NotNull BarDivision division) {
         this.title = title;
         this.color = color;
         this.division = division;
@@ -50,12 +51,12 @@ public class BossBar implements Viewable {
      * @param player the player to check the boss bars
      * @return all the visible boss bars of the player, null if not any
      */
-    public static Set<BossBar> getBossBars(Player player) {
+    public static Set<BossBar> getBossBars(@NotNull Player player) {
         return PLAYER_BOSSBAR_MAP.getOrDefault(player, null);
     }
 
     @Override
-    public synchronized boolean addViewer(Player player) {
+    public synchronized boolean addViewer(@NotNull Player player) {
         // Check already viewer
         if (isViewer(player)) {
             return false;
@@ -71,7 +72,7 @@ public class BossBar implements Viewable {
     }
 
     @Override
-    public synchronized boolean removeViewer(Player player) {
+    public synchronized boolean removeViewer(@NotNull Player player) {
         // Check not viewer
         final boolean result = this.viewers.remove(player);
         if (result) {
@@ -82,6 +83,7 @@ public class BossBar implements Viewable {
         return result;
     }
 
+    @NotNull
     @Override
     public Set<Player> getViewers() {
         return Collections.unmodifiableSet(viewers);
@@ -133,6 +135,7 @@ public class BossBar implements Viewable {
      *
      * @return the current bossbar color
      */
+    @NotNull
     public BarColor getColor() {
         return color;
     }
@@ -142,7 +145,7 @@ public class BossBar implements Viewable {
      *
      * @param color the new color of the bossbar
      */
-    public void setColor(BarColor color) {
+    public void setColor(@NotNull BarColor color) {
         this.color = color;
         updateStyle();
     }
@@ -152,6 +155,7 @@ public class BossBar implements Viewable {
      *
      * @return the current bossbar division
      */
+    @NotNull
     public BarDivision getDivision() {
         return division;
     }
@@ -161,7 +165,7 @@ public class BossBar implements Viewable {
      *
      * @param division the new bossbar division count
      */
-    public void setDivision(BarDivision division) {
+    public void setDivision(@NotNull BarDivision division) {
         this.division = division;
         updateStyle();
     }
@@ -213,7 +217,7 @@ public class BossBar implements Viewable {
      *
      * @param player the player to remove from the map
      */
-    private void removePlayer(Player player) {
+    private void removePlayer(@NotNull Player player) {
         if (!PLAYER_BOSSBAR_MAP.containsKey(player)) {
             return;
         }
@@ -231,7 +235,7 @@ public class BossBar implements Viewable {
      *
      * @param player the player to create the bossbar to
      */
-    private void addToPlayer(Player player) {
+    private void addToPlayer(@NotNull Player player) {
         // Add to the map
         Set<BossBar> bossBars = PLAYER_BOSSBAR_MAP.computeIfAbsent(player, p -> new HashSet<>());
         bossBars.add(this);
@@ -252,7 +256,7 @@ public class BossBar implements Viewable {
      *
      * @param player the player to remove the bossbar to
      */
-    private void removeToPlayer(Player player) {
+    private void removeToPlayer(@NotNull Player player) {
         BossBarPacket bossBarPacket = new BossBarPacket();
         bossBarPacket.uuid = uuid;
         bossBarPacket.action = BossBarPacket.Action.REMOVE;

src/main/java/net/minestom/server/data/Data.java

@@ -1,5 +1,8 @@
 package net.minestom.server.data;
 
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
 import java.util.Collections;
 import java.util.Set;
 
@@ -12,19 +15,20 @@ public interface Data {
 
     Data EMPTY = new Data() {
         @Override
-        public <T> void set(String key, T value, Class<T> type) {
+        public <T> void set(@NotNull String key, @NotNull T value, @NotNull Class<T> type) {
         }
 
         @Override
-        public <T> T get(String key) {
+        public <T> T get(@NotNull String key) {
             return null;
         }
 
         @Override
-        public boolean hasKey(String key) {
+        public boolean hasKey(@NotNull String key) {
             return false;
         }
 
+        @NotNull
         @Override
         public Set<String> getKeys() {
             return Collections.emptySet();
@@ -41,7 +45,7 @@ public interface Data {
         }
 
         @Override
-        public <T> T getOrDefault(String key, T defaultValue) {
+        public <T> T getOrDefault(@NotNull String key, T defaultValue) {
             return defaultValue;
         }
     };
@@ -54,7 +58,7 @@ public interface Data {
      * @param type  the value type
      * @param <T>   the value generic
      */
-    <T> void set(String key, T value, Class<T> type);
+    <T> void set(@NotNull String key, @NotNull T value, @NotNull Class<T> type);
 
     /**
      * Retrieves a value based on its key.
@@ -63,7 +67,8 @@ public interface Data {
      * @param <T> the value type
      * @return the data associated with the key or null
      */
-    <T> T get(String key);
+    @Nullable
+    <T> T get(@NotNull String key);
 
     /**
      * Retrieves a value based on its key, give a default value if not found.
@@ -73,7 +78,8 @@ public interface Data {
      * @param <T>          the value type
      * @return {@link #get(String)} if found, {@code defaultValue} otherwise
      */
-    <T> T getOrDefault(String key, T defaultValue);
+    @Nullable
+    <T> T getOrDefault(@NotNull String key, T defaultValue);
 
     /**
      * Gets if the data has a key.
@@ -81,13 +87,14 @@ public interface Data {
      * @param key the key to check
      * @return true if the data contains the key
      */
-    boolean hasKey(String key);
+    boolean hasKey(@NotNull String key);
 
     /**
      * Gets the list of data keys.
      *
      * @return an unmodifiable {@link Set} containing all the keys
      */
+    @NotNull
     Set<String> getKeys();
 
     /**

src/main/java/net/minestom/server/data/DataContainer.java

@@ -1,5 +1,7 @@
 package net.minestom.server.data;
 
+import org.jetbrains.annotations.Nullable;
+
 /**
  * Represents an element which can have a {@link Data} attached to it.
  * <p>
@@ -15,6 +17,7 @@ public interface DataContainer {
      *
      * @return the {@link Data} of this container, can be null
      */
+    @Nullable
     Data getData();
 
     /**
@@ -22,6 +25,6 @@ public interface DataContainer {
      *
      * @param data the {@link Data} of this container, null to remove it
      */
-    void setData(Data data);
+    void setData(@Nullable Data data);
 
 }

src/main/java/net/minestom/server/data/DataImpl.java

@@ -1,5 +1,7 @@
 package net.minestom.server.data;
 
+import org.jetbrains.annotations.NotNull;
+
 import java.util.Collections;
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
@@ -12,25 +14,26 @@ public class DataImpl implements Data {
     protected final ConcurrentHashMap<String, Object> data = new ConcurrentHashMap<>();
 
     @Override
-    public <T> void set(String key, T value, Class<T> type) {
+    public <T> void set(@NotNull String key, @NotNull T value, @NotNull Class<T> type) {
         this.data.put(key, value);
     }
 
     @Override
-    public <T> T get(String key) {
+    public <T> T get(@NotNull String key) {
         return (T) data.get(key);
     }
 
     @Override
-    public <T> T getOrDefault(String key, T defaultValue) {
+    public <T> T getOrDefault(@NotNull String key, T defaultValue) {
         return (T) data.getOrDefault(key, defaultValue);
     }
 
     @Override
-    public boolean hasKey(String key) {
+    public boolean hasKey(@NotNull String key) {
         return data.containsKey(key);
     }
 
+    @NotNull
     @Override
     public Set<String> getKeys() {
         return Collections.unmodifiableSet(data.keySet());

src/main/java/net/minestom/server/data/SerializableDataImpl.java

@@ -6,6 +6,7 @@ import it.unimi.dsi.fastutil.shorts.Short2ObjectOpenHashMap;
 import net.minestom.server.utils.PrimitiveConversion;
 import net.minestom.server.utils.binary.BinaryReader;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;
@@ -39,7 +40,7 @@ public class SerializableDataImpl extends DataImpl implements SerializableData {
      * @throws UnsupportedOperationException if {@code type} is not registered in {@link DataManager}
      */
     @Override
-    public <T> void set(String key, T value, Class<T> type) {
+    public <T> void set(@NotNull String key, @NotNull T value, @NotNull Class<T> type) {
         if (DATA_MANAGER.getDataType(type) == null) {
             throw new UnsupportedOperationException("Type " + type.getName() + " hasn't been registered in DataManager#registerType");
         }

src/main/java/net/minestom/server/entity/Entity.java

@@ -32,6 +32,8 @@ import net.minestom.server.utils.entity.EntityUtils;
 import net.minestom.server.utils.player.PlayerUtils;
 import net.minestom.server.utils.time.TimeUnit;
 import net.minestom.server.utils.validate.Check;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 import java.util.*;
 import java.util.concurrent.ConcurrentHashMap;
@@ -124,7 +126,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
     private long ticks;
     private final EntityTickEvent tickEvent = new EntityTickEvent(this);
 
-    public Entity(EntityType entityType, Position spawnPosition) {
+    public Entity(@NotNull EntityType entityType, @NotNull Position spawnPosition) {
         this.id = generateId();
         this.entityType = entityType;
         this.uuid = UUID.randomUUID();
@@ -144,11 +146,11 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
      *
      * @param callback the task to execute during the next entity tick
      */
-    public void scheduleNextTick(Consumer<Entity> callback) {
+    public void scheduleNextTick(@NotNull Consumer<Entity> callback) {
         this.nextTick.add(callback);
     }
 
-    public Entity(EntityType entityType) {
+    public Entity(@NotNull EntityType entityType) {
         this(entityType, new Position());
     }
 
@@ -156,6 +158,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
      * @param id the entity unique id ({@link #getEntityId()})
      * @return the entity having the specified id, null if not found
      */
+    @Nullable
     public static Entity getEntity(int id) {
         return entityById.getOrDefault(id, null);
     }
@@ -213,9 +216,9 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
      * {@link Instance#hasEnabledAutoChunkLoad()} returns true.
      *
      * @param position the teleport position
-     * @param callback the callback executed, even if auto chunk is not enabled
+     * @param callback the optional callback executed, even if auto chunk is not enabled
      */
-    public void teleport(Position position, Runnable callback) {
+    public void teleport(@NotNull Position position, @Nullable Runnable callback) {
         Check.notNull(position, "Teleport position cannot be null");
         Check.stateCondition(instance == null, "You need to use Entity#setInstance before teleporting an entity!");
 
@@ -238,7 +241,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
         }
     }
 
-    public void teleport(Position position) {
+    public void teleport(@NotNull Position position) {
         teleport(position, null);
     }
 
@@ -271,7 +274,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
      *
      * @param position the new view
      */
-    public void setView(Position position) {
+    public void setView(@NotNull Position position) {
         setView(position.getYaw(), position.getPitch());
     }
 
@@ -291,14 +294,17 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
     }
 
     /**
+     * Makes the entity auto viewable or only manually.
+     *
      * @param autoViewable should the entity be automatically viewable for close players
+     * @see #isAutoViewable()
      */
     public void setAutoViewable(boolean autoViewable) {
         this.autoViewable = autoViewable;
     }
 
     @Override
-    public boolean addViewer(Player player) {
+    public boolean addViewer(@NotNull Player player) {
         Check.notNull(player, "Viewer cannot be null");
         boolean result = this.viewers.add(player);
         if (!result)
@@ -308,7 +314,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
     }
 
     @Override
-    public boolean removeViewer(Player player) {
+    public boolean removeViewer(@NotNull Player player) {
         Check.notNull(player, "Viewer cannot be null");
         if (!viewers.remove(player))
             return false;
@@ -320,6 +326,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
         return true;
     }
 
+    @NotNull
     @Override
     public Set<Player> getViewers() {
         return Collections.unmodifiableSet(viewers);
@@ -331,7 +338,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
     }
 
     @Override
-    public void setData(Data data) {
+    public void setData(@Nullable Data data) {
         this.data = data;
     }
 
@@ -591,6 +598,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
      *
      * @return the entity unique id
      */
+    @NotNull
     public UUID getUuid() {
         return uuid;
     }
@@ -600,7 +608,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
      *
      * @param uuid the new entity uuid
      */
-    protected void setUuid(UUID uuid) {
+    protected void setUuid(@NotNull UUID uuid) {
         this.uuid = uuid;
     }
 
@@ -618,6 +626,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
      *
      * @return the entity bounding box
      */
+    @NotNull
     public BoundingBox getBoundingBox() {
         return boundingBox;
     }
@@ -640,6 +649,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
      *
      * @return the entity chunk
      */
+    @NotNull
     public Chunk getChunk() {
         return instance.getChunkAt(lastX, lastZ);
     }
@@ -660,7 +670,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
      * @throws NullPointerException  if {@code instance} is null
      * @throws IllegalStateException if {@code instance} has not been registered in {@link InstanceManager}
      */
-    public void setInstance(Instance instance) {
+    public void setInstance(@NotNull Instance instance) {
         Check.notNull(instance, "instance cannot be null!");
         Check.stateCondition(!instance.isRegistered(),
                 "Instances need to be registered, please use InstanceManager#registerInstance or InstanceManager#registerSharedInstance");
@@ -682,6 +692,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
      *
      * @return the entity current velocity
      */
+    @NotNull
     public Vector getVelocity() {
         return velocity;
     }
@@ -693,7 +704,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
      *
      * @param velocity the new entity velocity
      */
-    public void setVelocity(Vector velocity) {
+    public void setVelocity(@NotNull Vector velocity) {
         EntityVelocityEvent entityVelocityEvent = new EntityVelocityEvent(this, velocity);
         callCancellableEvent(EntityVelocityEvent.class, entityVelocityEvent, () -> {
             this.velocity.copy(entityVelocityEvent.getVelocity());
@@ -727,7 +738,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
      * @param entity the entity to get the distance from
      * @return the distance between this and {@code entity}
      */
-    public float getDistance(Entity entity) {
+    public float getDistance(@NotNull Entity entity) {
         Check.notNull(entity, "Entity cannot be null");
         return getPosition().getDistance(entity.getPosition());
     }
@@ -737,6 +748,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
      *
      * @return the entity vehicle, or null if there is not any
      */
+    @Nullable
     public Entity getVehicle() {
         return vehicle;
     }
@@ -748,7 +760,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
      * @throws NullPointerException  if {@code entity} is null
      * @throws IllegalStateException if {@link #getInstance()} returns null
      */
-    public void addPassenger(Entity entity) {
+    public void addPassenger(@NotNull Entity entity) {
         Check.notNull(entity, "Passenger cannot be null");
         Check.stateCondition(instance == null, "You need to set an instance using Entity#setInstance");
 
@@ -769,7 +781,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
      * @throws NullPointerException  if {@code entity} is null
      * @throws IllegalStateException if {@link #getInstance()} returns null
      */
-    public void removePassenger(Entity entity) {
+    public void removePassenger(@NotNull Entity entity) {
         Check.notNull(entity, "Passenger cannot be null");
         Check.stateCondition(instance == null, "You need to set an instance using Entity#setInstance");
 
@@ -793,10 +805,12 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
      *
      * @return an unmodifiable list containing all the entity passengers
      */
+    @NotNull
     public Set<Entity> getPassengers() {
         return Collections.unmodifiableSet(passengers);
     }
 
+    @NotNull
     protected SetPassengersPacket getPassengersPacket() {
         SetPassengersPacket passengersPacket = new SetPassengersPacket();
         passengersPacket.vehicleEntityId = getEntityId();
@@ -998,7 +1012,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
      * @param position the new position
      * @see #refreshPosition(float, float, float)
      */
-    public void refreshPosition(Position position) {
+    public void refreshPosition(@NotNull Position position) {
         refreshPosition(position.getX(), position.getY(), position.getZ());
     }
 
@@ -1010,7 +1024,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
      * @param lastChunk the previous {@link Chunk} of this entity
      * @param newChunk  the new {@link Chunk} of this entity
      */
-    private void updateView(Chunk lastChunk, Chunk newChunk) {
+    private void updateView(@NotNull Chunk lastChunk, @NotNull Chunk newChunk) {
         final boolean isPlayer = this instanceof Player;
 
         if (isPlayer)
@@ -1143,7 +1157,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
      * @param position the checked position chunk
      * @return true if the entity is in the same chunk as {@code position}
      */
-    public boolean sameChunk(Position position) {
+    public boolean sameChunk(@NotNull Position position) {
         Check.notNull(position, "Position cannot be null");
         final Position pos = getPosition();
         final int chunkX1 = ChunkUtils.getChunkCoordinate((int) Math.floor(pos.getX()));
@@ -1161,7 +1175,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
      * @param entity the entity to check
      * @return true if both entities are in the same chunk, false otherwise
      */
-    public boolean sameChunk(Entity entity) {
+    public boolean sameChunk(@NotNull Entity entity) {
         return sameChunk(entity.getPosition());
     }
 
@@ -1193,7 +1207,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
      * @param delay    the time before removing the entity
      * @param timeUnit the unit of the delay
      */
-    public void scheduleRemove(long delay, TimeUnit timeUnit) {
+    public void scheduleRemove(long delay, @NotNull TimeUnit timeUnit) {
         delay = timeUnit.toMilliseconds(delay);
 
         if (delay == 0) { // Cancel the scheduled remove
@@ -1212,6 +1226,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
         return scheduledRemoveTime != 0;
     }
 
+    @NotNull
     protected EntityVelocityPacket getVelocityPacket() {
         final float strength = 8000f / MinecraftServer.TICK_PER_SECOND;
         EntityVelocityPacket velocityPacket = new EntityVelocityPacket();
@@ -1227,6 +1242,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
      *
      * @return The {@link EntityMetaDataPacket} related to this entity
      */
+    @NotNull
     public EntityMetaDataPacket getMetadataPacket() {
         EntityMetaDataPacket metaDataPacket = new EntityMetaDataPacket();
         metaDataPacket.entityId = getEntityId();
@@ -1239,6 +1255,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
      *
      * @return The consumer used to write {@link EntityMetaDataPacket} in {@link #getMetadataPacket()}
      */
+    @NotNull
     public Consumer<BinaryWriter> getMetadataConsumer() {
         return packet -> {
             fillMetadataIndex(packet, 0);
@@ -1273,7 +1290,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
      * @param packet the packet writer
      * @param index  the index to fill/write
      */
-    protected void fillMetadataIndex(BinaryWriter packet, int index) {
+    protected void fillMetadataIndex(@NotNull BinaryWriter packet, int index) {
         switch (index) {
             case 0:
                 fillStateMetadata(packet);
@@ -1299,7 +1316,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
         }
     }
 
-    private void fillStateMetadata(BinaryWriter packet) {
+    private void fillStateMetadata(@NotNull BinaryWriter packet) {
         packet.writeByte((byte) 0);
         packet.writeByte(METADATA_BYTE);
         byte index0 = 0;
@@ -1322,13 +1339,13 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
         packet.writeByte(index0);
     }
 
-    private void fillAirTickMetaData(BinaryWriter packet) {
+    private void fillAirTickMetaData(@NotNull BinaryWriter packet) {
         packet.writeByte((byte) 1);
         packet.writeByte(METADATA_VARINT);
         packet.writeVarInt(air);
     }
 
-    private void fillCustomNameMetaData(BinaryWriter packet) {
+    private void fillCustomNameMetaData(@NotNull BinaryWriter packet) {
         boolean hasCustomName = customName != null;
 
         packet.writeByte((byte) 2);
@@ -1339,25 +1356,25 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
         }
     }
 
-    private void fillCustomNameVisibleMetaData(BinaryWriter packet) {
+    private void fillCustomNameVisibleMetaData(@NotNull BinaryWriter packet) {
         packet.writeByte((byte) 3);
         packet.writeByte(METADATA_BOOLEAN);
         packet.writeBoolean(customNameVisible);
     }
 
-    private void fillSilentMetaData(BinaryWriter packet) {
+    private void fillSilentMetaData(@NotNull BinaryWriter packet) {
         packet.writeByte((byte) 4);
         packet.writeByte(METADATA_BOOLEAN);
         packet.writeBoolean(silent);
     }
 
-    private void fillNoGravityMetaData(BinaryWriter packet) {
+    private void fillNoGravityMetaData(@NotNull BinaryWriter packet) {
         packet.writeByte((byte) 5);
         packet.writeByte(METADATA_BOOLEAN);
         packet.writeBoolean(noGravity);
     }
 
-    private void fillPoseMetaData(BinaryWriter packet) {
+    private void fillPoseMetaData(@NotNull BinaryWriter packet) {
         packet.writeByte((byte) 6);
         packet.writeByte(METADATA_POSE);
         packet.writeVarInt(pose.ordinal());

src/main/java/net/minestom/server/entity/EntityCreature.java

@@ -23,6 +23,7 @@ import net.minestom.server.utils.chunk.ChunkUtils;
 import net.minestom.server.utils.item.ItemStackUtils;
 import net.minestom.server.utils.time.TimeUnit;
 import net.minestom.server.utils.validate.Check;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.ArrayList;
 import java.util.List;
@@ -162,7 +163,7 @@ public abstract class EntityCreature extends LivingEntity {
     }
 
     @Override
-    public boolean addViewer(Player player) {
+    public boolean addViewer(@NotNull Player player) {
         final boolean result = super.addViewer(player);
 
         final PlayerConnection playerConnection = player.getPlayerConnection();

src/main/java/net/minestom/server/entity/ExperienceOrb.java

@@ -2,6 +2,7 @@ package net.minestom.server.entity;
 
 import net.minestom.server.network.packet.server.play.SpawnExperienceOrbPacket;
 import net.minestom.server.network.player.PlayerConnection;
+import org.jetbrains.annotations.NotNull;
 
 public class ExperienceOrb extends Entity {
 
@@ -25,7 +26,7 @@ public class ExperienceOrb extends Entity {
     }
 
     @Override
-    public boolean addViewer(Player player) {
+    public boolean addViewer(@NotNull Player player) {
         final boolean result = super.addViewer(player); // Add player to viewers list
         if (!result)
             return false;

src/main/java/net/minestom/server/entity/ItemEntity.java

@@ -9,6 +9,8 @@ import net.minestom.server.utils.binary.BinaryWriter;
 import net.minestom.server.utils.time.CooldownUtils;
 import net.minestom.server.utils.time.TimeUnit;
 import net.minestom.server.utils.time.UpdateOption;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 import java.util.Set;
 import java.util.function.Consumer;
@@ -48,6 +50,7 @@ public class ItemEntity extends ObjectEntity {
      *
      * @return the merge update option
      */
+    @Nullable
     public static UpdateOption getMergeUpdateOption() {
         return mergeUpdateOption;
     }
@@ -58,7 +61,7 @@ public class ItemEntity extends ObjectEntity {
      *
      * @param mergeUpdateOption the new merge update option
      */
-    public static void setMergeUpdateOption(UpdateOption mergeUpdateOption) {
+    public static void setMergeUpdateOption(@Nullable UpdateOption mergeUpdateOption) {
         ItemEntity.mergeUpdateOption = mergeUpdateOption;
     }
 
@@ -117,6 +120,7 @@ public class ItemEntity extends ObjectEntity {
         this.spawnTime = System.currentTimeMillis();
     }
 
+    @NotNull
     @Override
     public Consumer<BinaryWriter> getMetadataConsumer() {
         return packet -> {
@@ -126,7 +130,7 @@ public class ItemEntity extends ObjectEntity {
     }
 
     @Override
-    protected void fillMetadataIndex(BinaryWriter packet, int index) {
+    protected void fillMetadataIndex(@NotNull BinaryWriter packet, int index) {
         super.fillMetadataIndex(packet, index);
         if (index == 7) {
             packet.writeByte((byte) 7);
@@ -233,7 +237,7 @@ public class ItemEntity extends ObjectEntity {
      * @param delay    the pickup delay
      * @param timeUnit the unit of the delay
      */
-    public void setPickupDelay(long delay, TimeUnit timeUnit) {
+    public void setPickupDelay(long delay, @NotNull TimeUnit timeUnit) {
         this.pickupDelay = timeUnit.toMilliseconds(delay);
     }
 

src/main/java/net/minestom/server/entity/LivingEntity.java

@@ -21,6 +21,7 @@ import net.minestom.server.utils.Position;
 import net.minestom.server.utils.binary.BinaryWriter;
 import net.minestom.server.utils.time.TimeUnit;
 import net.minestom.server.utils.validate.Check;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.Set;
 import java.util.function.Consumer;
@@ -122,6 +123,7 @@ public abstract class LivingEntity extends Entity implements EquipmentHandler {
         }
     }
 
+    @NotNull
     @Override
     public Consumer<BinaryWriter> getMetadataConsumer() {
         return packet -> {
@@ -133,7 +135,7 @@ public abstract class LivingEntity extends Entity implements EquipmentHandler {
     }
 
     @Override
-    protected void fillMetadataIndex(BinaryWriter packet, int index) {
+    protected void fillMetadataIndex(@NotNull BinaryWriter packet, int index) {
         super.fillMetadataIndex(packet, index);
         if (index == 7) {
             packet.writeByte((byte) 7);

src/main/java/net/minestom/server/entity/ObjectEntity.java

@@ -3,6 +3,7 @@ package net.minestom.server.entity;
 import net.minestom.server.network.packet.server.play.SpawnEntityPacket;
 import net.minestom.server.network.player.PlayerConnection;
 import net.minestom.server.utils.Position;
+import org.jetbrains.annotations.NotNull;
 
 public abstract class ObjectEntity extends Entity {
 
@@ -30,7 +31,7 @@ public abstract class ObjectEntity extends Entity {
     }
 
     @Override
-    public boolean addViewer(Player player) {
+    public boolean addViewer(@NotNull Player player) {
         final boolean result = super.addViewer(player);
         if (!result)
             return false;

src/main/java/net/minestom/server/entity/Player.java

@@ -56,6 +56,8 @@ import net.minestom.server.utils.chunk.ChunkUtils;
 import net.minestom.server.utils.validate.Check;
 import net.minestom.server.world.DimensionType;
 import net.minestom.server.world.LevelType;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 import java.util.*;
 import java.util.concurrent.ConcurrentLinkedQueue;
@@ -542,7 +544,7 @@ public class Player extends LivingEntity implements CommandSender {
     }
 
     @Override
-    public boolean addViewer(Player player) {
+    public boolean addViewer(@NotNull Player player) {
         if (player == this)
             return false;
 
@@ -556,7 +558,7 @@ public class Player extends LivingEntity implements CommandSender {
     }
 
     @Override
-    public boolean removeViewer(Player player) {
+    public boolean removeViewer(@NotNull Player player) {
         if (player == this)
             return false;
 
@@ -571,7 +573,7 @@ public class Player extends LivingEntity implements CommandSender {
     }
 
     @Override
-    public void setInstance(Instance instance) {
+    public void setInstance(@NotNull Instance instance) {
         Check.notNull(instance, "instance cannot be null!");
         Check.argCondition(this.instance == instance, "Instance should be different than the current one");
 
@@ -619,6 +621,7 @@ public class Player extends LivingEntity implements CommandSender {
         }
     }
 
+    @NotNull
     @Override
     public Consumer<BinaryWriter> getMetadataConsumer() {
         return packet -> {
@@ -629,7 +632,7 @@ public class Player extends LivingEntity implements CommandSender {
     }
 
     @Override
-    protected void fillMetadataIndex(BinaryWriter packet, int index) {
+    protected void fillMetadataIndex(@NotNull BinaryWriter packet, int index) {
         super.fillMetadataIndex(packet, index);
         if (index == 14) {
             packet.writeByte((byte) 14);
@@ -643,12 +646,12 @@ public class Player extends LivingEntity implements CommandSender {
     }
 
     /**
-     * Send a plugin message to the player
+     * Sends a plugin message to the player.
      *
      * @param channel the message channel
      * @param data    the message data
      */
-    public void sendPluginMessage(String channel, byte[] data) {
+    public void sendPluginMessage(@NotNull String channel, @NotNull byte[] data) {
         PluginMessagePacket pluginMessagePacket = new PluginMessagePacket();
         pluginMessagePacket.channel = channel;
         pluginMessagePacket.data = data;
@@ -656,12 +659,12 @@ public class Player extends LivingEntity implements CommandSender {
     }
 
     /**
-     * Send a plugin message to the player
+     * Sends a plugin message to the player.
      *
      * @param channel the message channel
      * @param message the message
      */
-    public void sendPluginMessage(String channel, String message) {
+    public void sendPluginMessage(@NotNull String channel, @NotNull String message) {
         // Write the data
         BinaryWriter writer = new BinaryWriter();
         writer.writeSizedString(message);
@@ -672,7 +675,7 @@ public class Player extends LivingEntity implements CommandSender {
     }
 
     @Override
-    public void sendMessage(String message) {
+    public void sendMessage(@NotNull String message) {
         sendMessage(ColoredText.of(message));
     }
 
@@ -687,7 +690,7 @@ public class Player extends LivingEntity implements CommandSender {
      * @param message the message to send,
      *                you can use {@link ColoredText} and/or {@link RichMessage} to create it easily
      */
-    public void sendMessage(JsonMessage message) {
+    public void sendMessage(@NotNull JsonMessage message) {
         sendJsonMessage(message.toString());
     }
 
@@ -697,7 +700,7 @@ public class Player extends LivingEntity implements CommandSender {
      * @param text      the text with the legacy color formatting
      * @param colorChar the color character
      */
-    public void sendLegacyMessage(String text, char colorChar) {
+    public void sendLegacyMessage(@NotNull String text, char colorChar) {
         ColoredText coloredText = ColoredText.ofLegacy(text, colorChar);
         sendJsonMessage(coloredText.toString());
     }
@@ -707,12 +710,12 @@ public class Player extends LivingEntity implements CommandSender {
      *
      * @param text the text with the legacy color formatting
      */
-    public void sendLegacyMessage(String text) {
+    public void sendLegacyMessage(@NotNull String text) {
         ColoredText coloredText = ColoredText.ofLegacy(text, ChatParser.COLOR_CHAR);
         sendJsonMessage(coloredText.toString());
     }
 
-    public void sendJsonMessage(String json) {
+    public void sendJsonMessage(@NotNull String json) {
         ChatMessagePacket chatMessagePacket =
                 new ChatMessagePacket(json, ChatMessagePacket.Position.CHAT);
         playerConnection.sendPacket(chatMessagePacket);
@@ -723,13 +726,13 @@ public class Player extends LivingEntity implements CommandSender {
      *
      * @param message the message that the player will send
      */
-    public void chat(String message) {
+    public void chat(@NotNull String message) {
         ClientChatMessagePacket chatMessagePacket = new ClientChatMessagePacket();
         chatMessagePacket.message = message;
         addPacketToQueue(chatMessagePacket);
     }
 
-    public void playSound(Sound sound, SoundCategory soundCategory, int x, int y, int z, float volume, float pitch) {
+    public void playSound(@NotNull Sound sound, @NotNull SoundCategory soundCategory, int x, int y, int z, float volume, float pitch) {
         SoundEffectPacket soundEffectPacket = new SoundEffectPacket();
         soundEffectPacket.soundId = sound.getId();
         soundEffectPacket.soundCategory = soundCategory;
@@ -751,7 +754,7 @@ public class Player extends LivingEntity implements CommandSender {
      * @param data                  data for the effect
      * @param disableRelativeVolume disable volume scaling based on distance
      */
-    public void playEffect(Effects effect, int x, int y, int z, int data, boolean disableRelativeVolume) {
+    public void playEffect(@NotNull Effects effect, int x, int y, int z, int data, boolean disableRelativeVolume) {
         EffectPacket packet = new EffectPacket();
         packet.effectId = effect.getId();
         packet.position = new BlockPosition(x, y, z);
@@ -775,7 +778,7 @@ public class Player extends LivingEntity implements CommandSender {
      * @param header the header text
      * @param footer the footer text
      */
-    public void sendHeaderFooter(ColoredText header, ColoredText footer) {
+    public void sendHeaderFooter(@NotNull ColoredText header, @NotNull ColoredText footer) {
         PlayerListHeaderAndFooterPacket playerListHeaderAndFooterPacket = new PlayerListHeaderAndFooterPacket();
         playerListHeaderAndFooterPacket.emptyHeader = header == null;
         playerListHeaderAndFooterPacket.emptyFooter = footer == null;
@@ -792,7 +795,7 @@ public class Player extends LivingEntity implements CommandSender {
      * @param action the action of the title (where to show it)
      * @see #sendTitleTime(int, int, int) to specify the display time
      */
-    private void sendTitle(ColoredText text, TitlePacket.Action action) {
+    private void sendTitle(@NotNull ColoredText text, @NotNull TitlePacket.Action action) {
         TitlePacket titlePacket = new TitlePacket();
         titlePacket.action = action;
 
@@ -820,7 +823,7 @@ public class Player extends LivingEntity implements CommandSender {
      * @param subtitle the subtitle message
      * @see #sendTitleTime(int, int, int) to specify the display time
      */
-    public void sendTitleSubtitleMessage(ColoredText title, ColoredText subtitle) {
+    public void sendTitleSubtitleMessage(@NotNull ColoredText title, @NotNull ColoredText subtitle) {
         sendTitle(title, TitlePacket.Action.SET_TITLE);
         sendTitle(subtitle, TitlePacket.Action.SET_SUBTITLE);
     }
@@ -831,7 +834,7 @@ public class Player extends LivingEntity implements CommandSender {
      * @param title the title message
      * @see #sendTitleTime(int, int, int) to specify the display time
      */
-    public void sendTitleMessage(ColoredText title) {
+    public void sendTitleMessage(@NotNull ColoredText title) {
         sendTitle(title, TitlePacket.Action.SET_TITLE);
     }
 
@@ -841,7 +844,7 @@ public class Player extends LivingEntity implements CommandSender {
      * @param subtitle the subtitle message
      * @see #sendTitleTime(int, int, int) to specify the display time
      */
-    public void sendSubtitleMessage(ColoredText subtitle) {
+    public void sendSubtitleMessage(@NotNull ColoredText subtitle) {
         sendTitle(subtitle, TitlePacket.Action.SET_SUBTITLE);
     }
 
@@ -851,7 +854,7 @@ public class Player extends LivingEntity implements CommandSender {
      * @param actionBar the action bar message
      * @see #sendTitleTime(int, int, int) to specify the display time
      */
-    public void sendActionBarMessage(ColoredText actionBar) {
+    public void sendActionBarMessage(@NotNull ColoredText actionBar) {
         sendTitle(actionBar, TitlePacket.Action.SET_ACTION_BAR);
     }
 
@@ -890,7 +893,7 @@ public class Player extends LivingEntity implements CommandSender {
     }
 
     @Override
-    public boolean isImmune(DamageType type) {
+    public boolean isImmune(@NotNull DamageType type) {
         if (!getGameMode().canTakeDamage()) {
             return type != DamageType.VOID;
         }
@@ -898,7 +901,7 @@ public class Player extends LivingEntity implements CommandSender {
     }
 
     @Override
-    public void setAttribute(Attribute attribute, float value) {
+    public void setAttribute(@NotNull Attribute attribute, float value) {
         super.setAttribute(attribute, value);
         if (playerConnection != null)
             playerConnection.sendPacket(getPropertiesPacket());
@@ -1022,6 +1025,7 @@ public class Player extends LivingEntity implements CommandSender {
      * @return the player skin object,
      * null means that the player has his {@link #getUuid()} default skin
      */
+    @Nullable
     public PlayerSkin getSkin() {
         return skin;
     }
@@ -1034,7 +1038,7 @@ public class Player extends LivingEntity implements CommandSender {
      * @param skin the player skin, null to reset it to his {@link #getUuid()} default skin
      * @see PlayerSkinInitEvent if you want to apply the skin at connection
      */
-    public synchronized void setSkin(PlayerSkin skin) {
+    public synchronized void setSkin(@Nullable PlayerSkin skin) {
         this.skin = skin;
 
         DestroyEntitiesPacket destroyEntitiesPacket = new DestroyEntitiesPacket();
@@ -1090,6 +1094,7 @@ public class Player extends LivingEntity implements CommandSender {
      *
      * @return the player username
      */
+    @NotNull
     public String getUsername() {
         return username;
     }
@@ -1100,11 +1105,11 @@ public class Player extends LivingEntity implements CommandSender {
      *
      * @param username the new player name
      */
-    protected void setUsername(String username) {
+    protected void setUsername(@NotNull String username) {
         this.username = username;
     }
 
-    private void sendChangeGameStatePacket(ChangeGameStatePacket.Reason reason, float value) {
+    private void sendChangeGameStatePacket(@NotNull ChangeGameStatePacket.Reason reason, float value) {
         ChangeGameStatePacket changeGameStatePacket = new ChangeGameStatePacket();
         changeGameStatePacket.reason = reason;
         changeGameStatePacket.value = value;
@@ -1117,7 +1122,7 @@ public class Player extends LivingEntity implements CommandSender {
      * @param item the item to drop
      * @return true if player can drop the item (event not cancelled), false otherwise
      */
-    public boolean dropItem(ItemStack item) {
+    public boolean dropItem(@NotNull ItemStack item) {
         ItemDropEvent itemDropEvent = new ItemDropEvent(item);
         callEvent(ItemDropEvent.class, itemDropEvent);
         return !itemDropEvent.isCancelled();
@@ -1145,7 +1150,7 @@ public class Player extends LivingEntity implements CommandSender {
      * @param facePoint      the point from where the player should aim
      * @param targetPosition the target position to face
      */
-    public void facePosition(FacePoint facePoint, Position targetPosition) {
+    public void facePosition(@NotNull FacePoint facePoint, @NotNull Position targetPosition) {
         facePosition(facePoint, targetPosition, null, null);
     }
 
@@ -1156,11 +1161,11 @@ public class Player extends LivingEntity implements CommandSender {
      * @param entity      the entity to face
      * @param targetPoint the point to aim at {@code entity} position
      */
-    public void facePosition(FacePoint facePoint, Entity entity, FacePoint targetPoint) {
+    public void facePosition(@NotNull FacePoint facePoint, Entity entity, FacePoint targetPoint) {
         facePosition(facePoint, entity.getPosition(), entity, targetPoint);
     }
 
-    private void facePosition(FacePoint facePoint, Position targetPosition, Entity entity, FacePoint targetPoint) {
+    private void facePosition(@NotNull FacePoint facePoint, @NotNull Position targetPosition, @Nullable Entity entity, @Nullable FacePoint targetPoint) {
         FacePlayerPacket facePlayerPacket = new FacePlayerPacket();
         facePlayerPacket.entityFacePosition = facePoint == FacePoint.EYE ?
                 FacePlayerPacket.FacePosition.EYES : FacePlayerPacket.FacePosition.FEET;
@@ -1180,7 +1185,7 @@ public class Player extends LivingEntity implements CommandSender {
      *
      * @param entity the entity to spectate
      */
-    public void spectate(Entity entity) {
+    public void spectate(@NotNull Entity entity) {
         CameraPacket cameraPacket = new CameraPacket();
         cameraPacket.cameraId = entity.getEntityId();
         playerConnection.sendPacket(cameraPacket);
@@ -1200,6 +1205,7 @@ public class Player extends LivingEntity implements CommandSender {
      *
      * @return the default respawn point
      */
+    @NotNull
     public Position getRespawnPoint() {
         return respawnPoint;
     }
@@ -1209,7 +1215,7 @@ public class Player extends LivingEntity implements CommandSender {
      *
      * @param respawnPoint the player respawn point
      */
-    public void setRespawnPoint(Position respawnPoint) {
+    public void setRespawnPoint(@NotNull Position respawnPoint) {
         this.respawnPoint = respawnPoint;
     }
 
@@ -1320,7 +1326,7 @@ public class Player extends LivingEntity implements CommandSender {
      *
      * @param newChunk the current/new player chunk
      */
-    protected void onChunkChange(Chunk newChunk) {
+    protected void onChunkChange(@NotNull Chunk newChunk) {
         // Previous chunks indexes
         final long[] lastVisibleChunks = viewableChunks.stream().mapToLong(viewableChunks ->
                 ChunkUtils.getChunkIndex(viewableChunks.getChunkX(), viewableChunks.getChunkZ())
@@ -1368,7 +1374,7 @@ public class Player extends LivingEntity implements CommandSender {
     }
 
     @Override
-    public void teleport(Position position, Runnable callback) {
+    public void teleport(@NotNull Position position, @Nullable Runnable callback) {
         super.teleport(position, () -> {
             updatePlayerPosition();
             OptionalCallback.execute(callback);
@@ -1376,7 +1382,7 @@ public class Player extends LivingEntity implements CommandSender {
     }
 
     @Override
-    public void teleport(Position position) {
+    public void teleport(@NotNull Position position) {
         teleport(position, null);
     }
 
@@ -1387,6 +1393,7 @@ public class Player extends LivingEntity implements CommandSender {
      *
      * @return the player connection
      */
+    @NotNull
     public PlayerConnection getPlayerConnection() {
         return playerConnection;
     }
@@ -1405,6 +1412,7 @@ public class Player extends LivingEntity implements CommandSender {
      *
      * @return the player settings
      */
+    @NotNull
     public PlayerSettings getSettings() {
         return settings;
     }
@@ -1418,6 +1426,7 @@ public class Player extends LivingEntity implements CommandSender {
         return dimensionType;
     }
 
+    @NotNull
     public PlayerInventory getInventory() {
         return inventory;
     }
@@ -1446,7 +1455,7 @@ public class Player extends LivingEntity implements CommandSender {
      *
      * @param gameMode the new player GameMode
      */
-    public void setGameMode(GameMode gameMode) {
+    public void setGameMode(@NotNull GameMode gameMode) {
         Check.notNull(gameMode, "GameMode cannot be null");
         this.gameMode = gameMode;
         sendChangeGameStatePacket(ChangeGameStatePacket.Reason.CHANGE_GAMEMODE, gameMode.getId());
@@ -1471,7 +1480,7 @@ public class Player extends LivingEntity implements CommandSender {
      *
      * @param dimensionType the new player dimension
      */
-    protected void sendDimension(DimensionType dimensionType) {
+    protected void sendDimension(@NotNull DimensionType dimensionType) {
         Check.notNull(dimensionType, "Dimension cannot be null!");
         Check.argCondition(dimensionType.equals(getDimensionType()), "The dimension needs to be different than the current one!");
 
@@ -1488,7 +1497,7 @@ public class Player extends LivingEntity implements CommandSender {
      *
      * @param text the kick reason
      */
-    public void kick(ColoredText text) {
+    public void kick(@NotNull ColoredText text) {
         DisconnectPacket disconnectPacket = new DisconnectPacket();
         disconnectPacket.message = text;
         playerConnection.sendPacket(disconnectPacket);
@@ -1501,7 +1510,7 @@ public class Player extends LivingEntity implements CommandSender {
      *
      * @param message the kick reason
      */
-    public void kick(String message) {
+    public void kick(@NotNull String message) {
         kick(ColoredText.of(message));
     }
 
@@ -1581,6 +1590,7 @@ public class Player extends LivingEntity implements CommandSender {
      *
      * @return the currently mined {@link CustomBlock} by the player, null if there is not
      */
+    @Nullable
     public CustomBlock getCustomBlockTarget() {
         return targetCustomBlock;
     }
@@ -1590,6 +1600,7 @@ public class Player extends LivingEntity implements CommandSender {
      *
      * @return the currently open inventory, null if there is not (player inventory is not detected)
      */
+    @Nullable
     public Inventory getOpenInventory() {
         return openInventory;
     }
@@ -1600,7 +1611,7 @@ public class Player extends LivingEntity implements CommandSender {
      * @param inventory the inventory to open
      * @return true if the inventory has been opened/sent to the player, false otherwise (cancelled by event)
      */
-    public boolean openInventory(Inventory inventory) {
+    public boolean openInventory(@NotNull Inventory inventory) {
         Check.notNull(inventory, "Inventory cannot be null, use Player#closeInventory() to close current");
 
         InventoryOpenEvent inventoryOpenEvent = new InventoryOpenEvent(this, inventory);
@@ -1702,7 +1713,7 @@ public class Player extends LivingEntity implements CommandSender {
      *
      * @param chunk the chunk to update the view
      */
-    public void updateViewPosition(Chunk chunk) {
+    public void updateViewPosition(@NotNull Chunk chunk) {
         UpdateViewPositionPacket updateViewPositionPacket = new UpdateViewPositionPacket();
         updateViewPositionPacket.chunkX = chunk.getChunkX();
         updateViewPositionPacket.chunkZ = chunk.getChunkZ();

src/main/java/net/minestom/server/entity/hologram/Hologram.java

@@ -7,6 +7,7 @@ import net.minestom.server.entity.type.decoration.EntityArmorStand;
 import net.minestom.server.instance.Instance;
 import net.minestom.server.utils.Position;
 import net.minestom.server.utils.validate.Check;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.Set;
 
@@ -105,15 +106,16 @@ public class Hologram implements Viewable {
     }
 
     @Override
-    public boolean addViewer(Player player) {
+    public boolean addViewer(@NotNull Player player) {
         return entity.addViewer(player);
     }
 
     @Override
-    public boolean removeViewer(Player player) {
+    public boolean removeViewer(@NotNull Player player) {
         return entity.removeViewer(player);
     }
 
+    @NotNull
     @Override
     public Set<Player> getViewers() {
         return entity.getViewers();

src/main/java/net/minestom/server/entity/type/ambient/EntityBat.java

@@ -5,6 +5,7 @@ import net.minestom.server.entity.EntityType;
 import net.minestom.server.entity.type.Animal;
 import net.minestom.server.utils.Position;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.function.Consumer;
 
@@ -17,6 +18,7 @@ public class EntityBat extends EntityCreature implements Animal {
         setBoundingBox(0.5f, 0.9f, 0.5f);
     }
 
+    @NotNull
     @Override
     public Consumer<BinaryWriter> getMetadataConsumer() {
         return packet -> {
@@ -26,7 +28,7 @@ public class EntityBat extends EntityCreature implements Animal {
     }
 
     @Override
-    protected void fillMetadataIndex(BinaryWriter packet, int index) {
+    protected void fillMetadataIndex(@NotNull BinaryWriter packet, int index) {
         super.fillMetadataIndex(packet, index);
         if (index == 15) {
             packet.writeByte((byte) 15);

src/main/java/net/minestom/server/entity/type/animal/EntityMooshroom.java

@@ -19,6 +19,7 @@ public class EntityMooshroom extends EntityCreature implements Animal {
         setMooshroomType(MooshroomType.RED);
     }
 
+    @NotNull
     @Override
     public Consumer<BinaryWriter> getMetadataConsumer() {
         return packet -> {
@@ -28,7 +29,7 @@ public class EntityMooshroom extends EntityCreature implements Animal {
     }
 
     @Override
-    protected void fillMetadataIndex(BinaryWriter packet, int index) {
+    protected void fillMetadataIndex(@NotNull BinaryWriter packet, int index) {
         super.fillMetadataIndex(packet, index);
         if (index == 16) {
             packet.writeByte((byte) 16);

src/main/java/net/minestom/server/entity/type/animal/EntityPig.java

@@ -5,6 +5,7 @@ import net.minestom.server.entity.EntityType;
 import net.minestom.server.entity.type.Animal;
 import net.minestom.server.utils.Position;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.function.Consumer;
 
@@ -17,6 +18,7 @@ public class EntityPig extends EntityCreature implements Animal {
         setBoundingBox(0.9f, 0.9f, 0.9f);
     }
 
+    @NotNull
     @Override
     public Consumer<BinaryWriter> getMetadataConsumer() {
         return packet -> {
@@ -26,7 +28,7 @@ public class EntityPig extends EntityCreature implements Animal {
     }
 
     @Override
-    protected void fillMetadataIndex(BinaryWriter packet, int index) {
+    protected void fillMetadataIndex(@NotNull BinaryWriter packet, int index) {
         super.fillMetadataIndex(packet, index);
         if (index == 16) {
             packet.writeByte((byte) 16);

src/main/java/net/minestom/server/entity/type/animal/EntityPolarBear.java

@@ -5,6 +5,7 @@ import net.minestom.server.entity.EntityType;
 import net.minestom.server.entity.type.Animal;
 import net.minestom.server.utils.Position;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.function.Consumer;
 
@@ -17,6 +18,7 @@ public class EntityPolarBear extends EntityCreature implements Animal {
         setBoundingBox(1.3f, 1.4f, 1.3f);
     }
 
+    @NotNull
     @Override
     public Consumer<BinaryWriter> getMetadataConsumer() {
         return packet -> {
@@ -26,7 +28,7 @@ public class EntityPolarBear extends EntityCreature implements Animal {
     }
 
     @Override
-    protected void fillMetadataIndex(BinaryWriter packet, int index) {
+    protected void fillMetadataIndex(@NotNull BinaryWriter packet, int index) {
         super.fillMetadataIndex(packet, index);
         if (index == 16) {
             packet.writeByte((byte) 16);

src/main/java/net/minestom/server/entity/type/animal/EntityRabbit.java

@@ -5,6 +5,7 @@ import net.minestom.server.entity.EntityType;
 import net.minestom.server.entity.type.Animal;
 import net.minestom.server.utils.Position;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.function.Consumer;
 
@@ -17,6 +18,7 @@ public class EntityRabbit extends EntityCreature implements Animal {
         setBoundingBox(0.4f, 0.5f, 0.4f);
     }
 
+    @NotNull
     @Override
     public Consumer<BinaryWriter> getMetadataConsumer() {
         return packet -> {
@@ -26,7 +28,7 @@ public class EntityRabbit extends EntityCreature implements Animal {
     }
 
     @Override
-    protected void fillMetadataIndex(BinaryWriter packet, int index) {
+    protected void fillMetadataIndex(@NotNull BinaryWriter packet, int index) {
         super.fillMetadataIndex(packet, index);
         if (index == 16) {
             packet.writeByte((byte) 16);

src/main/java/net/minestom/server/entity/type/decoration/EntityArmorStand.java

@@ -11,6 +11,7 @@ import net.minestom.server.utils.Position;
 import net.minestom.server.utils.Vector;
 import net.minestom.server.utils.binary.BinaryWriter;
 import net.minestom.server.utils.item.ItemStackUtils;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.function.Consumer;
 
@@ -61,7 +62,7 @@ public class EntityArmorStand extends ObjectEntity implements EquipmentHandler {
     }
 
     @Override
-    public boolean addViewer(Player player) {
+    public boolean addViewer(@NotNull Player player) {
         boolean result = super.addViewer(player);
         syncEquipments(player.getPlayerConnection());
         return result;
@@ -72,6 +73,7 @@ public class EntityArmorStand extends ObjectEntity implements EquipmentHandler {
         return 0;
     }
 
+    @NotNull
     @Override
     public Consumer<BinaryWriter> getMetadataConsumer() {
         return packet -> {
@@ -87,7 +89,7 @@ public class EntityArmorStand extends ObjectEntity implements EquipmentHandler {
     }
 
     @Override
-    protected void fillMetadataIndex(BinaryWriter packet, int index) {
+    protected void fillMetadataIndex(@NotNull BinaryWriter packet, int index) {
         super.fillMetadataIndex(packet, index);
         if (index == 14) {
             packet.writeByte((byte) 14);

src/main/java/net/minestom/server/entity/type/decoration/EntityItemFrame.java

@@ -7,6 +7,7 @@ import net.minestom.server.utils.Position;
 import net.minestom.server.utils.Rotation;
 import net.minestom.server.utils.binary.BinaryWriter;
 import net.minestom.server.utils.item.ItemStackUtils;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.function.Consumer;
 
@@ -26,6 +27,7 @@ public class EntityItemFrame extends ObjectEntity {
         setGravity(0f);
     }
 
+    @NotNull
     @Override
     public Consumer<BinaryWriter> getMetadataConsumer() {
         return packet -> {
@@ -36,7 +38,7 @@ public class EntityItemFrame extends ObjectEntity {
     }
 
     @Override
-    protected void fillMetadataIndex(BinaryWriter packet, int index) {
+    protected void fillMetadataIndex(@NotNull BinaryWriter packet, int index) {
         super.fillMetadataIndex(packet, index);
         if (index == 7) {
             packet.writeByte((byte) 7);

src/main/java/net/minestom/server/entity/type/monster/EntityBlaze.java

@@ -5,6 +5,7 @@ import net.minestom.server.entity.EntityType;
 import net.minestom.server.entity.type.Monster;
 import net.minestom.server.utils.Position;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.function.Consumer;
 
@@ -15,6 +16,7 @@ public class EntityBlaze extends EntityCreature implements Monster {
         setBoundingBox(0.6f, 1.8f, 0.6f);
     }
 
+    @NotNull
     @Override
     public Consumer<BinaryWriter> getMetadataConsumer() {
         return packet -> {
@@ -24,7 +26,7 @@ public class EntityBlaze extends EntityCreature implements Monster {
     }
 
     @Override
-    protected void fillMetadataIndex(BinaryWriter packet, int index) {
+    protected void fillMetadataIndex(@NotNull BinaryWriter packet, int index) {
         super.fillMetadataIndex(packet, index);
         if (index == 15) {
             packet.writeByte((byte) 15);

src/main/java/net/minestom/server/entity/type/monster/EntityCreeper.java

@@ -5,6 +5,7 @@ import net.minestom.server.entity.EntityType;
 import net.minestom.server.entity.type.Monster;
 import net.minestom.server.utils.Position;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.function.Consumer;
 
@@ -19,6 +20,7 @@ public class EntityCreeper extends EntityCreature implements Monster {
         setBoundingBox(0.6f, 1.7f, 0.6f);
     }
 
+    @NotNull
     @Override
     public Consumer<BinaryWriter> getMetadataConsumer() {
         return packet -> {
@@ -28,7 +30,7 @@ public class EntityCreeper extends EntityCreature implements Monster {
     }
 
     @Override
-    protected void fillMetadataIndex(BinaryWriter packet, int index) {
+    protected void fillMetadataIndex(@NotNull BinaryWriter packet, int index) {
         super.fillMetadataIndex(packet, index);
         if (index == 15) {
             packet.writeByte((byte) 15);

src/main/java/net/minestom/server/entity/type/monster/EntityGhast.java

@@ -5,6 +5,7 @@ import net.minestom.server.entity.EntityType;
 import net.minestom.server.entity.type.Monster;
 import net.minestom.server.utils.Position;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.function.Consumer;
 
@@ -18,6 +19,7 @@ public class EntityGhast extends EntityCreature implements Monster {
         setEyeHeight(2.6f);
     }
 
+    @NotNull
     @Override
     public Consumer<BinaryWriter> getMetadataConsumer() {
         return packet -> {
@@ -27,7 +29,7 @@ public class EntityGhast extends EntityCreature implements Monster {
     }
 
     @Override
-    protected void fillMetadataIndex(BinaryWriter packet, int index) {
+    protected void fillMetadataIndex(@NotNull BinaryWriter packet, int index) {
         super.fillMetadataIndex(packet, index);
         if (index == 15) {
             packet.writeByte((byte) 15);

src/main/java/net/minestom/server/entity/type/monster/EntityGuardian.java

@@ -6,6 +6,7 @@ import net.minestom.server.entity.EntityType;
 import net.minestom.server.entity.type.Monster;
 import net.minestom.server.utils.Position;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.function.Consumer;
 
@@ -19,6 +20,7 @@ public class EntityGuardian extends EntityCreature implements Monster {
         setBoundingBox(0.85f, 0.85f, 0.85f);
     }
 
+    @NotNull
     @Override
     public Consumer<BinaryWriter> getMetadataConsumer() {
         return packet -> {
@@ -29,7 +31,7 @@ public class EntityGuardian extends EntityCreature implements Monster {
     }
 
     @Override
-    protected void fillMetadataIndex(BinaryWriter packet, int index) {
+    protected void fillMetadataIndex(@NotNull BinaryWriter packet, int index) {
         super.fillMetadataIndex(packet, index);
         if (index == 15) {
             packet.writeByte((byte) 15);

src/main/java/net/minestom/server/entity/type/monster/EntityPhantom.java

@@ -5,6 +5,7 @@ import net.minestom.server.entity.EntityType;
 import net.minestom.server.entity.type.Monster;
 import net.minestom.server.utils.Position;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.function.Consumer;
 
@@ -17,6 +18,7 @@ public class EntityPhantom extends EntityCreature implements Monster {
         setBoundingBox(0.9f, 0.5f, 0.9f); // TODO change based on size
     }
 
+    @NotNull
     @Override
     public Consumer<BinaryWriter> getMetadataConsumer() {
         return packet -> {
@@ -26,7 +28,7 @@ public class EntityPhantom extends EntityCreature implements Monster {
     }
 
     @Override
-    protected void fillMetadataIndex(BinaryWriter packet, int index) {
+    protected void fillMetadataIndex(@NotNull BinaryWriter packet, int index) {
         super.fillMetadataIndex(packet, index);
         if (index == 15) {
             packet.writeByte((byte) 15);

src/main/java/net/minestom/server/entity/type/monster/EntitySlime.java

@@ -5,6 +5,7 @@ import net.minestom.server.entity.EntityType;
 import net.minestom.server.entity.type.Monster;
 import net.minestom.server.utils.Position;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.function.Consumer;
 
@@ -17,6 +18,7 @@ public class EntitySlime extends EntityCreature implements Monster {
         setSize(1);
     }
 
+    @NotNull
     @Override
     public Consumer<BinaryWriter> getMetadataConsumer() {
         return packet -> {
@@ -26,7 +28,7 @@ public class EntitySlime extends EntityCreature implements Monster {
     }
 
     @Override
-    protected void fillMetadataIndex(BinaryWriter packet, int index) {
+    protected void fillMetadataIndex(@NotNull BinaryWriter packet, int index) {
         super.fillMetadataIndex(packet, index);
         if (index == 15) {
             packet.writeByte((byte) 15);

src/main/java/net/minestom/server/entity/type/monster/EntitySpider.java

@@ -5,6 +5,7 @@ import net.minestom.server.entity.EntityType;
 import net.minestom.server.entity.type.Monster;
 import net.minestom.server.utils.Position;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.function.Consumer;
 
@@ -18,6 +19,7 @@ public class EntitySpider extends EntityCreature implements Monster {
         setEyeHeight(0.65f);
     }
 
+    @NotNull
     @Override
     public Consumer<BinaryWriter> getMetadataConsumer() {
         return packet -> {
@@ -27,7 +29,7 @@ public class EntitySpider extends EntityCreature implements Monster {
     }
 
     @Override
-    protected void fillMetadataIndex(BinaryWriter packet, int index) {
+    protected void fillMetadataIndex(@NotNull BinaryWriter packet, int index) {
         super.fillMetadataIndex(packet, index);
         if (index == 15) {
             packet.writeByte((byte) 15);

src/main/java/net/minestom/server/entity/type/monster/EntityWitch.java

@@ -5,6 +5,7 @@ import net.minestom.server.entity.EntityType;
 import net.minestom.server.entity.type.Monster;
 import net.minestom.server.utils.Position;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.function.Consumer;
 
@@ -18,6 +19,7 @@ public class EntityWitch extends EntityCreature implements Monster {
         setEyeHeight(1.62f);
     }
 
+    @NotNull
     @Override
     public Consumer<BinaryWriter> getMetadataConsumer() {
         return packet -> {
@@ -27,7 +29,7 @@ public class EntityWitch extends EntityCreature implements Monster {
     }
 
     @Override
-    protected void fillMetadataIndex(BinaryWriter packet, int index) {
+    protected void fillMetadataIndex(@NotNull BinaryWriter packet, int index) {
         super.fillMetadataIndex(packet, index);
         if (index == 16) {
             packet.writeByte((byte) 16);

src/main/java/net/minestom/server/entity/type/monster/EntityZombie.java

@@ -5,6 +5,7 @@ import net.minestom.server.entity.EntityType;
 import net.minestom.server.entity.type.Monster;
 import net.minestom.server.utils.Position;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.function.Consumer;
 
@@ -18,6 +19,7 @@ public class EntityZombie extends EntityCreature implements Monster {
         setBoundingBox(0.6f, 1.95f, 0.6f);
     }
 
+    @NotNull
     @Override
     public Consumer<BinaryWriter> getMetadataConsumer() {
         return packet -> {
@@ -28,7 +30,7 @@ public class EntityZombie extends EntityCreature implements Monster {
     }
 
     @Override
-    protected void fillMetadataIndex(BinaryWriter packet, int index) {
+    protected void fillMetadataIndex(@NotNull BinaryWriter packet, int index) {
         super.fillMetadataIndex(packet, index);
         if (index == 15) {
             packet.writeByte((byte) 15);

src/main/java/net/minestom/server/entity/type/monster/EntityZombifiedPiglin.java

@@ -5,6 +5,7 @@ import net.minestom.server.entity.EntityType;
 import net.minestom.server.entity.type.Monster;
 import net.minestom.server.utils.Position;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.function.Consumer;
 
@@ -18,6 +19,7 @@ public class EntityZombifiedPiglin extends EntityCreature implements Monster {
         setBoundingBox(0.6f, 1.95f, 0.6f);
     }
 
+    @NotNull
     @Override
     public Consumer<BinaryWriter> getMetadataConsumer() {
         return packet -> {
@@ -28,7 +30,7 @@ public class EntityZombifiedPiglin extends EntityCreature implements Monster {
     }
 
     @Override
-    protected void fillMetadataIndex(BinaryWriter packet, int index) {
+    protected void fillMetadataIndex(@NotNull BinaryWriter packet, int index) {
         super.fillMetadataIndex(packet, index);
         if (index == 15) {
             packet.writeByte((byte) 15);

src/main/java/net/minestom/server/entity/type/other/EntityAreaEffectCloud.java

@@ -5,6 +5,7 @@ import net.minestom.server.entity.ObjectEntity;
 import net.minestom.server.particle.Particle;
 import net.minestom.server.utils.Position;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.function.Consumer;
 
@@ -26,6 +27,7 @@ public class EntityAreaEffectCloud extends ObjectEntity {
         });
     }
 
+    @NotNull
     @Override
     public Consumer<BinaryWriter> getMetadataConsumer() {
         return packet -> {
@@ -38,7 +40,7 @@ public class EntityAreaEffectCloud extends ObjectEntity {
     }
 
     @Override
-    protected void fillMetadataIndex(BinaryWriter packet, int index) {
+    protected void fillMetadataIndex(@NotNull BinaryWriter packet, int index) {
         super.fillMetadataIndex(packet, index);
         if (index == 7) {
             packet.writeByte((byte) 7);

src/main/java/net/minestom/server/entity/type/other/EntityIronGolem.java

@@ -5,6 +5,7 @@ import net.minestom.server.entity.EntityType;
 import net.minestom.server.entity.type.Constructable;
 import net.minestom.server.utils.Position;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.function.Consumer;
 
@@ -17,6 +18,7 @@ public class EntityIronGolem extends EntityCreature implements Constructable {
         setBoundingBox(1.4f, 2.7f, 1.4f);
     }
 
+    @NotNull
     @Override
     public Consumer<BinaryWriter> getMetadataConsumer() {
         return packet -> {
@@ -26,7 +28,7 @@ public class EntityIronGolem extends EntityCreature implements Constructable {
     }
 
     @Override
-    protected void fillMetadataIndex(BinaryWriter packet, int index) {
+    protected void fillMetadataIndex(@NotNull BinaryWriter packet, int index) {
         super.fillMetadataIndex(packet, index);
         if (index == 15) {
             packet.writeByte((byte) 15);

src/main/java/net/minestom/server/entity/type/other/EntitySnowman.java

@@ -5,6 +5,7 @@ import net.minestom.server.entity.EntityType;
 import net.minestom.server.entity.type.Constructable;
 import net.minestom.server.utils.Position;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.function.Consumer;
 
@@ -17,6 +18,7 @@ public class EntitySnowman extends EntityCreature implements Constructable {
         setBoundingBox(0.7f, 1.9f, 0.7f);
     }
 
+    @NotNull
     @Override
     public Consumer<BinaryWriter> getMetadataConsumer() {
         return packet -> {
@@ -26,7 +28,7 @@ public class EntitySnowman extends EntityCreature implements Constructable {
     }
 
     @Override
-    protected void fillMetadataIndex(BinaryWriter packet, int index) {
+    protected void fillMetadataIndex(@NotNull BinaryWriter packet, int index) {
         super.fillMetadataIndex(packet, index);
         if (index == 15) {
             packet.writeByte((byte) 15);

src/main/java/net/minestom/server/entity/type/projectile/EntityEyeOfEnder.java

@@ -7,6 +7,7 @@ import net.minestom.server.item.ItemStack;
 import net.minestom.server.item.Material;
 import net.minestom.server.utils.Position;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.function.Consumer;
 
@@ -18,6 +19,7 @@ public class EntityEyeOfEnder extends ObjectEntity implements Projectile {
         super(EntityType.EYE_OF_ENDER, spawnPosition);
     }
 
+    @NotNull
     @Override
     public Consumer<BinaryWriter> getMetadataConsumer() {
         return packet -> {
@@ -27,7 +29,7 @@ public class EntityEyeOfEnder extends ObjectEntity implements Projectile {
     }
 
     @Override
-    protected void fillMetadataIndex(BinaryWriter packet, int index) {
+    protected void fillMetadataIndex(@NotNull BinaryWriter packet, int index) {
         super.fillMetadataIndex(packet, index);
         if (index == 7) {
             packet.writeByte((byte) 7);

src/main/java/net/minestom/server/entity/type/projectile/EntityPotion.java

@@ -6,6 +6,7 @@ import net.minestom.server.entity.type.Projectile;
 import net.minestom.server.item.ItemStack;
 import net.minestom.server.utils.Position;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.function.Consumer;
 
@@ -19,6 +20,7 @@ public class EntityPotion extends ObjectEntity implements Projectile {
         setPotion(potion);
     }
 
+    @NotNull
     @Override
     public Consumer<BinaryWriter> getMetadataConsumer() {
         return packet -> {
@@ -28,7 +30,7 @@ public class EntityPotion extends ObjectEntity implements Projectile {
     }
 
     @Override
-    protected void fillMetadataIndex(BinaryWriter packet, int index) {
+    protected void fillMetadataIndex(@NotNull BinaryWriter packet, int index) {
         super.fillMetadataIndex(packet, index);
         if (index == 7) {
             packet.writeByte((byte) 7);

src/main/java/net/minestom/server/entity/type/vehicle/EntityBoat.java

@@ -6,6 +6,7 @@ import net.minestom.server.entity.type.Vehicle;
 import net.minestom.server.utils.Position;
 import net.minestom.server.utils.binary.BinaryWriter;
 import net.minestom.server.utils.validate.Check;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.function.Consumer;
 
@@ -26,6 +27,7 @@ public class EntityBoat extends ObjectEntity implements Vehicle {
         return 0;
     }
 
+    @NotNull
     @Override
     public Consumer<BinaryWriter> getMetadataConsumer() {
         return packet -> {
@@ -39,7 +41,7 @@ public class EntityBoat extends ObjectEntity implements Vehicle {
     }
 
     @Override
-    protected void fillMetadataIndex(BinaryWriter packet, int index) {
+    protected void fillMetadataIndex(@NotNull BinaryWriter packet, int index) {
         super.fillMetadataIndex(packet, index);
         if (index == 10) {
             packet.writeByte((byte) 10);

src/main/java/net/minestom/server/instance/Chunk.java

@@ -27,6 +27,8 @@ import net.minestom.server.utils.player.PlayerUtils;
 import net.minestom.server.utils.validate.Check;
 import net.minestom.server.world.biomes.Biome;
 import net.minestom.server.world.biomes.BiomeManager;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 import java.util.*;
 import java.util.concurrent.CopyOnWriteArraySet;
@@ -79,7 +81,7 @@ public abstract class Chunk implements Viewable, DataContainer {
     // Data
     protected Data data;
 
-    public Chunk(Instance instance, Biome[] biomes, int chunkX, int chunkZ, boolean shouldGenerate) {
+    public Chunk(@NotNull Instance instance, @Nullable Biome[] biomes, int chunkX, int chunkZ, boolean shouldGenerate) {
         this.instance = instance;
         this.chunkX = chunkX;
         this.chunkZ = chunkZ;
@@ -114,7 +116,7 @@ public abstract class Chunk implements Viewable, DataContainer {
      *                      Warning: <code>customBlockId</code> cannot be 0 and needs to be valid since the update delay and method
      *                      will be retrieved from the associated {@link CustomBlock} object
      */
-    public abstract void UNSAFE_setBlock(int x, int y, int z, short blockStateId, short customBlockId, Data data, boolean updatable);
+    public abstract void UNSAFE_setBlock(int x, int y, int z, short blockStateId, short customBlockId, @Nullable Data data, boolean updatable);
 
     /**
      * Executes a chunk tick.
@@ -126,7 +128,7 @@ public abstract class Chunk implements Viewable, DataContainer {
      * @param time     the time of the update in milliseconds
      * @param instance the {@link Instance} linked to this chunk
      */
-    public abstract void tick(long time, Instance instance);
+    public abstract void tick(long time, @NotNull Instance instance);
 
     /**
      * Gets the block state id at a position.
@@ -175,6 +177,7 @@ public abstract class Chunk implements Viewable, DataContainer {
      * @param index the block index
      * @return the {@link Data} at the block index, null if none
      */
+    @Nullable
     public abstract Data getBlockData(int index);
 
     /**
@@ -192,6 +195,7 @@ public abstract class Chunk implements Viewable, DataContainer {
      *
      * @return the block entities in this chunk
      */
+    @NotNull
     public abstract Set<Integer> getBlockEntities();
 
     /**
@@ -210,17 +214,18 @@ public abstract class Chunk implements Viewable, DataContainer {
      * @param reader   the data reader
      *                 WARNING: the data will not necessary be read directly in the same thread,
      *                 be sure that the data is only used for this reading.
-     * @param callback the callback to execute once the chunk is done reading
+     * @param callback the optional callback to execute once the chunk is done reading
      *                 WARNING: this need to be called to notify the instance.
      * @see #getSerializedData() which is responsible for the serialized data given
      */
-    public abstract void readChunk(BinaryReader reader, ChunkCallback callback);
+    public abstract void readChunk(@NotNull BinaryReader reader, @Nullable ChunkCallback callback);
 
     /**
      * Creates a {@link ChunkDataPacket} with this chunk data ready to be written.
      *
      * @return a new chunk data packet
      */
+    @NotNull
     protected abstract ChunkDataPacket createFreshPacket();
 
     /**
@@ -395,6 +400,7 @@ public abstract class Chunk implements Viewable, DataContainer {
      *
      * @return a fresh non-full chunk data packet
      */
+    @NotNull
     public ChunkDataPacket getFreshPartialDataPacket() {
         ChunkDataPacket fullDataPacket = createFreshPacket();
         fullDataPacket.fullChunk = false;
@@ -423,7 +429,7 @@ public abstract class Chunk implements Viewable, DataContainer {
      * @return true if the player has just been added to the viewer collection
      */
     @Override
-    public boolean addViewer(Player player) {
+    public boolean addViewer(@NotNull Player player) {
         final boolean result = this.viewers.add(player);
 
         // Send the chunk data & light packets to the player
@@ -444,7 +450,7 @@ public abstract class Chunk implements Viewable, DataContainer {
      * @return true if the player has just been removed to the viewer collection
      */
     @Override
-    public boolean removeViewer(Player player) {
+    public boolean removeViewer(@NotNull Player player) {
         final boolean result = this.viewers.remove(player);
 
         // Remove from the viewable chunks set
@@ -455,6 +461,7 @@ public abstract class Chunk implements Viewable, DataContainer {
         return result;
     }
 
+    @NotNull
     @Override
     public Set<Player> getViewers() {
         return Collections.unmodifiableSet(viewers);
@@ -563,6 +570,7 @@ public abstract class Chunk implements Viewable, DataContainer {
      * @param section the chunk section to update
      * @return the {@link ChunkDataPacket} to update a single chunk section
      */
+    @NotNull
     protected ChunkDataPacket getChunkSectionUpdatePacket(int section) {
         ChunkDataPacket chunkDataPacket = getFreshPartialDataPacket();
         chunkDataPacket.fullChunk = false;

src/main/java/net/minestom/server/instance/DynamicChunk.java

@@ -23,6 +23,7 @@ import net.minestom.server.utils.time.CooldownUtils;
 import net.minestom.server.utils.time.UpdateOption;
 import net.minestom.server.utils.validate.Check;
 import net.minestom.server.world.biomes.Biome;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.HashSet;
 import java.util.Set;
@@ -123,7 +124,7 @@ public class DynamicChunk extends Chunk {
     }
 
     @Override
-    public void tick(long time, Instance instance) {
+    public void tick(long time, @NotNull Instance instance) {
         if (updatableBlocks.isEmpty())
             return;
 
@@ -202,6 +203,7 @@ public class DynamicChunk extends Chunk {
         }
     }
 
+    @NotNull
     @Override
     public Set<Integer> getBlockEntities() {
         return blockEntities;
@@ -302,7 +304,7 @@ public class DynamicChunk extends Chunk {
     }
 
     @Override
-    public void readChunk(BinaryReader reader, ChunkCallback callback) {
+    public void readChunk(@NotNull BinaryReader reader, ChunkCallback callback) {
         // Check the buffer length
         final int length = reader.available();
         Check.argCondition(length == 0, "The length of the buffer must be > 0");
@@ -385,6 +387,7 @@ public class DynamicChunk extends Chunk {
         }).schedule();
     }
 
+    @NotNull
     @Override
     protected ChunkDataPacket createFreshPacket() {
         ChunkDataPacket fullDataPacket = new ChunkDataPacket();

src/main/java/net/minestom/server/instance/IChunkLoader.java

@@ -4,6 +4,8 @@ import net.minestom.server.MinecraftServer;
 import net.minestom.server.utils.callback.OptionalCallback;
 import net.minestom.server.utils.chunk.ChunkCallback;
 import net.minestom.server.utils.thread.MinestomThread;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 import java.util.Collection;
 import java.util.concurrent.ExecutorService;
@@ -26,7 +28,7 @@ public interface IChunkLoader {
      *                 never called if the method returns false. Can be null.
      * @return true if the chunk loaded successfully, false otherwise
      */
-    boolean loadChunk(Instance instance, int chunkX, int chunkZ, ChunkCallback callback);
+    boolean loadChunk(@NotNull Instance instance, int chunkX, int chunkZ, @Nullable ChunkCallback callback);
 
     /**
      * Saves a {@link Chunk} with an optional callback for when it is done.
@@ -36,7 +38,7 @@ public interface IChunkLoader {
      *                 should be called even if the saving failed (you can throw an exception).
      *                 Can be null.
      */
-    void saveChunk(Chunk chunk, Runnable callback);
+    void saveChunk(@NotNull Chunk chunk, @Nullable Runnable callback);
 
     /**
      * Saves multiple chunks with an optional callback for when it is done.
@@ -48,7 +50,7 @@ public interface IChunkLoader {
      *                 should be called even if the saving failed (you can throw an exception).
      *                 Can be null.
      */
-    default void saveChunks(Collection<Chunk> chunks, Runnable callback) {
+    default void saveChunks(@NotNull Collection<Chunk> chunks, @Nullable Runnable callback) {
         if (supportsParallelSaving()) {
             ExecutorService parallelSavingThreadPool = new MinestomThread(MinecraftServer.THREAD_COUNT_PARALLEL_CHUNK_SAVING, MinecraftServer.THREAD_NAME_PARALLEL_CHUNK_SAVING, true);
             chunks.forEach(c -> parallelSavingThreadPool.execute(() -> saveChunk(c, null)));

src/main/java/net/minestom/server/instance/Instance.java

@@ -30,6 +30,8 @@ import net.minestom.server.utils.time.TimeUnit;
 import net.minestom.server.utils.time.UpdateOption;
 import net.minestom.server.utils.validate.Check;
 import net.minestom.server.world.DimensionType;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 import java.util.*;
 import java.util.concurrent.ConcurrentHashMap;
@@ -145,7 +147,7 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      * @param callback optional consumer called after the chunk has been generated,
      *                 the returned chunk will never be null
      */
-    public abstract void loadChunk(int chunkX, int chunkZ, ChunkCallback callback);
+    public abstract void loadChunk(int chunkX, int chunkZ, @Nullable ChunkCallback callback);
 
     /**
      * Loads the chunk if the chunk is already loaded or if
@@ -156,7 +158,7 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      * @param callback optional consumer called after the chunk has tried to be loaded,
      *                 contains a chunk if it is successful, null otherwise
      */
-    public abstract void loadOptionalChunk(int chunkX, int chunkZ, ChunkCallback callback);
+    public abstract void loadOptionalChunk(int chunkX, int chunkZ, @Nullable ChunkCallback callback);
 
     /**
      * Schedules the removal of a {@link Chunk}, this method does not promise when it will be done.
@@ -167,7 +169,7 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      *
      * @param chunk the chunk to unload
      */
-    public abstract void unloadChunk(Chunk chunk);
+    public abstract void unloadChunk(@NotNull Chunk chunk);
 
     /**
      * Gets the loaded {@link Chunk} at a position.
@@ -178,6 +180,7 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      * @param chunkZ the chunk Z
      * @return the chunk at the specified position, null if not loaded
      */
+    @Nullable
     public abstract Chunk getChunk(int chunkX, int chunkZ);
 
     /**
@@ -186,14 +189,14 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      * @param chunk    the {@link Chunk} to save
      * @param callback optional callback called when the {@link Chunk} is done saving
      */
-    public abstract void saveChunkToStorage(Chunk chunk, Runnable callback);
+    public abstract void saveChunkToStorage(Chunk chunk, @Nullable Runnable callback);
 
     /**
      * Saves multiple chunks to permanent storage.
      *
      * @param callback optional callback called when the chunks are done saving
      */
-    public abstract void saveChunksToStorage(Runnable callback);
+    public abstract void saveChunksToStorage(@Nullable Runnable callback);
 
     /**
      * Creates a new {@link BlockBatch} linked to this instance.
@@ -256,7 +259,7 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      * @param chunkZ   the chunk X
      * @param callback the optional callback executed once the {@link Chunk} has been retrieved
      */
-    protected abstract void retrieveChunk(int chunkX, int chunkZ, ChunkCallback callback);
+    protected abstract void retrieveChunk(int chunkX, int chunkZ, @Nullable ChunkCallback callback);
 
     /**
      * Called to generated a new {@link Chunk} from scratch.
@@ -267,7 +270,7 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      * @param chunkZ   the chunk Z
      * @param callback the optional callback executed with the newly created {@link Chunk}
      */
-    protected abstract void createChunk(int chunkX, int chunkZ, ChunkCallback callback);
+    protected abstract void createChunk(int chunkX, int chunkZ, @Nullable ChunkCallback callback);
 
     /**
      * When set to true, chunks will load automatically when requested.
@@ -543,7 +546,7 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      * @param position the chunk position
      * @param callback the optional callback to run when the chunk is loaded
      */
-    public void loadChunk(Position position, ChunkCallback callback) {
+    public void loadChunk(Position position, @Nullable ChunkCallback callback) {
         final int chunkX = ChunkUtils.getChunkCoordinate((int) position.getX());
         final int chunkZ = ChunkUtils.getChunkCoordinate((int) position.getZ());
         loadChunk(chunkX, chunkZ, callback);
@@ -556,7 +559,7 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      * @param position the chunk position
      * @param callback the optional callback executed when the chunk is loaded (or with a null chunk if not)
      */
-    public void loadOptionalChunk(Position position, ChunkCallback callback) {
+    public void loadOptionalChunk(Position position, @Nullable ChunkCallback callback) {
         final int chunkX = ChunkUtils.getChunkCoordinate((int) position.getX());
         final int chunkZ = ChunkUtils.getChunkCoordinate((int) position.getZ());
         loadOptionalChunk(chunkX, chunkZ, callback);
@@ -606,7 +609,7 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      * @param blockPosition the block position
      * @return the block state id at the position
      */
-    public short getBlockStateId(BlockPosition blockPosition) {
+    public short getBlockStateId(@NotNull BlockPosition blockPosition) {
         return getBlockStateId(blockPosition.getX(), blockPosition.getY(), blockPosition.getZ());
     }
 
@@ -618,6 +621,7 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      * @param z the Z position
      * @return the custom block object at the position, null if not any
      */
+    @Nullable
     public CustomBlock getCustomBlock(int x, int y, int z) {
         final Chunk chunk = getChunkAt(x, z);
         Check.notNull(chunk, "The chunk at " + x + ":" + z + " is not loaded");
@@ -630,7 +634,8 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      * @param blockPosition the block position
      * @return the custom block object at the position, null if not any
      */
-    public CustomBlock getCustomBlock(BlockPosition blockPosition) {
+    @Nullable
+    public CustomBlock getCustomBlock(@NotNull BlockPosition blockPosition) {
         return getCustomBlock(blockPosition.getX(), blockPosition.getY(), blockPosition.getZ());
     }
 
@@ -642,7 +647,7 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      * @param actionParam
      * @see <a href="https://wiki.vg/Protocol#Block_Action">BlockActionPacket</a> for the action id &amp; param
      */
-    public void sendBlockAction(BlockPosition blockPosition, byte actionId, byte actionParam) {
+    public void sendBlockAction(@NotNull BlockPosition blockPosition, byte actionId, byte actionParam) {
         final short blockStateId = getBlockStateId(blockPosition);
         final Block block = Block.fromStateId(blockStateId);
 
@@ -677,7 +682,7 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      * @param blockPosition the block position
      * @return the block data at the position, null if not any
      */
-    public Data getBlockData(BlockPosition blockPosition) {
+    public Data getBlockData(@NotNull BlockPosition blockPosition) {
         return getBlockData(blockPosition.getX(), blockPosition.getY(), blockPosition.getZ());
     }
 
@@ -689,7 +694,7 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      * @param z    the Z position
      * @param data the data to be set, can be null
      */
-    public void setBlockData(int x, int y, int z, Data data) {
+    public void setBlockData(int x, int y, int z, @Nullable Data data) {
         final Chunk chunk = getChunkAt(x, z);
         Check.notNull(chunk, "The chunk at " + x + ":" + z + " is not loaded");
         synchronized (chunk) {
@@ -703,7 +708,7 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      * @param blockPosition the block position
      * @param data          the data to be set, can be null
      */
-    public void setBlockData(BlockPosition blockPosition, Data data) {
+    public void setBlockData(@NotNull BlockPosition blockPosition, Data data) {
         setBlockData(blockPosition.getX(), (byte) blockPosition.getY(), blockPosition.getZ(), data);
     }
 
@@ -714,6 +719,7 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      * @param z the Z position
      * @return the chunk at the given position, null if not loaded
      */
+    @Nullable
     public Chunk getChunkAt(float x, float z) {
         final int chunkX = ChunkUtils.getChunkCoordinate((int) x);
         final int chunkZ = ChunkUtils.getChunkCoordinate((int) z);
@@ -737,7 +743,8 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      * @param blockPosition the chunk position
      * @return the chunk at the given position, null if not loaded
      */
-    public Chunk getChunkAt(BlockPosition blockPosition) {
+    @Nullable
+    public Chunk getChunkAt(@NotNull BlockPosition blockPosition) {
         return getChunkAt(blockPosition.getX(), blockPosition.getZ());
     }
 
@@ -747,7 +754,8 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      * @param position the chunk position
      * @return the chunk at the given position, null if not loaded
      */
-    public Chunk getChunkAt(Position position) {
+    @Nullable
+    public Chunk getChunkAt(@NotNull Position position) {
         return getChunkAt(position.getX(), position.getZ());
     }
 
@@ -756,7 +764,7 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      *
      * @param chunk the chunk to save
      */
-    public void saveChunkToStorage(Chunk chunk) {
+    public void saveChunkToStorage(@NotNull Chunk chunk) {
         saveChunkToStorage(chunk, null);
     }
 
@@ -772,6 +780,7 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      *
      * @return the instance unique id
      */
+    @NotNull
     public UUID getUniqueId() {
         return uniqueId;
     }
@@ -782,7 +791,7 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
     }
 
     @Override
-    public void setData(Data data) {
+    public void setData(@Nullable Data data) {
         this.data = data;
     }
 
@@ -801,7 +810,7 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      *
      * @param entity the entity to add
      */
-    public void addEntity(Entity entity) {
+    public void addEntity(@NotNull Entity entity) {
         final Instance lastInstance = entity.getInstance();
         if (lastInstance != null && lastInstance != this) {
             lastInstance.removeEntity(entity); // If entity is in another instance, remove it from there and add it to this
@@ -844,7 +853,7 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      *
      * @param entity the entity to remove
      */
-    public void removeEntity(Entity entity) {
+    public void removeEntity(@NotNull Entity entity) {
         final Instance entityInstance = entity.getInstance();
         if (entityInstance != this)
             return;
@@ -868,7 +877,7 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      * @param entity the entity to add
      * @param chunk  the chunk where the entity will be added
      */
-    public void addEntityToChunk(Entity entity, Chunk chunk) {
+    public void addEntityToChunk(@NotNull Entity entity, @NotNull Chunk chunk) {
         Check.notNull(chunk,
                 "The chunk " + chunk + " is not loaded, you can make it automatic by using Instance#enableAutoChunkLoad(true)");
         Check.argCondition(!chunk.isLoaded(), "Chunk " + chunk + " has been unloaded previously");
@@ -899,7 +908,7 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      * @param entity the entity to remove
      * @param chunk  the chunk where the entity will be removed
      */
-    public void removeEntityFromChunk(Entity entity, Chunk chunk) {
+    public void removeEntityFromChunk(@NotNull Entity entity, @NotNull Chunk chunk) {
         synchronized (chunkEntities) {
             if (chunk != null) {
                 final long chunkIndex = ChunkUtils.getChunkIndex(chunk.getChunkX(), chunk.getChunkZ());
@@ -925,13 +934,14 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
         }
     }
 
+    @NotNull
     private Set<Entity> getEntitiesInChunk(long index) {
         return chunkEntities.computeIfAbsent(index, i -> new CopyOnWriteArraySet<>());
     }
 
     /**
      * Schedules a block update at a given {@link BlockPosition}.
-     * Does nothing if no {@link CustomBlock} is present at 'position'.
+     * Does nothing if no {@link CustomBlock} is present at {@code position}.
      * <p>
      * Cancelled if the block changes between this call and the actual update.
      *
@@ -939,7 +949,7 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      * @param unit     in what unit is the time expressed
      * @param position the location of the block to update
      */
-    public abstract void scheduleUpdate(int time, TimeUnit unit, BlockPosition position);
+    public abstract void scheduleUpdate(int time, @NotNull TimeUnit unit, @NotNull BlockPosition position);
 
     /**
      * Performs a single tick in the instance, including scheduled tasks from {@link #scheduleNextTick(Consumer)}.
@@ -999,7 +1009,7 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      * @param additionalData data to pass to the explosion supplier
      * @throws IllegalStateException If no {@link ExplosionSupplier} was supplied
      */
-    public void explode(float centerX, float centerY, float centerZ, float strength, Data additionalData) {
+    public void explode(float centerX, float centerY, float centerZ, float strength, @Nullable Data additionalData) {
         final ExplosionSupplier explosionSupplier = getExplosionSupplier();
         Check.stateCondition(explosionSupplier == null, "Tried to create an explosion with no explosion supplier");
         final Explosion explosion = explosionSupplier.createExplosion(centerX, centerY, centerZ, strength, additionalData);
@@ -1011,6 +1021,7 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      *
      * @return the instance explosion supplier, null if none was provided
      */
+    @Nullable
     public ExplosionSupplier getExplosionSupplier() {
         return explosionSupplier;
     }
@@ -1020,7 +1031,7 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      *
      * @param supplier the explosion supplier
      */
-    public void setExplosionSupplier(ExplosionSupplier supplier) {
+    public void setExplosionSupplier(@Nullable ExplosionSupplier supplier) {
         this.explosionSupplier = supplier;
     }
 
@@ -1031,6 +1042,7 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      *
      * @return the instance space
      */
+    @NotNull
     public PFInstanceSpace getInstanceSpace() {
         return instanceSpace;
     }

src/main/java/net/minestom/server/instance/InstanceContainer.java

@@ -29,6 +29,8 @@ import net.minestom.server.utils.time.TimeUnit;
 import net.minestom.server.utils.validate.Check;
 import net.minestom.server.world.DimensionType;
 import net.minestom.server.world.biomes.Biome;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 import java.util.*;
 import java.util.concurrent.ConcurrentHashMap;
@@ -393,7 +395,7 @@ public class InstanceContainer extends Instance {
     }
 
     @Override
-    public void loadChunk(int chunkX, int chunkZ, ChunkCallback callback) {
+    public void loadChunk(int chunkX, int chunkZ, @Nullable ChunkCallback callback) {
         final Chunk chunk = getChunk(chunkX, chunkZ);
         if (chunk != null) {
             // Chunk already loaded
@@ -405,7 +407,7 @@ public class InstanceContainer extends Instance {
     }
 
     @Override
-    public void loadOptionalChunk(int chunkX, int chunkZ, ChunkCallback callback) {
+    public void loadOptionalChunk(int chunkX, int chunkZ, @Nullable ChunkCallback callback) {
         final Chunk chunk = getChunk(chunkX, chunkZ);
         if (chunk != null) {
             // Chunk already loaded
@@ -422,7 +424,7 @@ public class InstanceContainer extends Instance {
     }
 
     @Override
-    public void unloadChunk(Chunk chunk) {
+    public void unloadChunk(@NotNull Chunk chunk) {
         // Already unloaded chunk
         if (!ChunkUtils.isLoaded(chunk)) {
             return;
@@ -445,9 +447,9 @@ public class InstanceContainer extends Instance {
      * <p>
      * WARNING: {@link #getData()} needs to be a {@link SerializableData} in order to be saved.
      *
-     * @param callback the callback once the saving is done. Can be null.
+     * @param callback the optional callback once the saving is done
      */
-    public void saveInstance(Runnable callback) {
+    public void saveInstance(@Nullable Runnable callback) {
         Check.notNull(getStorageLocation(), "You cannot save the instance if no StorageLocation has been defined");
 
         this.storageLocation.set(UUID_KEY, getUniqueId(), UUID.class);
@@ -493,7 +495,7 @@ public class InstanceContainer extends Instance {
     }
 
     @Override
-    protected void retrieveChunk(int chunkX, int chunkZ, ChunkCallback callback) {
+    protected void retrieveChunk(int chunkX, int chunkZ, @Nullable ChunkCallback callback) {
         final boolean loaded = chunkLoader.loadChunk(this, chunkX, chunkZ, chunk -> {
             cacheChunk(chunk);
             UPDATE_MANAGER.signalChunkLoad(this, chunkX, chunkZ);
@@ -511,7 +513,7 @@ public class InstanceContainer extends Instance {
     }
 
     @Override
-    protected void createChunk(int chunkX, int chunkZ, ChunkCallback callback) {
+    protected void createChunk(int chunkX, int chunkZ, @Nullable ChunkCallback callback) {
         Biome[] biomes = new Biome[Chunk.BIOME_COUNT];
         if (chunkGenerator == null) {
             Arrays.fill(biomes, MinecraftServer.getBiomeManager().getById(0));
@@ -566,7 +568,7 @@ public class InstanceContainer extends Instance {
      * @param chunkSupplier the new {@link ChunkSupplier} of this instance, chunks need to be non-null
      * @throws NullPointerException if {@code chunkSupplier} is null
      */
-    public void setChunkSupplier(ChunkSupplier chunkSupplier) {
+    public void setChunkSupplier(@NotNull ChunkSupplier chunkSupplier) {
         Check.notNull(chunkSupplier, "The chunk supplier cannot be null, you can use a StaticChunk for a lightweight implementation");
         this.chunkSupplier = chunkSupplier;
     }
@@ -676,7 +678,7 @@ public class InstanceContainer extends Instance {
     }
 
     @Override
-    public void scheduleUpdate(int time, TimeUnit unit, BlockPosition position) {
+    public void scheduleUpdate(int time, @NotNull TimeUnit unit, @NotNull BlockPosition position) {
         final CustomBlock toUpdate = getCustomBlock(position);
         if (toUpdate == null) {
             return;

src/main/java/net/minestom/server/instance/MinestomBasicChunkLoader.java

@@ -5,6 +5,7 @@ import net.minestom.server.utils.binary.BinaryReader;
 import net.minestom.server.utils.callback.OptionalCallback;
 import net.minestom.server.utils.chunk.ChunkCallback;
 import net.minestom.server.utils.chunk.ChunkSupplier;
+import org.jetbrains.annotations.NotNull;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -36,7 +37,7 @@ public class MinestomBasicChunkLoader implements IChunkLoader {
     }
 
     @Override
-    public void saveChunk(Chunk chunk, Runnable callback) {
+    public void saveChunk(@NotNull Chunk chunk, Runnable callback) {
         final StorageLocation storageLocation = instanceContainer.getStorageLocation();
         if (storageLocation == null) {
             callback.run();
@@ -64,7 +65,7 @@ public class MinestomBasicChunkLoader implements IChunkLoader {
     }
 
     @Override
-    public boolean loadChunk(Instance instance, int chunkX, int chunkZ, ChunkCallback callback) {
+    public boolean loadChunk(@NotNull Instance instance, int chunkX, int chunkZ, ChunkCallback callback) {
         final StorageLocation storageLocation = instanceContainer.getStorageLocation();
         final byte[] bytes = storageLocation == null ? null : storageLocation.get(getChunkKey(chunkX, chunkZ));
 

src/main/java/net/minestom/server/instance/SharedInstance.java

@@ -9,6 +9,8 @@ import net.minestom.server.utils.BlockPosition;
 import net.minestom.server.utils.Position;
 import net.minestom.server.utils.chunk.ChunkCallback;
 import net.minestom.server.utils.time.TimeUnit;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 import java.util.Collection;
 import java.util.UUID;
@@ -37,17 +39,17 @@ public class SharedInstance extends Instance {
     }
 
     @Override
-    public void loadChunk(int chunkX, int chunkZ, ChunkCallback callback) {
+    public void loadChunk(int chunkX, int chunkZ, @Nullable ChunkCallback callback) {
         this.instanceContainer.loadChunk(chunkX, chunkZ, callback);
     }
 
     @Override
-    public void loadOptionalChunk(int chunkX, int chunkZ, ChunkCallback callback) {
+    public void loadOptionalChunk(int chunkX, int chunkZ, @Nullable ChunkCallback callback) {
         this.instanceContainer.loadOptionalChunk(chunkX, chunkZ, callback);
     }
 
     @Override
-    public void unloadChunk(Chunk chunk) {
+    public void unloadChunk(@NotNull Chunk chunk) {
         instanceContainer.unloadChunk(chunk);
     }
 
@@ -57,12 +59,12 @@ public class SharedInstance extends Instance {
     }
 
     @Override
-    public void saveChunkToStorage(Chunk chunk, Runnable callback) {
+    public void saveChunkToStorage(Chunk chunk, @Nullable Runnable callback) {
         this.instanceContainer.saveChunkToStorage(chunk, callback);
     }
 
     @Override
-    public void saveChunksToStorage(Runnable callback) {
+    public void saveChunksToStorage(@Nullable Runnable callback) {
         instanceContainer.saveChunksToStorage(callback);
     }
 
@@ -102,7 +104,7 @@ public class SharedInstance extends Instance {
     }
 
     @Override
-    public void retrieveChunk(int chunkX, int chunkZ, ChunkCallback callback) {
+    public void retrieveChunk(int chunkX, int chunkZ, @Nullable ChunkCallback callback) {
         this.instanceContainer.retrieveChunk(chunkX, chunkZ, callback);
     }
 
@@ -142,12 +144,12 @@ public class SharedInstance extends Instance {
     }
 
     @Override
-    public void scheduleUpdate(int time, TimeUnit unit, BlockPosition position) {
+    public void scheduleUpdate(int time, @NotNull TimeUnit unit, @NotNull BlockPosition position) {
         this.instanceContainer.scheduleUpdate(time, unit, position);
     }
 
     /**
-     * Gets the {@link InstanceContainer} from where this instance takes its {@link Chunk} from.
+     * Gets the {@link InstanceContainer} from where this instance takes its chunks from.
      *
      * @return the associated {@link InstanceContainer}
      */

src/main/java/net/minestom/server/instance/StaticChunk.java

@@ -5,9 +5,12 @@ import net.minestom.server.data.Data;
 import net.minestom.server.instance.block.BlockProvider;
 import net.minestom.server.network.packet.server.play.ChunkDataPacket;
 import net.minestom.server.utils.binary.BinaryReader;
+import net.minestom.server.utils.callback.OptionalCallback;
 import net.minestom.server.utils.chunk.ChunkCallback;
 import net.minestom.server.utils.chunk.ChunkUtils;
 import net.minestom.server.world.biomes.Biome;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 import java.util.HashSet;
 import java.util.Set;
@@ -36,7 +39,7 @@ public class StaticChunk extends Chunk {
     }
 
     @Override
-    public void tick(long time, Instance instance) {
+    public void tick(long time, @NotNull Instance instance) {
         //noop
     }
 
@@ -71,6 +74,7 @@ public class StaticChunk extends Chunk {
         //noop
     }
 
+    @NotNull
     @Override
     public Set<Integer> getBlockEntities() {
         return new HashSet<>();
@@ -82,10 +86,11 @@ public class StaticChunk extends Chunk {
     }
 
     @Override
-    public void readChunk(BinaryReader reader, ChunkCallback callback) {
+    public void readChunk(@NotNull BinaryReader reader, @Nullable ChunkCallback callback) {
-        callback.accept(this);
+        OptionalCallback.execute(callback, this);
     }
 
+    @NotNull
     @Override
     protected ChunkDataPacket createFreshPacket() {
         ChunkDataPacket fullDataPacket = new ChunkDataPacket();

src/main/java/net/minestom/server/instance/batch/ChunkBatch.java

@@ -6,6 +6,7 @@ import net.minestom.server.instance.*;
 import net.minestom.server.instance.block.CustomBlock;
 import net.minestom.server.utils.block.CustomBlockUtils;
 import net.minestom.server.utils.chunk.ChunkCallback;
+import org.jetbrains.annotations.Nullable;
 
 import java.util.List;
 
@@ -57,7 +58,7 @@ public class ChunkBatch implements InstanceBatch {
         }
     }
 
-    public void flushChunkGenerator(ChunkGenerator chunkGenerator, ChunkCallback callback) {
+    public void flushChunkGenerator(ChunkGenerator chunkGenerator, @Nullable ChunkCallback callback) {
         BLOCK_BATCH_POOL.execute(() -> {
             final List<ChunkPopulator> populators = chunkGenerator.getPopulators();
             final boolean hasPopulator = populators != null && !populators.isEmpty();
@@ -84,7 +85,7 @@ public class ChunkBatch implements InstanceBatch {
      *
      * @param callback the callback to execute once the blocks are placed
      */
-    public void flush(ChunkCallback callback) {
+    public void flush(@Nullable ChunkCallback callback) {
         BLOCK_BATCH_POOL.execute(() -> singleThreadFlush(callback, true));
     }
 
@@ -95,7 +96,7 @@ public class ChunkBatch implements InstanceBatch {
      *
      * @param callback the callback to execute once the blocks are placed
      */
-    public void unsafeFlush(ChunkCallback callback) {
+    public void unsafeFlush(@Nullable ChunkCallback callback) {
         BLOCK_BATCH_POOL.execute(() -> singleThreadFlush(callback, false));
     }
 
@@ -114,7 +115,7 @@ public class ChunkBatch implements InstanceBatch {
      * @param callback     the callback to execute once the blocks are placed
      * @param safeCallback true to run the callback in the instance update thread, otherwise run in the current one
      */
-    private void singleThreadFlush(ChunkCallback callback, boolean safeCallback) {
+    private void singleThreadFlush(@Nullable ChunkCallback callback, boolean safeCallback) {
         synchronized (chunk) {
             if (!chunk.isLoaded())
                 return;
@@ -145,7 +146,7 @@ public class ChunkBatch implements InstanceBatch {
         private final short customBlockId;
         private final Data data;
 
-        private BlockData(int x, int y, int z, short blockStateId, short customBlockId, Data data) {
+        private BlockData(int x, int y, int z, short blockStateId, short customBlockId, @Nullable Data data) {
             this.x = x;
             this.y = y;
             this.z = z;

src/main/java/net/minestom/server/inventory/Inventory.java

@@ -19,6 +19,7 @@ import net.minestom.server.utils.MathUtils;
 import net.minestom.server.utils.inventory.PlayerInventoryUtils;
 import net.minestom.server.utils.item.ItemStackUtils;
 import net.minestom.server.utils.validate.Check;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.Arrays;
 import java.util.Collections;
@@ -238,20 +239,21 @@ public class Inventory implements InventoryModifier, InventoryClickHandler, View
         sendSlotRefresh(slot, getItemStack(slot));
     }
 
+    @NotNull
     @Override
     public Set<Player> getViewers() {
         return Collections.unmodifiableSet(viewers);
     }
 
     @Override
-    public boolean addViewer(Player player) {
+    public boolean addViewer(@NotNull Player player) {
         final boolean result = this.viewers.add(player);
         update(player);
         return result;
     }
 
     @Override
-    public boolean removeViewer(Player player) {
+    public boolean removeViewer(@NotNull Player player) {
         final boolean result = this.viewers.remove(player);
         this.cursorPlayersItem.remove(player);
         this.clickProcessor.clearCache(player);

src/main/java/net/minestom/server/item/metadata/CompassMeta.java

@@ -1,5 +1,6 @@
 package net.minestom.server.item.metadata;
 
+import org.jetbrains.annotations.NotNull;
 import org.jglrxavpok.hephaistos.nbt.NBTCompound;
 
 public class CompassMeta implements ItemMeta {
@@ -54,7 +55,7 @@ public class CompassMeta implements ItemMeta {
     }
 
     @Override
-    public boolean isSimilar(ItemMeta itemMeta) {
+    public boolean isSimilar(@NotNull ItemMeta itemMeta) {
         if (!(itemMeta instanceof CompassMeta))
             return false;
         CompassMeta compassMeta = (CompassMeta) itemMeta;
@@ -64,7 +65,7 @@ public class CompassMeta implements ItemMeta {
     }
 
     @Override
-    public void read(NBTCompound compound) {
+    public void read(@NotNull NBTCompound compound) {
         if (compound.containsKey("LodestoneTracked")) {
             this.lodestoneTracked = compound.getByte("LodestoneTracked") == 1;
             // TODO get boolean
@@ -81,7 +82,7 @@ public class CompassMeta implements ItemMeta {
     }
 
     @Override
-    public void write(NBTCompound compound) {
+    public void write(@NotNull NBTCompound compound) {
         compound.setByte("LodestoneTracked", (byte) (lodestoneTracked ? 1 : 0));
         if(lodestoneDimension != null) {
             compound.setString("LodestoneDimension", lodestoneDimension);
@@ -96,6 +97,7 @@ public class CompassMeta implements ItemMeta {
         }
     }
 
+    @NotNull
     @Override
     public ItemMeta clone() {
         CompassMeta compassMeta = new CompassMeta();

src/main/java/net/minestom/server/item/metadata/CrossbowMeta.java

@@ -6,6 +6,7 @@ import net.minestom.server.registry.Registries;
 import net.minestom.server.utils.NBTUtils;
 import net.minestom.server.utils.item.ItemStackUtils;
 import net.minestom.server.utils.validate.Check;
+import org.jetbrains.annotations.NotNull;
 import org.jglrxavpok.hephaistos.nbt.NBTCompound;
 import org.jglrxavpok.hephaistos.nbt.NBTList;
 import org.jglrxavpok.hephaistos.nbt.NBTTypes;
@@ -106,7 +107,7 @@ public class CrossbowMeta implements ItemMeta {
     }
 
     @Override
-    public boolean isSimilar(ItemMeta itemMeta) {
+    public boolean isSimilar(@NotNull ItemMeta itemMeta) {
         if (!(itemMeta instanceof CrossbowMeta))
             return false;
 
@@ -125,7 +126,7 @@ public class CrossbowMeta implements ItemMeta {
     }
 
     @Override
-    public void read(NBTCompound compound) {
+    public void read(@NotNull NBTCompound compound) {
         if (compound.containsKey("ChargedProjectiles")) {
             final NBTList<NBTCompound> projectilesList = compound.getList("ChargedProjectiles");
             int index = 0;
@@ -158,7 +159,7 @@ public class CrossbowMeta implements ItemMeta {
     }
 
     @Override
-    public void write(NBTCompound compound) {
+    public void write(@NotNull NBTCompound compound) {
         if (projectile1 != null || projectile2 != null || projectile3 != null) {
             NBTList<NBTCompound> chargedProjectiles = new NBTList<>(NBTTypes.TAG_Compound);
             if (projectile1 != null) {
@@ -178,6 +179,7 @@ public class CrossbowMeta implements ItemMeta {
         }
     }
 
+    @NotNull
     @Override
     public ItemMeta clone() {
         CrossbowMeta crossbowMeta = new CrossbowMeta();

src/main/java/net/minestom/server/item/metadata/EnchantedBookMeta.java

@@ -2,6 +2,7 @@ package net.minestom.server.item.metadata;
 
 import net.minestom.server.item.Enchantment;
 import net.minestom.server.utils.NBTUtils;
+import org.jetbrains.annotations.NotNull;
 import org.jglrxavpok.hephaistos.nbt.NBTCompound;
 
 import java.util.Collections;
@@ -18,6 +19,7 @@ public class EnchantedBookMeta implements ItemMeta {
      *
      * @return an unmodifiable map containing the item stored enchantments
      */
+    @NotNull
     public Map<Enchantment, Short> getStoredEnchantmentMap() {
         return Collections.unmodifiableMap(storedEnchantmentMap);
     }
@@ -28,7 +30,7 @@ public class EnchantedBookMeta implements ItemMeta {
      * @param enchantment the enchantment type
      * @param level       the enchantment level
      */
-    public void setStoredEnchantment(Enchantment enchantment, short level) {
+    public void setStoredEnchantment(@NotNull Enchantment enchantment, short level) {
         if (level < 1) {
             removeStoredEnchantment(enchantment);
             return;
@@ -42,7 +44,7 @@ public class EnchantedBookMeta implements ItemMeta {
      *
      * @param enchantment the enchantment type
      */
-    public void removeStoredEnchantment(Enchantment enchantment) {
+    public void removeStoredEnchantment(@NotNull Enchantment enchantment) {
         this.storedEnchantmentMap.remove(enchantment);
     }
 
@@ -52,7 +54,7 @@ public class EnchantedBookMeta implements ItemMeta {
      * @param enchantment the enchantment type
      * @return the stored enchantment level, 0 if not present
      */
-    public int getStoredEnchantmentLevel(Enchantment enchantment) {
+    public int getStoredEnchantmentLevel(@NotNull Enchantment enchantment) {
         return this.storedEnchantmentMap.getOrDefault(enchantment, (short) 0);
     }
 
@@ -62,25 +64,26 @@ public class EnchantedBookMeta implements ItemMeta {
     }
 
     @Override
-    public boolean isSimilar(ItemMeta itemMeta) {
+    public boolean isSimilar(@NotNull ItemMeta itemMeta) {
         return itemMeta instanceof EnchantedBookMeta &&
                 ((EnchantedBookMeta) itemMeta).storedEnchantmentMap.equals(storedEnchantmentMap);
     }
 
     @Override
-    public void read(NBTCompound compound) {
+    public void read(@NotNull NBTCompound compound) {
         if (compound.containsKey("StoredEnchantments")) {
             NBTUtils.loadEnchantments(compound.getList("StoredEnchantments"), this::setStoredEnchantment);
         }
     }
 
     @Override
-    public void write(NBTCompound compound) {
+    public void write(@NotNull NBTCompound compound) {
         if (!storedEnchantmentMap.isEmpty()) {
             NBTUtils.writeEnchant(compound, "StoredEnchantments", storedEnchantmentMap);
         }
     }
 
+    @NotNull
     @Override
     public ItemMeta clone() {
         EnchantedBookMeta enchantedBookMeta = new EnchantedBookMeta();

src/main/java/net/minestom/server/item/metadata/FireworkMeta.java

@@ -1,5 +1,6 @@
 package net.minestom.server.item.metadata;
 
+import org.jetbrains.annotations.NotNull;
 import org.jglrxavpok.hephaistos.nbt.NBTCompound;
 
 public class FireworkMeta implements ItemMeta {
@@ -19,20 +20,21 @@ public class FireworkMeta implements ItemMeta {
     }
 
     @Override
-    public boolean isSimilar(ItemMeta itemMeta) {
+    public boolean isSimilar(@NotNull ItemMeta itemMeta) {
         return false;
     }
 
     @Override
-    public void read(NBTCompound compound) {
+    public void read(@NotNull NBTCompound compound) {
 
     }
 
     @Override
-    public void write(NBTCompound compound) {
+    public void write(@NotNull NBTCompound compound) {
 
     }
 
+    @NotNull
     @Override
     public ItemMeta clone() {
         return null;

src/main/java/net/minestom/server/item/metadata/ItemMeta.java

@@ -1,6 +1,7 @@
 package net.minestom.server.item.metadata;
 
 import net.minestom.server.item.ItemStack;
+import org.jetbrains.annotations.NotNull;
 import org.jglrxavpok.hephaistos.nbt.NBTCompound;
 
 /**
@@ -23,7 +24,7 @@ public interface ItemMeta {
      * @param itemMeta the second item meta to check
      * @return true if the two meta are similar, false otherwise
      */
-    boolean isSimilar(ItemMeta itemMeta);
+    boolean isSimilar(@NotNull ItemMeta itemMeta);
 
     /**
      * Reads nbt data from a compound.
@@ -33,14 +34,14 @@ public interface ItemMeta {
      *
      * @param compound the compound containing the data
      */
-    void read(NBTCompound compound);
+    void read(@NotNull NBTCompound compound);
 
     /**
      * Writes nbt data to a compound.
      *
      * @param compound the compound receiving the item meta data
      */
-    void write(NBTCompound compound);
+    void write(@NotNull NBTCompound compound);
 
     /**
      * Clones this item meta.
@@ -49,6 +50,7 @@ public interface ItemMeta {
      *
      * @return the cloned item meta
      */
+    @NotNull
     ItemMeta clone();
 
 }

src/main/java/net/minestom/server/item/metadata/LeatherArmorMeta.java

@@ -1,6 +1,7 @@
 package net.minestom.server.item.metadata;
 
 import net.minestom.server.chat.ChatColor;
+import org.jetbrains.annotations.NotNull;
 import org.jglrxavpok.hephaistos.nbt.NBTCompound;
 
 public class LeatherArmorMeta implements ItemMeta {
@@ -73,7 +74,7 @@ public class LeatherArmorMeta implements ItemMeta {
     }
 
     @Override
-    public boolean isSimilar(ItemMeta itemMeta) {
+    public boolean isSimilar(@NotNull ItemMeta itemMeta) {
         if (!(itemMeta instanceof LeatherArmorMeta))
             return false;
         final LeatherArmorMeta leatherArmorMeta = (LeatherArmorMeta) itemMeta;
@@ -84,7 +85,7 @@ public class LeatherArmorMeta implements ItemMeta {
     }
 
     @Override
-    public void read(NBTCompound compound) {
+    public void read(@NotNull NBTCompound compound) {
         if (compound.containsKey("display")) {
             final NBTCompound nbtCompound = compound.getCompound("display");
             if (nbtCompound.containsKey("color")) {
@@ -97,7 +98,7 @@ public class LeatherArmorMeta implements ItemMeta {
     }
 
     @Override
-    public void write(NBTCompound compound) {
+    public void write(@NotNull NBTCompound compound) {
         if (modified) {
             NBTCompound displayCompound;
             if (!compound.containsKey("display")) {
@@ -110,6 +111,7 @@ public class LeatherArmorMeta implements ItemMeta {
         }
     }
 
+    @NotNull
     @Override
     public ItemMeta clone() {
         LeatherArmorMeta leatherArmorMeta = new LeatherArmorMeta();

src/main/java/net/minestom/server/item/metadata/MapMeta.java

@@ -1,6 +1,7 @@
 package net.minestom.server.item.metadata;
 
 import net.minestom.server.chat.ChatColor;
+import org.jetbrains.annotations.NotNull;
 import org.jglrxavpok.hephaistos.nbt.NBTCompound;
 import org.jglrxavpok.hephaistos.nbt.NBTList;
 import org.jglrxavpok.hephaistos.nbt.NBTTypes;
@@ -103,7 +104,7 @@ public class MapMeta implements ItemMeta {
     }
 
     @Override
-    public boolean isSimilar(ItemMeta itemMeta) {
+    public boolean isSimilar(@NotNull ItemMeta itemMeta) {
         if (!(itemMeta instanceof MapMeta))
             return false;
 
@@ -115,7 +116,7 @@ public class MapMeta implements ItemMeta {
     }
 
     @Override
-    public void read(NBTCompound compound) {
+    public void read(@NotNull NBTCompound compound) {
         if (compound.containsKey("map")) {
             this.mapId = compound.getAsInt("map");
         }
@@ -160,7 +161,7 @@ public class MapMeta implements ItemMeta {
     }
 
     @Override
-    public void write(NBTCompound compound) {
+    public void write(@NotNull NBTCompound compound) {
         compound.setInt("map", mapId);
 
         compound.setInt("map_scale_direction", mapScaleDirection);
@@ -191,6 +192,7 @@ public class MapMeta implements ItemMeta {
         }
     }
 
+    @NotNull
     @Override
     public ItemMeta clone() {
         MapMeta mapMeta = new MapMeta();

src/main/java/net/minestom/server/item/metadata/PlayerHeadMeta.java

@@ -1,6 +1,7 @@
 package net.minestom.server.item.metadata;
 
 import net.minestom.server.entity.PlayerSkin;
+import org.jetbrains.annotations.NotNull;
 import org.jglrxavpok.hephaistos.nbt.NBTCompound;
 
 public class PlayerHeadMeta implements ItemMeta {
@@ -14,7 +15,7 @@ public class PlayerHeadMeta implements ItemMeta {
     }
 
     @Override
-    public boolean isSimilar(ItemMeta itemMeta) {
+    public boolean isSimilar(@NotNull ItemMeta itemMeta) {
         if (!(itemMeta instanceof PlayerHeadMeta))
             return false;
         final PlayerHeadMeta playerHeadMeta = (PlayerHeadMeta) itemMeta;
@@ -22,15 +23,16 @@ public class PlayerHeadMeta implements ItemMeta {
     }
 
     @Override
-    public void read(NBTCompound compound) {
+    public void read(@NotNull NBTCompound compound) {
 
     }
 
     @Override
-    public void write(NBTCompound compound) {
+    public void write(@NotNull NBTCompound compound) {
 
     }
 
+    @NotNull
     @Override
     public ItemMeta clone() {
         return null;

src/main/java/net/minestom/server/item/metadata/PotionMeta.java

@@ -4,6 +4,7 @@ import net.minestom.server.chat.ChatColor;
 import net.minestom.server.potion.CustomPotionEffect;
 import net.minestom.server.potion.PotionType;
 import net.minestom.server.registry.Registries;
+import org.jetbrains.annotations.NotNull;
 import org.jglrxavpok.hephaistos.nbt.NBTCompound;
 import org.jglrxavpok.hephaistos.nbt.NBTList;
 import org.jglrxavpok.hephaistos.nbt.NBTTypes;
@@ -78,7 +79,7 @@ public class PotionMeta implements ItemMeta {
     }
 
     @Override
-    public boolean isSimilar(ItemMeta itemMeta) {
+    public boolean isSimilar(@NotNull ItemMeta itemMeta) {
         if (!(itemMeta instanceof PotionMeta))
             return false;
         PotionMeta potionMeta = (PotionMeta) itemMeta;
@@ -91,7 +92,7 @@ public class PotionMeta implements ItemMeta {
     }
 
     @Override
-    public void read(NBTCompound compound) {
+    public void read(@NotNull NBTCompound compound) {
         if (compound.containsKey("Potion")) {
             this.potionType = Registries.getPotionType(compound.getString("Potion"));
         }
@@ -120,7 +121,7 @@ public class PotionMeta implements ItemMeta {
     }
 
     @Override
-    public void write(NBTCompound compound) {
+    public void write(@NotNull NBTCompound compound) {
         if (potionType != null) {
             compound.setString("Potion", potionType.getNamespaceID());
         }
@@ -149,6 +150,7 @@ public class PotionMeta implements ItemMeta {
 
     }
 
+    @NotNull
     @Override
     public ItemMeta clone() {
         PotionMeta potionMeta = new PotionMeta();

src/main/java/net/minestom/server/item/metadata/SpawnEggMeta.java

@@ -1,6 +1,7 @@
 package net.minestom.server.item.metadata;
 
 import net.minestom.server.entity.EntityType;
+import org.jetbrains.annotations.NotNull;
 import org.jglrxavpok.hephaistos.nbt.NBTCompound;
 
 // TODO for which item
@@ -14,7 +15,7 @@ public class SpawnEggMeta implements ItemMeta {
     }
 
     @Override
-    public boolean isSimilar(ItemMeta itemMeta) {
+    public boolean isSimilar(@NotNull ItemMeta itemMeta) {
         if (!(itemMeta instanceof SpawnEggMeta))
             return false;
         final SpawnEggMeta spawnEggMeta = (SpawnEggMeta) itemMeta;
@@ -22,14 +23,14 @@ public class SpawnEggMeta implements ItemMeta {
     }
 
     @Override
-    public void read(NBTCompound compound) {
+    public void read(@NotNull NBTCompound compound) {
         if (compound.containsKey("EntityTag")) {
             // TODO
         }
     }
 
     @Override
-    public void write(NBTCompound compound) {
+    public void write(@NotNull NBTCompound compound) {
         if (!hasNbt())
             return;
         NBTCompound entityCompound = new NBTCompound();
@@ -39,6 +40,7 @@ public class SpawnEggMeta implements ItemMeta {
 
     }
 
+    @NotNull
     @Override
     public ItemMeta clone() {
         SpawnEggMeta spawnEggMeta = new SpawnEggMeta();

src/main/java/net/minestom/server/item/metadata/WritableBookMeta.java

@@ -1,5 +1,6 @@
 package net.minestom.server.item.metadata;
 
+import org.jetbrains.annotations.NotNull;
 import org.jglrxavpok.hephaistos.nbt.NBTCompound;
 import org.jglrxavpok.hephaistos.nbt.NBTList;
 import org.jglrxavpok.hephaistos.nbt.NBTString;
@@ -37,7 +38,7 @@ public class WritableBookMeta implements ItemMeta {
     }
 
     @Override
-    public boolean isSimilar(ItemMeta itemMeta) {
+    public boolean isSimilar(@NotNull ItemMeta itemMeta) {
         if (!(itemMeta instanceof WritableBookMeta))
             return false;
         final WritableBookMeta writableBookMeta = (WritableBookMeta) itemMeta;
@@ -45,7 +46,7 @@ public class WritableBookMeta implements ItemMeta {
     }
 
     @Override
-    public void read(NBTCompound compound) {
+    public void read(@NotNull NBTCompound compound) {
         if (compound.containsKey("pages")) {
             final NBTList<NBTString> list = compound.getList("pages");
             for (NBTString page : list) {
@@ -55,7 +56,7 @@ public class WritableBookMeta implements ItemMeta {
     }
 
     @Override
-    public void write(NBTCompound compound) {
+    public void write(@NotNull NBTCompound compound) {
         if (!pages.isEmpty()) {
             NBTList<NBTString> list = new NBTList<>(NBTTypes.TAG_String);
             for (String page : pages) {
@@ -65,6 +66,7 @@ public class WritableBookMeta implements ItemMeta {
         }
     }
 
+    @NotNull
     @Override
     public ItemMeta clone() {
         WritableBookMeta writableBookMeta = new WritableBookMeta();

src/main/java/net/minestom/server/item/metadata/WrittenBookMeta.java

@@ -2,6 +2,7 @@ package net.minestom.server.item.metadata;
 
 import net.minestom.server.chat.ChatParser;
 import net.minestom.server.chat.ColoredText;
+import org.jetbrains.annotations.NotNull;
 import org.jglrxavpok.hephaistos.nbt.NBTCompound;
 import org.jglrxavpok.hephaistos.nbt.NBTList;
 import org.jglrxavpok.hephaistos.nbt.NBTString;
@@ -118,7 +119,7 @@ public class WrittenBookMeta implements ItemMeta {
     }
 
     @Override
-    public boolean isSimilar(ItemMeta itemMeta) {
+    public boolean isSimilar(@NotNull ItemMeta itemMeta) {
         if (!(itemMeta instanceof WrittenBookMeta))
             return false;
         final WrittenBookMeta writtenBookMeta = (WrittenBookMeta) itemMeta;
@@ -130,7 +131,7 @@ public class WrittenBookMeta implements ItemMeta {
     }
 
     @Override
-    public void read(NBTCompound compound) {
+    public void read(@NotNull NBTCompound compound) {
         if (compound.containsKey("resolved")) {
             this.resolved = compound.getByte("resolved") == 1;
         }
@@ -154,7 +155,7 @@ public class WrittenBookMeta implements ItemMeta {
     }
 
     @Override
-    public void write(NBTCompound compound) {
+    public void write(@NotNull NBTCompound compound) {
         if (resolved) {
             compound.setByte("resolved", (byte) 1);
         }
@@ -176,6 +177,7 @@ public class WrittenBookMeta implements ItemMeta {
         }
     }
 
+    @NotNull
     @Override
     public ItemMeta clone() {
         WrittenBookMeta writtenBookMeta = new WrittenBookMeta();

src/main/java/net/minestom/server/scoreboard/BelowNameTag.java

@@ -3,6 +3,7 @@ package net.minestom.server.scoreboard;
 import net.minestom.server.entity.Player;
 import net.minestom.server.network.packet.server.play.ScoreboardObjectivePacket;
 import net.minestom.server.network.player.PlayerConnection;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.Collections;
 import java.util.Set;
@@ -41,7 +42,7 @@ public class BelowNameTag implements Scoreboard {
     }
 
     @Override
-    public boolean addViewer(Player player) {
+    public boolean addViewer(@NotNull Player player) {
         boolean result = this.viewers.add(player);
         PlayerConnection connection = player.getPlayerConnection();
 
@@ -56,7 +57,7 @@ public class BelowNameTag implements Scoreboard {
     }
 
     @Override
-    public boolean removeViewer(Player player) {
+    public boolean removeViewer(@NotNull Player player) {
         boolean result = this.viewers.remove(player);
         PlayerConnection connection = player.getPlayerConnection();
 
@@ -68,6 +69,7 @@ public class BelowNameTag implements Scoreboard {
         return result;
     }
 
+    @NotNull
     @Override
     public Set<Player> getViewers() {
         return Collections.unmodifiableSet(viewers);

src/main/java/net/minestom/server/scoreboard/Sidebar.java

@@ -10,6 +10,7 @@ import net.minestom.server.network.packet.server.play.TeamsPacket;
 import net.minestom.server.network.packet.server.play.UpdateScorePacket;
 import net.minestom.server.network.player.PlayerConnection;
 import net.minestom.server.utils.validate.Check;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.Iterator;
 import java.util.Set;
@@ -177,7 +178,7 @@ public class Sidebar implements Scoreboard {
     }
 
     @Override
-    public boolean addViewer(Player player) {
+    public boolean addViewer(@NotNull Player player) {
         final boolean result = this.viewers.add(player);
         PlayerConnection playerConnection = player.getPlayerConnection();
 
@@ -195,7 +196,7 @@ public class Sidebar implements Scoreboard {
     }
 
     @Override
-    public boolean removeViewer(Player player) {
+    public boolean removeViewer(@NotNull Player player) {
         final boolean result = this.viewers.remove(player);
         PlayerConnection playerConnection = player.getPlayerConnection();
         ScoreboardObjectivePacket scoreboardObjectivePacket = this.getDestructionObjectivePacket();
@@ -208,6 +209,7 @@ public class Sidebar implements Scoreboard {
         return result;
     }
 
+    @NotNull
     @Override
     public Set<Player> getViewers() {
         return viewers;

src/main/java/net/minestom/server/scoreboard/TabList.java

@@ -3,6 +3,7 @@ package net.minestom.server.scoreboard;
 import net.minestom.server.entity.Player;
 import net.minestom.server.network.packet.server.play.ScoreboardObjectivePacket;
 import net.minestom.server.network.player.PlayerConnection;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.Collections;
 import java.util.Set;
@@ -49,7 +50,7 @@ public class TabList implements Scoreboard {
     }
 
     @Override
-    public boolean addViewer(Player player) {
+    public boolean addViewer(@NotNull Player player) {
         boolean result = this.viewers.add(player);
         PlayerConnection connection = player.getPlayerConnection();
 
@@ -62,7 +63,7 @@ public class TabList implements Scoreboard {
     }
 
     @Override
-    public boolean removeViewer(Player player) {
+    public boolean removeViewer(@NotNull Player player) {
         boolean result = this.viewers.remove(player);
         PlayerConnection connection = player.getPlayerConnection();
 
@@ -73,6 +74,7 @@ public class TabList implements Scoreboard {
         return result;
     }
 
+    @NotNull
     @Override
     public Set<Player> getViewers() {
         return Collections.unmodifiableSet(this.viewers);