Use the Contract annotation to prevent useless NPE warnings

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

@@ -338,6 +338,7 @@ public class Player extends LivingEntity implements CommandSender {
                         final Chunk chunk = instance.getChunkAt(targetBlockPosition);
                         final int entityId = targetCustomBlock.getBreakEntityId(this);
                         final BlockBreakAnimationPacket blockBreakAnimationPacket = new BlockBreakAnimationPacket(entityId, targetBlockPosition, targetStage);
+                        Check.notNull(chunk, "Tried to interact with an unloaded chunk.");
                         chunk.sendPacketToViewers(blockBreakAnimationPacket);
 
                         refreshBreakDelay(targetBreakers);
@@ -374,6 +375,8 @@ public class Player extends LivingEntity implements CommandSender {
                 triggerStatus((byte) 9); // Mark item use as finished
                 ItemUpdateStateEvent itemUpdateStateEvent = callItemUpdateStateEvent(true);
 
+                Check.notNull(itemUpdateStateEvent, "#callItemUpdateStateEvent returned null.");
+
                 // Refresh hand
                 final boolean isOffHand = itemUpdateStateEvent.getHand() == Player.Hand.OFF;
                 refreshActiveHand(false, isOffHand, false);
@@ -780,7 +783,7 @@ public class Player extends LivingEntity implements CommandSender {
     }
 
     /**
-     * Plays a sound from an identifier (represents a custom sound in a resource pack)
+     * Plays a sound from an identifier (represents a custom sound in a resource pack).
      *
      * @param identifier    the identifier of the sound to play
      * @param soundCategory the sound category
@@ -1299,7 +1302,9 @@ public class Player extends LivingEntity implements CommandSender {
         {
             // Send new chunks
             final BlockPosition pos = position.toBlockPosition();
-            onChunkChange(instance.getChunk(pos.getX() >> 4, pos.getZ() >> 4));
+            final Chunk chunk = instance.getChunk(pos.getX() >> 4, pos.getZ() >> 4);
+            Check.notNull(chunk, "Tried to interact with an unloaded chunk.");
+            onChunkChange(chunk);
         }
     }
 

src/main/java/net/minestom/server/entity/damage/DamageType.java

@@ -64,7 +64,7 @@ public class DamageType implements DataContainer {
      * @return the death message, null to do not send anything.
      * Can be for instance, of type {@link ColoredText} or {@link RichMessage}.
      */
-    @NotNull
+    @Nullable
     public JsonMessage buildDeathMessage(@NotNull Player killed) {
         return ColoredText.of("{@death." + identifier + "," + killed.getUsername() + "}");
     }
@@ -109,7 +109,7 @@ public class DamageType implements DataContainer {
      * @param killed the player who has been killed
      * @return the death screen text, null to do not send anything
      */
-    @NotNull
+    @Nullable
     public ColoredText buildDeathScreenText(@NotNull Player killed) {
         return ColoredText.of("{@death." + identifier + "}");
     }

src/main/java/net/minestom/server/gamedata/loottables/LootTableManager.java

@@ -13,7 +13,7 @@ import java.io.*;
 /**
  * Handles loading and configuration of loot tables
  */
-public class LootTableManager {
+public final class LootTableManager {
 
     private NamespaceIDHashMap<JsonDeserializer<? extends Condition>> conditionDeserializers = new NamespaceIDHashMap<>();
     private NamespaceIDHashMap<LootTableType> tableTypes = new NamespaceIDHashMap<>();

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

@@ -9,6 +9,8 @@ import net.minestom.server.instance.block.BlockManager;
 import net.minestom.server.instance.block.CustomBlock;
 import net.minestom.server.utils.BlockPosition;
 import net.minestom.server.utils.validate.Check;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 /**
  * Represents an element which can place blocks at position.
@@ -30,7 +32,7 @@ public interface BlockModifier {
      * @param blockStateId the block state id
      * @param data         the block {@link Data}, can be null
      */
-    void setBlockStateId(int x, int y, int z, short blockStateId, Data data);
+    void setBlockStateId(int x, int y, int z, short blockStateId, @Nullable Data data);
 
     /**
      * Sets a {@link CustomBlock} at a position.
@@ -43,7 +45,7 @@ public interface BlockModifier {
      * @param customBlockId the custom block id
      * @param data          the block {@link Data}, can be null
      */
-    void setCustomBlock(int x, int y, int z, short customBlockId, Data data);
+    void setCustomBlock(int x, int y, int z, short customBlockId, @Nullable Data data);
 
     /**
      * Sets a {@link CustomBlock} at a position with a custom state id.
@@ -58,17 +60,17 @@ public interface BlockModifier {
      * @param customBlockId the custom block id
      * @param data          the block {@link Data}, can be null
      */
-    void setSeparateBlocks(int x, int y, int z, short blockStateId, short customBlockId, Data data);
+    void setSeparateBlocks(int x, int y, int z, short blockStateId, short customBlockId, @Nullable Data data);
 
     default void setBlockStateId(int x, int y, int z, short blockStateId) {
         setBlockStateId(x, y, z, blockStateId, null);
     }
 
-    default void setBlock(int x, int y, int z, Block block) {
+    default void setBlock(int x, int y, int z, @NotNull Block block) {
         setBlockStateId(x, y, z, block.getBlockId(), null);
     }
 
-    default void setBlock(BlockPosition blockPosition, Block block) {
+    default void setBlock(@NotNull BlockPosition blockPosition, @NotNull Block block) {
         Check.notNull(blockPosition, "The block position cannot be null");
         Check.notNull(block, "The block cannot be null");
         setBlock(blockPosition.getX(), blockPosition.getY(), blockPosition.getZ(), block);
@@ -83,18 +85,18 @@ public interface BlockModifier {
         setCustomBlock(x, y, z, customBlockId, null);
     }
 
-    default void setCustomBlock(int x, int y, int z, String customBlockId, Data data) {
+    default void setCustomBlock(int x, int y, int z, @NotNull String customBlockId, @Nullable Data data) {
         CustomBlock customBlock = BLOCK_MANAGER.getCustomBlock(customBlockId);
         Check.notNull(customBlock, "The CustomBlock " + customBlockId + " is not registered");
 
         setCustomBlock(x, y, z, customBlock.getCustomBlockId(), data);
     }
 
-    default void setCustomBlock(int x, int y, int z, String customBlockId) {
+    default void setCustomBlock(int x, int y, int z, @NotNull String customBlockId) {
         setCustomBlock(x, y, z, customBlockId, null);
     }
 
-    default void setCustomBlock(BlockPosition blockPosition, String customBlockId) {
+    default void setCustomBlock(@NotNull BlockPosition blockPosition, @NotNull String customBlockId) {
         Check.notNull(blockPosition, "The block position cannot be null");
         setCustomBlock(blockPosition.getX(), blockPosition.getY(), blockPosition.getZ(), customBlockId);
     }
@@ -103,7 +105,7 @@ public interface BlockModifier {
         setSeparateBlocks(x, y, z, blockStateId, customBlockId, null);
     }
 
-    default void setSeparateBlocks(BlockPosition blockPosition, short blockStateId, short customBlockId) {
+    default void setSeparateBlocks(@NotNull BlockPosition blockPosition, short blockStateId, short customBlockId) {
         Check.notNull(blockPosition, "The block position cannot be null");
         setSeparateBlocks(blockPosition.getX(), blockPosition.getY(), blockPosition.getZ(), blockStateId, customBlockId, null);
     }

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

@@ -886,6 +886,7 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
 
             // Remove the entity from cache
             final Chunk chunk = getChunkAt(entity.getPosition());
+            Check.notNull(chunk, "Tried to interact with an unloaded chunk.");
             removeEntityFromChunk(entity, chunk);
         });
     }
@@ -931,15 +932,13 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      */
     public void removeEntityFromChunk(@NotNull Entity entity, @NotNull Chunk chunk) {
         synchronized (chunkEntities) {
-            if (chunk != null) {
+            final long chunkIndex = ChunkUtils.getChunkIndex(chunk.getChunkX(), chunk.getChunkZ());
-                final long chunkIndex = ChunkUtils.getChunkIndex(chunk.getChunkX(), chunk.getChunkZ());
+            Set<Entity> entities = getEntitiesInChunk(chunkIndex);
-                Set<Entity> entities = getEntitiesInChunk(chunkIndex);
+            entities.remove(entity);
-                entities.remove(entity);
+            if (entities.isEmpty()) {
-                if (entities.isEmpty()) {
+                this.chunkEntities.remove(chunkIndex);
-                    this.chunkEntities.remove(chunkIndex);
+            } else {
-                } else {
+                this.chunkEntities.put(chunkIndex, entities);
-                    this.chunkEntities.put(chunkIndex, entities);
-                }
             }
 
             this.entities.remove(entity);

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

@@ -4,6 +4,8 @@ import net.minestom.server.MinecraftServer;
 import net.minestom.server.storage.StorageLocation;
 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.Collections;
 import java.util.Set;
@@ -25,7 +27,7 @@ public final class InstanceManager {
      *
      * @param instance the {@link Instance} to register
      */
-    public void registerInstance(Instance instance) {
+    public void registerInstance(@NotNull Instance instance) {
         Check.stateCondition(instance instanceof SharedInstance,
                 "Please use InstanceManager#registerSharedInstance to register a shared instance");
         UNSAFE_registerInstance(instance);
@@ -39,7 +41,8 @@ public final class InstanceManager {
      * @param storageLocation the {@link StorageLocation} of the instance, can be null
      * @return the created {@link InstanceContainer}
      */
-    public InstanceContainer createInstanceContainer(DimensionType dimensionType, StorageLocation storageLocation) {
+    @NotNull
+    public InstanceContainer createInstanceContainer(@NotNull DimensionType dimensionType, @Nullable StorageLocation storageLocation) {
         final InstanceContainer instanceContainer = new InstanceContainer(UUID.randomUUID(), dimensionType, storageLocation);
         registerInstance(instanceContainer);
         return instanceContainer;
@@ -51,7 +54,8 @@ public final class InstanceManager {
      * @param storageLocation the {@link StorageLocation} of the instance, can be null
      * @return the created {@link InstanceContainer}
      */
-    public InstanceContainer createInstanceContainer(StorageLocation storageLocation) {
+    @NotNull
+    public InstanceContainer createInstanceContainer(@Nullable StorageLocation storageLocation) {
         return createInstanceContainer(DimensionType.OVERWORLD, storageLocation);
     }
 
@@ -61,7 +65,8 @@ public final class InstanceManager {
      * @param dimensionType the {@link DimensionType} of the instance
      * @return the created {@link InstanceContainer}
      */
-    public InstanceContainer createInstanceContainer(DimensionType dimensionType) {
+    @NotNull
+    public InstanceContainer createInstanceContainer(@NotNull DimensionType dimensionType) {
         return createInstanceContainer(dimensionType, null);
     }
 
@@ -70,6 +75,7 @@ public final class InstanceManager {
      *
      * @return the created {@link InstanceContainer}
      */
+    @NotNull
     public InstanceContainer createInstanceContainer() {
         return createInstanceContainer(DimensionType.OVERWORLD);
     }
@@ -83,7 +89,8 @@ public final class InstanceManager {
      * @return the registered {@link SharedInstance}
      * @throws NullPointerException if {@code sharedInstance} doesn't have an {@link InstanceContainer} assigned to it
      */
-    public SharedInstance registerSharedInstance(SharedInstance sharedInstance) {
+    @NotNull
+    public SharedInstance registerSharedInstance(@NotNull SharedInstance sharedInstance) {
         final InstanceContainer instanceContainer = sharedInstance.getInstanceContainer();
         Check.notNull(instanceContainer, "SharedInstance needs to have an InstanceContainer to be created!");
 
@@ -99,7 +106,8 @@ public final class InstanceManager {
      * @return the created {@link SharedInstance}
      * @throws IllegalStateException if {@code instanceContainer} is not registered
      */
-    public SharedInstance createSharedInstance(InstanceContainer instanceContainer) {
+    @NotNull
+    public SharedInstance createSharedInstance(@NotNull InstanceContainer instanceContainer) {
         Check.notNull(instanceContainer, "Instance container cannot be null when creating a SharedInstance!");
         Check.stateCondition(!instanceContainer.isRegistered(), "The container needs to be register in the InstanceManager");
 
@@ -114,8 +122,8 @@ public final class InstanceManager {
      *
      * @param instance the {@link Instance} to unregister
      */
-    public void unregisterInstance(Instance instance) {
+    public void unregisterInstance(@NotNull Instance instance) {
-        Check.stateCondition(!instance.getPlayers().isEmpty(), "You cannot unregister an instance with players");
+        Check.stateCondition(!instance.getPlayers().isEmpty(), "You cannot unregister an instance with players inside.");
 
         synchronized (instance) {
             // Unload all chunks
@@ -136,6 +144,7 @@ public final class InstanceManager {
      *
      * @return an unmodifiable {@link Set} containing all the registered instances
      */
+    @NotNull
     public Set<Instance> getInstances() {
         return Collections.unmodifiableSet(instances);
     }
@@ -147,7 +156,7 @@ public final class InstanceManager {
      *
      * @param instance the {@link Instance} to register
      */
-    private void UNSAFE_registerInstance(Instance instance) {
+    private void UNSAFE_registerInstance(@NotNull Instance instance) {
         instance.setRegistered(true);
         this.instances.add(instance);
         MinecraftServer.getUpdateManager().signalInstanceCreate(instance);

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

@@ -23,7 +23,7 @@ public class SharedInstance extends Instance {
 
     private final InstanceContainer instanceContainer;
 
-    public SharedInstance(UUID uniqueId, InstanceContainer instanceContainer) {
+    public SharedInstance(@NotNull UUID uniqueId, @NotNull InstanceContainer instanceContainer) {
         super(uniqueId, instanceContainer.getDimensionType());
         this.instanceContainer = instanceContainer;
     }
@@ -154,6 +154,7 @@ public class SharedInstance extends Instance {
      *
      * @return the associated {@link InstanceContainer}
      */
+    @NotNull
     public InstanceContainer getInstanceContainer() {
         return instanceContainer;
     }

src/main/java/net/minestom/server/instance/block/CustomBlock.java

@@ -16,6 +16,8 @@ import net.minestom.server.network.packet.server.play.BlockBreakAnimationPacket;
 import net.minestom.server.utils.BlockPosition;
 import net.minestom.server.utils.time.UpdateOption;
 import net.minestom.server.utils.validate.Check;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 import org.jglrxavpok.hephaistos.nbt.NBTCompound;
 
 import java.util.HashMap;
@@ -24,7 +26,7 @@ import java.util.Map;
 import java.util.Set;
 
 /**
- * Represents the handler of a custom block type.
+ * Represents the handler of a custom block type which can be registered with {@link BlockManager#registerCustomBlock(CustomBlock)}.
  * <p>
  * There should be only one instance of this class for each custom block type,
  * every individual blocks will execute the callbacks present there. Each of which contains the
@@ -55,12 +57,12 @@ public abstract class CustomBlock {
      * @param defaultBlockStateId the default block state id
      * @param identifier          the custom block identifier
      */
-    public CustomBlock(short defaultBlockStateId, String identifier) {
+    public CustomBlock(short defaultBlockStateId, @NotNull String identifier) {
         this.defaultBlockStateId = defaultBlockStateId;
         this.identifier = identifier;
     }
 
-    public CustomBlock(Block block, String identifier) {
+    public CustomBlock(@NotNull Block block, @NotNull String identifier) {
         this(block.getBlockId(), identifier);
     }
 
@@ -73,7 +75,7 @@ public abstract class CustomBlock {
      * @throws UnsupportedOperationException if {@link #getUpdateOption()}
      *                                       is not null but the update method is not overridden
      */
-    public void update(Instance instance, BlockPosition blockPosition, Data data) {
+    public void update(@NotNull Instance instance, @NotNull BlockPosition blockPosition, @Nullable Data data) {
         throw new UnsupportedOperationException("Update method not overridden, check #getUpdateOption()");
     }
 
@@ -86,6 +88,7 @@ public abstract class CustomBlock {
      *
      * @return the update option of the block
      */
+    @Nullable
     public UpdateOption getUpdateOption() {
         return null;
     }
@@ -97,7 +100,7 @@ public abstract class CustomBlock {
      * @param blockPosition the position of the block
      * @param data          the data associated with the block
      */
-    public abstract void onPlace(Instance instance, BlockPosition blockPosition, Data data);
+    public abstract void onPlace(@NotNull Instance instance, @NotNull BlockPosition blockPosition, @Nullable Data data);
 
     /**
      * Called when a custom block has been destroyed or replaced.
@@ -106,7 +109,7 @@ public abstract class CustomBlock {
      * @param blockPosition the position of the block
      * @param data          the data associated with the block
      */
-    public abstract void onDestroy(Instance instance, BlockPosition blockPosition, Data data);
+    public abstract void onDestroy(@NotNull Instance instance, @NotNull BlockPosition blockPosition, @Nullable Data data);
 
     /**
      * Handles interactions with this block. Can also block normal item use (containers should block when opening the
@@ -118,7 +121,8 @@ public abstract class CustomBlock {
      * @param data          the data at this position
      * @return true if this block blocks normal item use, false otherwise
      */
-    public abstract boolean onInteract(Player player, Player.Hand hand, BlockPosition blockPosition, Data data);
+    public abstract boolean onInteract(@NotNull Player player, @NotNull Player.Hand hand,
+                                       @NotNull BlockPosition blockPosition, @Nullable Data data);
 
     /**
      * This id can be serialized in chunk file, meaning no duplicate should exist
@@ -139,7 +143,7 @@ public abstract class CustomBlock {
      * negative value allow to skip stages (-2 will skip 2 stages per tick)
      * @see #enableCustomBreakDelay() to enable/disable it
      */
-    public int getBreakDelay(Player player, BlockPosition position, byte stage, Set<Player> breakers) {
+    public int getBreakDelay(@NotNull Player player, @NotNull BlockPosition position, byte stage, Set<Player> breakers) {
         return 0;
     }
 
@@ -186,7 +190,7 @@ public abstract class CustomBlock {
      * @param position the position at which the block is
      * @param touching the entity currently touching the block
      */
-    public void handleContact(Instance instance, BlockPosition position, Entity touching) {
+    public void handleContact(@NotNull Instance instance, @NotNull BlockPosition position, @NotNull Entity touching) {
     }
 
     /**
@@ -208,6 +212,7 @@ public abstract class CustomBlock {
      *
      * @return the custom block identifier
      */
+    @NotNull
     public String getIdentifier() {
         return identifier;
     }
@@ -219,7 +224,8 @@ public abstract class CustomBlock {
      * @param data          data given to 'setBlock', can be null
      * @return Data for this block. Can be null, 'data', or a new object
      */
-    public Data createData(Instance instance, BlockPosition blockPosition, Data data) {
+    @Nullable
+    public Data createData(@NotNull Instance instance, @NotNull BlockPosition blockPosition, @Nullable Data data) {
         return data;
     }
 
@@ -231,7 +237,8 @@ public abstract class CustomBlock {
      * @param neighborPosition the neighboring block which triggered the update
      * @param directNeighbor   is the neighbor directly connected to this block? (No diagonals)
      */
-    public void updateFromNeighbor(Instance instance, BlockPosition thisPosition, BlockPosition neighborPosition, boolean directNeighbor) {
+    public void updateFromNeighbor(@NotNull Instance instance, @NotNull BlockPosition thisPosition,
+                                   @NotNull BlockPosition neighborPosition, boolean directNeighbor) {
         if (directNeighbor && hasUpdate()) {
             update(instance, thisPosition, instance.getBlockData(thisPosition));
         }
@@ -244,7 +251,7 @@ public abstract class CustomBlock {
      * @param position  the position of the block
      * @param blockData the data of the block
      */
-    public void scheduledUpdate(Instance instance, BlockPosition position, Data blockData) {
+    public void scheduledUpdate(@NotNull Instance instance, @NotNull BlockPosition position, @Nullable Data blockData) {
         update(instance, position, blockData);
     }
 
@@ -255,7 +262,7 @@ public abstract class CustomBlock {
      *
      * @return the drag of this block
      */
-    public float getDrag(Instance instance, BlockPosition blockPosition) {
+    public float getDrag(@NotNull Instance instance, @NotNull BlockPosition blockPosition) {
         return 0.5f;
     }
 
@@ -266,8 +273,9 @@ public abstract class CustomBlock {
      *
      * @param position  position of the block
      * @param blockData equivalent to <pre>instance.getBlockData(position)</pre>
+     * @param nbt       the nbt to write in the {@link net.minestom.server.network.packet.server.play.ChunkDataPacket}
      */
-    public void writeBlockEntity(BlockPosition position, Data blockData, NBTCompound nbt) {
+    public void writeBlockEntity(@NotNull BlockPosition position, @Nullable Data blockData, @NotNull NBTCompound nbt) {
     }
 
     /**
@@ -278,7 +286,7 @@ public abstract class CustomBlock {
      * @return 'true' if the explosion should happen on this block, 'false' to cancel the destruction.
      * Returning true does NOT block the explosion rays, ie it does not change the block explosion resistance
      */
-    public boolean onExplode(Instance instance, BlockPosition position, Data lootTableArguments) {
+    public boolean onExplode(@NotNull Instance instance, @NotNull BlockPosition position, Data lootTableArguments) {
         return true;
     }
 
@@ -288,7 +296,8 @@ public abstract class CustomBlock {
      * @param tableManager the loot table manager
      * @return the loot table associated to this block
      */
-    public LootTable getLootTable(LootTableManager tableManager) {
+    @Nullable
+    public LootTable getLootTable(@NotNull LootTableManager tableManager) {
         return null;
     }
 
@@ -302,7 +311,7 @@ public abstract class CustomBlock {
      * @param blockPosition the position of the block
      * @param player        the player who started digging
      */
-    public void startDigging(Instance instance, BlockPosition blockPosition, Player player) {
+    public void startDigging(@NotNull Instance instance, @NotNull BlockPosition blockPosition, @NotNull Player player) {
         // Stay null if multi player breaking is disabled
         Set<Player> breakers = null;
 
@@ -339,7 +348,7 @@ public abstract class CustomBlock {
      * @param blockPosition the position of the block
      * @param player        the player who stopped digging
      */
-    public void stopDigging(Instance instance, BlockPosition blockPosition, Player player) {
+    public void stopDigging(@NotNull Instance instance, @NotNull BlockPosition blockPosition, @NotNull Player player) {
         if (enableMultiPlayerBreaking()) {
             // Remove cache data
             if (instanceBreakDataMap.containsKey(instance)) {
@@ -355,6 +364,7 @@ public abstract class CustomBlock {
                         final int entityId = instanceBreakData.breakIdMap.getInt(blockPosition);
 
                         final Chunk chunk = instance.getChunkAt(blockPosition);
+                        Check.notNull(chunk, "Tried to interact with an unloaded chunk.");
                         chunk.sendPacketToViewers(new BlockBreakAnimationPacket(entityId, blockPosition, (byte) -1));
 
                         // Clear cache
@@ -367,6 +377,7 @@ public abstract class CustomBlock {
             // Stop the breaking animation for the specific player id
             final Chunk chunk = instance.getChunkAt(blockPosition);
             final int entityId = getBreakEntityId(player);
+            Check.notNull(chunk, "Tried to interact with an unloaded chunk.");
             chunk.sendPacketToViewers(new BlockBreakAnimationPacket(entityId, blockPosition, (byte) -1));
         }
     }
@@ -382,7 +393,8 @@ public abstract class CustomBlock {
      * @return true if the block can continue being digged
      * @throws IllegalStateException if {@link #enableMultiPlayerBreaking()} is disabled
      */
-    public synchronized boolean processStage(Instance instance, BlockPosition blockPosition, Player player, byte stageIncrease) {
+    public synchronized boolean processStage(@NotNull Instance instance, @NotNull BlockPosition blockPosition,
+                                             @NotNull Player player, byte stageIncrease) {
         Check.stateCondition(!enableMultiPlayerBreaking(),
                 "CustomBlock#processState requires having the multi player breaking feature enabled");
 
@@ -400,6 +412,7 @@ public abstract class CustomBlock {
 
                 // Send the block break animation
                 final Chunk chunk = instance.getChunkAt(blockPosition);
+                Check.notNull(chunk, "Tried to interact with an unloaded chunk.");
                 chunk.sendPacketToViewers(new BlockBreakAnimationPacket(entityId, blockPosition, stage));
 
                 // Refresh the stage
@@ -411,7 +424,7 @@ public abstract class CustomBlock {
         return false;
     }
 
-    public void removeDiggingInformation(Instance instance, BlockPosition blockPosition) {
+    public void removeDiggingInformation(@NotNull Instance instance, @NotNull BlockPosition blockPosition) {
         if (!enableMultiPlayerBreaking()) {
             return;
         }
@@ -431,7 +444,8 @@ public abstract class CustomBlock {
      * @return the {@link Set} of breakers of a block
      * @throws IllegalStateException if {@link #enableMultiPlayerBreaking()} is disabled
      */
-    public Set<Player> getBreakers(Instance instance, BlockPosition blockPosition) {
+    @Nullable
+    public Set<Player> getBreakers(@NotNull Instance instance, @NotNull BlockPosition blockPosition) {
         Check.stateCondition(!enableMultiPlayerBreaking(),
                 "CustomBlock#getBreakers requires having the multi player breaking feature enabled");
 
@@ -450,7 +464,7 @@ public abstract class CustomBlock {
      * @param blockPosition the position of the custom block
      * @return the break stage at the position. Can also be 0 when nonexistent
      */
-    public byte getBreakStage(Instance instance, BlockPosition blockPosition) {
+    public byte getBreakStage(@NotNull Instance instance, @NotNull BlockPosition blockPosition) {
         Check.stateCondition(!enableMultiPlayerBreaking(),
                 "CustomBlock#getBreakStage requires having the multi player breaking feature enabled");
 
@@ -472,7 +486,7 @@ public abstract class CustomBlock {
         // Contains the entity id used by the block break packet
         private final Object2IntMap<BlockPosition> breakIdMap = new Object2IntOpenHashMap<>();
 
-        private void clear(BlockPosition blockPosition) {
+        private void clear(@NotNull BlockPosition blockPosition) {
             this.breakersMap.remove(blockPosition);
             this.breakStageMap.removeByte(blockPosition);
             this.breakIdMap.removeInt(blockPosition);

src/main/java/net/minestom/server/utils/validate/Check.java

@@ -1,27 +1,35 @@
 package net.minestom.server.utils.validate;
 
+import org.jetbrains.annotations.Contract;
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 
 import java.util.Objects;
 
+/**
+ * Convenient class to check for common exceptions types.
+ */
 public final class Check {
+
     private Check() {
 
     }
 
+    @Contract("null, _ -> fail")
     public static void notNull(@Nullable Object object, @NotNull String reason) {
         if (Objects.isNull(object)) {
             throw new NullPointerException(reason);
         }
     }
 
+    @Contract("true, _ -> fail")
     public static void argCondition(boolean condition, @NotNull String reason) {
         if (condition) {
             throw new IllegalArgumentException(reason);
         }
     }
 
+    @Contract("true, _ -> fail")
     public static void stateCondition(boolean condition, @NotNull String reason) {
         if (condition) {
             throw new IllegalStateException(reason);

src/test/java/demo/PlayerInit.java

@@ -19,7 +19,6 @@ import net.minestom.server.instance.InstanceContainer;
 import net.minestom.server.instance.block.Block;
 import net.minestom.server.inventory.Inventory;
 import net.minestom.server.inventory.InventoryType;
-import net.minestom.server.item.Enchantment;
 import net.minestom.server.item.ItemStack;
 import net.minestom.server.item.Material;
 import net.minestom.server.network.ConnectionManager;
@@ -43,7 +42,7 @@ public class PlayerInit {
         StorageLocation storageLocation = MinecraftServer.getStorageManager().getLocation("instance_data", new StorageOptions().setCompression(true));
         ChunkGeneratorDemo chunkGeneratorDemo = new ChunkGeneratorDemo();
         NoiseTestGenerator noiseTestGenerator = new NoiseTestGenerator();
-        instanceContainer = MinecraftServer.getInstanceManager().createInstanceContainer(DimensionType.OVERWORLD);
+        instanceContainer = MinecraftServer.getInstanceManager().createInstanceContainer(DimensionType.OVERWORLD, storageLocation);
         instanceContainer.enableAutoChunkLoad(true);
         instanceContainer.setChunkGenerator(noiseTestGenerator);
 
@@ -113,7 +112,7 @@ public class PlayerInit {
                 final Block block = Block.fromStateId(event.getBlockStateId());
 
                 if (block == Block.STONE) {
-                    event.setCustomBlock((short) 2); // custom stone block
+                    event.setCustomBlock("custom_block");
                     System.out.println("custom stone");
                 }
                 if (block == Block.TORCH) {
@@ -154,9 +153,6 @@ public class PlayerInit {
             player.addEventCallback(PlayerLoginEvent.class, event -> {
 
                 event.setSpawningInstance(instanceContainer);
-                player.setEnableRespawnScreen(false);
-
-                player.setPermissionLevel(4);
 
                 player.getInventory().addInventoryCondition((p, slot, clickType, inventoryConditionResult) -> {
                     if (slot == -999)
@@ -168,29 +164,6 @@ public class PlayerInit {
                     }
                     System.out.println("slot player: " + slot + " : " + itemStack.getMaterial() + " : " + (itemStack.getData() != null));
                 });
-
-                {
-                    /*AdvancementManager advancementManager = MinecraftServer.getAdvancementManager();
-                    AdvancementRoot root = new AdvancementRoot(ColoredText.of("title"), ColoredText.of(ChatColor.BLUE + "description"),
-                            Material.APPLE, FrameType.TASK, 0, 0,
-                            "minecraft:textures/block/red_wool.png");
-                    root.setAchieved(true);
-                    AdvancementTab tab = advancementManager.createTab("root", root);
-                    Advancement advancement = new Advancement(ColoredText.of("adv"), ColoredText.of("desc"),
-                            Material.WOODEN_AXE, FrameType.CHALLENGE, 1, 0)
-                            .showToast(true).setHidden(false);
-                    tab.createAdvancement("second", advancement, root);
-
-                    tab.addViewer(player);
-
-                    root.setTitle(ColoredText.of("test ttlechange"));
-
-                    Advancement advancement2 = new Advancement(ColoredText.of(ChatColor.BLUE + "Title"),
-                            ColoredText.of("description of the advancement"),
-                            Material.GOLD_BLOCK, FrameType.CHALLENGE, 3, 0)
-                            .showToast(true).setHidden(false);
-                    tab.createAdvancement("second2", advancement2, root);*/
-                }
             });
 
             player.addEventCallback(PlayerSpawnEvent.class, event -> {
@@ -199,10 +172,10 @@ public class PlayerInit {
 
                 Data data = new DataImpl();
                 data.set("test", 5, Integer.class);
-                ItemStack itemStack = new ItemStack(Material.NETHERITE_PICKAXE, (byte) 1);
+                ItemStack itemStack = new ItemStack(Material.DIAMOND_PICKAXE, (byte) 1);
-                itemStack.setEnchantment(Enchantment.EFFICIENCY, (short) 5);
                 itemStack.setData(data);
                 player.getInventory().addItemStack(itemStack);
+                //player.getInventory().addItemStack(new ItemStack(Material.STONE, (byte)64));
             });
 
             player.addEventCallback(PlayerRespawnEvent.class, event -> {

src/test/java/demo/blocks/BurningTorchBlock.java

@@ -9,6 +9,7 @@ import net.minestom.server.instance.Instance;
 import net.minestom.server.instance.block.Block;
 import net.minestom.server.instance.block.CustomBlock;
 import net.minestom.server.utils.BlockPosition;
+import org.jetbrains.annotations.NotNull;
 
 /**
  * Custom block which burns entities that touch it
@@ -19,7 +20,7 @@ public class BurningTorchBlock extends CustomBlock {
     }
 
     @Override
-    public void handleContact(Instance instance, BlockPosition position, Entity touching) {
+    public void handleContact(@NotNull Instance instance, @NotNull BlockPosition position, @NotNull Entity touching) {
         System.out.println("touching " + touching);
         if (touching instanceof LivingEntity) {
             ((LivingEntity) touching).damage(DamageType.GRAVITY, 0.1f);
@@ -27,17 +28,17 @@ public class BurningTorchBlock extends CustomBlock {
     }
 
     @Override
-    public void onPlace(Instance instance, BlockPosition blockPosition, Data data) {
+    public void onPlace(@NotNull Instance instance, @NotNull BlockPosition blockPosition, Data data) {
         System.out.println(blockPosition);
     }
 
     @Override
-    public void onDestroy(Instance instance, BlockPosition blockPosition, Data data) {
+    public void onDestroy(@NotNull Instance instance, @NotNull BlockPosition blockPosition, Data data) {
 
     }
 
     @Override
-    public boolean onInteract(Player player, Player.Hand hand, BlockPosition blockPosition, Data data) {
+    public boolean onInteract(@NotNull Player player, @NotNull Player.Hand hand, @NotNull BlockPosition blockPosition, Data data) {
         return false;
     }
 

src/test/java/demo/blocks/StoneBlock.java

@@ -6,22 +6,23 @@ import net.minestom.server.instance.Instance;
 import net.minestom.server.instance.block.Block;
 import net.minestom.server.instance.block.CustomBlock;
 import net.minestom.server.utils.BlockPosition;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.Set;
 
 public class StoneBlock extends CustomBlock {
 
     public StoneBlock() {
-        super(Block.STONE, "custom_block");
+        super(Block.GOLD_BLOCK, "custom_block");
     }
 
     @Override
-    public void onPlace(Instance instance, BlockPosition blockPosition, Data data) {
+    public void onPlace(@NotNull Instance instance, @NotNull BlockPosition blockPosition, Data data) {
 
     }
 
     @Override
-    public void onDestroy(Instance instance, BlockPosition blockPosition, Data data) {
+    public void onDestroy(@NotNull Instance instance, @NotNull BlockPosition blockPosition, Data data) {
         BlockPosition above = blockPosition.clone().add(0, 1, 0);
         CustomBlock blockAbove = instance.getCustomBlock(above);
         if (blockAbove == this) {
@@ -31,17 +32,17 @@ public class StoneBlock extends CustomBlock {
     }
 
     @Override
-    public void updateFromNeighbor(Instance instance, BlockPosition thisPosition, BlockPosition neighborPosition, boolean directNeighbor) {
+    public void updateFromNeighbor(@NotNull Instance instance, @NotNull BlockPosition thisPosition, @NotNull BlockPosition neighborPosition, boolean directNeighbor) {
 
     }
 
     @Override
-    public boolean onInteract(Player player, Player.Hand hand, BlockPosition blockPosition, Data data) {
+    public boolean onInteract(@NotNull Player player, @NotNull Player.Hand hand, @NotNull BlockPosition blockPosition, Data data) {
         return false;
     }
 
     @Override
-    public int getBreakDelay(Player player, BlockPosition position, byte stage, Set<Player> breakers) {
+    public int getBreakDelay(@NotNull Player player, @NotNull BlockPosition position, byte stage, Set<Player> breakers) {
         return 2;
     }
 

src/test/java/demo/blocks/UpdatableBlockDemo.java

@@ -8,6 +8,7 @@ import net.minestom.server.instance.block.CustomBlock;
 import net.minestom.server.utils.BlockPosition;
 import net.minestom.server.utils.time.TimeUnit;
 import net.minestom.server.utils.time.UpdateOption;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.Set;
 
@@ -20,22 +21,22 @@ public class UpdatableBlockDemo extends CustomBlock {
     }
 
     @Override
-    public void update(Instance instance, BlockPosition blockPosition, Data data) {
+    public void update(@NotNull Instance instance, @NotNull BlockPosition blockPosition, Data data) {
         System.out.println("BLOCK UPDATE");
     }
 
     @Override
-    public void onPlace(Instance instance, BlockPosition blockPosition, Data data) {
+    public void onPlace(@NotNull Instance instance, @NotNull BlockPosition blockPosition, Data data) {
 
     }
 
     @Override
-    public void onDestroy(Instance instance, BlockPosition blockPosition, Data data) {
+    public void onDestroy(@NotNull Instance instance, @NotNull BlockPosition blockPosition, Data data) {
 
     }
 
     @Override
-    public boolean onInteract(Player player, Player.Hand hand, BlockPosition blockPosition, Data data) {
+    public boolean onInteract(@NotNull Player player, @NotNull Player.Hand hand, @NotNull BlockPosition blockPosition, Data data) {
         return false;
     }
 
@@ -45,7 +46,7 @@ public class UpdatableBlockDemo extends CustomBlock {
     }
 
     @Override
-    public int getBreakDelay(Player player, BlockPosition position, byte stage, Set<Player> breakers) {
+    public int getBreakDelay(@NotNull Player player, @NotNull BlockPosition position, byte stage, Set<Player> breakers) {
         return 1;
     }
 

src/test/java/demo/commands/DimensionCommand.java

@@ -6,10 +6,12 @@ import net.minestom.server.command.CommandSender;
 import net.minestom.server.entity.Player;
 import net.minestom.server.instance.Instance;
 import net.minestom.server.world.DimensionType;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.Optional;
 
 public class DimensionCommand implements CommandProcessor {
+    @NotNull
     @Override
     public String getCommandName() {
         return "dimensiontest";
@@ -21,7 +23,7 @@ public class DimensionCommand implements CommandProcessor {
     }
 
     @Override
-    public boolean process(CommandSender sender, String command, String[] args) {
+    public boolean process(@NotNull CommandSender sender, @NotNull String command, @NotNull String[] args) {
 
         if (!sender.isPlayer())
             return false;
@@ -48,7 +50,7 @@ public class DimensionCommand implements CommandProcessor {
     }
 
     @Override
-    public boolean hasAccess(Player player) {
+    public boolean hasAccess(@NotNull Player player) {
         return true;
     }
 }

src/test/java/demo/commands/ShutdownCommand.java

@@ -4,12 +4,14 @@ import net.minestom.server.MinecraftServer;
 import net.minestom.server.command.CommandProcessor;
 import net.minestom.server.command.CommandSender;
 import net.minestom.server.entity.Player;
+import org.jetbrains.annotations.NotNull;
 
 /**
  * A simple shutdown command.
  */
 public class ShutdownCommand implements CommandProcessor {
 
+    @NotNull
     @Override
     public String getCommandName() {
         return "shutdown";
@@ -21,13 +23,13 @@ public class ShutdownCommand implements CommandProcessor {
     }
 
     @Override
-    public boolean process(CommandSender sender, String command, String[] args) {
+    public boolean process(@NotNull CommandSender sender, @NotNull String command, @NotNull String[] args) {
         MinecraftServer.stopCleanly();
         return true;
     }
 
     @Override
-    public boolean hasAccess(Player player) {
+    public boolean hasAccess(@NotNull Player player) {
         return true;
     }
 }

src/test/java/demo/commands/SimpleCommand.java

@@ -3,8 +3,10 @@ package demo.commands;
 import net.minestom.server.command.CommandProcessor;
 import net.minestom.server.command.CommandSender;
 import net.minestom.server.entity.Player;
+import org.jetbrains.annotations.NotNull;
 
 public class SimpleCommand implements CommandProcessor {
+    @NotNull
     @Override
     public String getCommandName() {
         return "test";
@@ -16,11 +18,11 @@ public class SimpleCommand implements CommandProcessor {
     }
 
     @Override
-    public boolean process(CommandSender sender, String command, String[] args) {
+    public boolean process(@NotNull CommandSender sender, @NotNull String command, @NotNull String[] args) {
         if (!sender.isPlayer())
             return false;
 
-        //sender.asPlayer().getInstance().saveChunksToStorage(() -> System.out.println("END SAVE"));
+        sender.asPlayer().getInstance().saveChunksToStorage(() -> System.out.println("END SAVE"));
 
         System.gc();
 
@@ -28,7 +30,7 @@ public class SimpleCommand implements CommandProcessor {
     }
 
     @Override
-    public boolean hasAccess(Player player) {
+    public boolean hasAccess(@NotNull Player player) {
         return true;
     }
 
@@ -38,7 +40,7 @@ public class SimpleCommand implements CommandProcessor {
     }
 
     @Override
-    public String[] onWrite(String text) {
+    public String[] onWrite(@NotNull String text) {
         return new String[]{"Complete1", "Complete2"};
     }
 }