diff --git a/src/main/java/net/minestom/server/advancements/AdvancementTab.java b/src/main/java/net/minestom/server/advancements/AdvancementTab.java
index 5f3d041a1..d7c69dbfe 100644
--- a/src/main/java/net/minestom/server/advancements/AdvancementTab.java
+++ b/src/main/java/net/minestom/server/advancements/AdvancementTab.java
@@ -9,6 +9,7 @@ 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 org.jetbrains.annotations.Nullable;
 
 import java.util.*;
 
@@ -38,7 +39,7 @@ public class AdvancementTab implements Viewable {
     // will never change (since the root identifier is always the same)
     protected final ByteBuf removeBuffer;
 
-    protected AdvancementTab(String rootIdentifier, AdvancementRoot root) {
+    protected AdvancementTab(@NotNull String rootIdentifier, @NotNull AdvancementRoot root) {
         this.root = root;
 
         cacheAdvancement(rootIdentifier, root, null);
@@ -53,7 +54,8 @@ public class AdvancementTab implements Viewable {
      * @param player the player to get the tabs from
      * @return all the advancement tabs that the player sees
      */
-    public static Set<AdvancementTab> getTabs(Player player) {
+    @NotNull
+    public static Set<AdvancementTab> getTabs(@NotNull Player player) {
         return PLAYER_TAB_MAP.getOrDefault(player, null);
     }
 
@@ -62,6 +64,7 @@ public class AdvancementTab implements Viewable {
      *
      * @return the root advancement
      */
+    @NotNull
     public AdvancementRoot getRoot() {
         return root;
     }
@@ -73,7 +76,7 @@ public class AdvancementTab implements Viewable {
      * @param advancement the advancement to add
      * @param parent      the parent of this advancement, it cannot be null
      */
-    public void createAdvancement(String identifier, Advancement advancement, Advancement parent) {
+    public void createAdvancement(@NotNull String identifier, @NotNull Advancement advancement, @NotNull Advancement parent) {
         Check.argCondition(identifier == null, "the advancement identifier cannot be null");
         Check.stateCondition(!advancementMap.containsKey(parent),
                 "You tried to set a parent which doesn't exist or isn't registered");
@@ -96,6 +99,7 @@ public class AdvancementTab implements Viewable {
      *
      * @return the packet adding this advancement tab and all its advancements
      */
+    @NotNull
     protected AdvancementsPacket createPacket() {
         AdvancementsPacket advancementsPacket = new AdvancementsPacket();
         advancementsPacket.resetAdvancements = false;
@@ -120,9 +124,9 @@ public class AdvancementTab implements Viewable {
      *
      * @param identifier  the identifier of the advancement
      * @param advancement the advancement
-     * @param parent      the parent of this advancement
+     * @param parent      the parent of this advancement, only null for the root advancement
      */
-    private void cacheAdvancement(String identifier, Advancement advancement, Advancement parent) {
+    private void cacheAdvancement(@NotNull String identifier, @NotNull Advancement advancement, @Nullable Advancement parent) {
         Check.stateCondition(advancement.getTab() != null,
                 "You tried to add an advancement already linked to a tab");
         advancement.setTab(this);
@@ -178,7 +182,7 @@ public class AdvancementTab implements Viewable {
      *
      * @param player the player
      */
-    private void addPlayer(Player player) {
+    private void addPlayer(@NotNull Player player) {
         Set<AdvancementTab> tabs = PLAYER_TAB_MAP.computeIfAbsent(player, p -> new HashSet<>());
         tabs.add(this);
     }
@@ -188,7 +192,7 @@ public class AdvancementTab implements Viewable {
      *
      * @param player the player
      */
-    private void removePlayer(Player player) {
+    private void removePlayer(@NotNull Player player) {
         if (!PLAYER_TAB_MAP.containsKey(player)) {
             return;
         }
diff --git a/src/main/java/net/minestom/server/attribute/Attribute.java b/src/main/java/net/minestom/server/attribute/Attribute.java
index 654743693..94a634a3a 100644
--- a/src/main/java/net/minestom/server/attribute/Attribute.java
+++ b/src/main/java/net/minestom/server/attribute/Attribute.java
@@ -23,7 +23,7 @@ public enum Attribute {
     private final float defaultValue;
     private final float maxVanillaValue;
 
-    Attribute(String key, float defaultValue, float maxVanillaValue) {
+    Attribute(@NotNull String key, float defaultValue, float maxVanillaValue) {
         this.key = key;
         this.defaultValue = defaultValue;
         this.maxVanillaValue = maxVanillaValue;
diff --git a/src/main/java/net/minestom/server/command/CommandManager.java b/src/main/java/net/minestom/server/command/CommandManager.java
index 34334ad44..c7454e99a 100644
--- a/src/main/java/net/minestom/server/command/CommandManager.java
+++ b/src/main/java/net/minestom/server/command/CommandManager.java
@@ -22,6 +22,8 @@ import net.minestom.server.network.packet.server.play.DeclareCommandsPacket;
 import net.minestom.server.utils.ArrayUtils;
 import net.minestom.server.utils.validate.Check;
 import org.apache.commons.lang3.tuple.Pair;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 import java.util.*;
 import java.util.function.Consumer;
@@ -75,7 +77,7 @@ public final class CommandManager {
      *
      * @param command the command to register
      */
-    public void register(Command command) {
+    public void register(@NotNull Command command) {
         this.dispatcher.register(command);
     }
 
@@ -85,7 +87,8 @@ public final class CommandManager {
      * @param commandName the command name
      * @return the command associated with the name, null if not any
      */
-    public Command getCommand(String commandName) {
+    @Nullable
+    public Command getCommand(@NotNull String commandName) {
         return dispatcher.findCommand(commandName);
     }
 
@@ -94,7 +97,7 @@ public final class CommandManager {
      *
      * @param commandProcessor the command to register
      */
-    public void register(CommandProcessor commandProcessor) {
+    public void register(@NotNull CommandProcessor commandProcessor) {
         this.commandProcessorMap.put(commandProcessor.getCommandName().toLowerCase(), commandProcessor);
         // Register aliases
         final String[] aliases = commandProcessor.getAliases();
@@ -111,7 +114,8 @@ public final class CommandManager {
      * @param commandName the command name
      * @return the command associated with the name, null if not any
      */
-    public CommandProcessor getCommandProcessor(String commandName) {
+    @Nullable
+    public CommandProcessor getCommandProcessor(@NotNull String commandName) {
         return commandProcessorMap.get(commandName.toLowerCase());
     }
 
@@ -122,7 +126,7 @@ public final class CommandManager {
      * @param command the raw command string (without the command prefix)
      * @return true if the command hadn't been cancelled and has been successful
      */
-    public boolean execute(CommandSender sender, String command) {
+    public boolean execute(@NotNull CommandSender sender, @NotNull String command) {
         Check.notNull(sender, "Source cannot be null");
         Check.notNull(command, "Command string cannot be null");
 
@@ -165,6 +169,7 @@ public final class CommandManager {
      *
      * @return the {@link ConsoleSender}
      */
+    @NotNull
     public ConsoleSender getConsoleSender() {
         return consoleSender;
     }
@@ -177,7 +182,8 @@ public final class CommandManager {
      * @param player the player to get the commands packet
      * @return the {@link DeclareCommandsPacket} for {@code player}
      */
-    public DeclareCommandsPacket createDeclareCommandsPacket(Player player) {
+    @NotNull
+    public DeclareCommandsPacket createDeclareCommandsPacket(@NotNull Player player) {
         return buildPacket(player);
     }
 
@@ -187,7 +193,8 @@ public final class CommandManager {
      * @param player the player to build the packet for
      * @return the commands packet for the specific player
      */
-    private DeclareCommandsPacket buildPacket(Player player) {
+    @NotNull
+    private DeclareCommandsPacket buildPacket(@NotNull Player player) {
         DeclareCommandsPacket declareCommandsPacket = new DeclareCommandsPacket();
 
         List<DeclareCommandsPacket.Node> nodes = new ArrayList<>();
@@ -282,7 +289,11 @@ public final class CommandManager {
      * @param syntaxes     the syntaxes of the command
      * @param rootChildren the children of the main node (all commands name)
      */
-    private void createCommand(List<DeclareCommandsPacket.Node> nodes, IntList cmdChildren, String name, Collection<CommandSyntax> syntaxes, IntList rootChildren) {
+    private void createCommand(@NotNull List<DeclareCommandsPacket.Node> nodes,
+                               @NotNull IntList cmdChildren,
+                               @NotNull String name,
+                               @NotNull Collection<CommandSyntax> syntaxes,
+                               @NotNull IntList rootChildren) {
 
         DeclareCommandsPacket.Node literalNode = createMainNode(name, syntaxes.isEmpty());
 
@@ -347,7 +358,8 @@ public final class CommandManager {
         }
     }
 
-    private DeclareCommandsPacket.Node createMainNode(String name, boolean executable) {
+    @NotNull
+    private DeclareCommandsPacket.Node createMainNode(@NotNull String name, boolean executable) {
         DeclareCommandsPacket.Node literalNode = new DeclareCommandsPacket.Node();
         literalNode.flags = getFlag(NodeType.LITERAL, executable, false, false);
         literalNode.name = name;
@@ -362,7 +374,8 @@ public final class CommandManager {
      * @param executable true if this is the last argument, false otherwise
      * @return the list of nodes that the argument require
      */
-    private List<DeclareCommandsPacket.Node> toNodes(Argument<?> argument, boolean executable) {
+    @NotNull
+    private List<DeclareCommandsPacket.Node> toNodes(@NotNull Argument<?> argument, boolean executable) {
         List<DeclareCommandsPacket.Node> nodes = new ArrayList<>();
 
         // You can uncomment this to test any brigadier parser on the client
@@ -526,7 +539,7 @@ public final class CommandManager {
         return nodes;
     }
 
-    private byte getNumberProperties(ArgumentNumber<? extends Number> argumentNumber) {
+    private byte getNumberProperties(@NotNull ArgumentNumber<? extends Number> argumentNumber) {
         byte result = 0;
         if (argumentNumber.hasMin())
             result += 1;
@@ -543,8 +556,9 @@ public final class CommandManager {
      * @param executable true if this will be the last argument, false otherwise
      * @return the created {@link DeclareCommandsPacket.Node}
      */
-    private DeclareCommandsPacket.Node simpleArgumentNode(List<DeclareCommandsPacket.Node> nodes,
-                                                          Argument<?> argument, boolean executable, boolean suggestion) {
+    @NotNull
+    private DeclareCommandsPacket.Node simpleArgumentNode(@NotNull List<DeclareCommandsPacket.Node> nodes,
+                                                          @NotNull Argument<?> argument, boolean executable, boolean suggestion) {
         DeclareCommandsPacket.Node argumentNode = new DeclareCommandsPacket.Node();
         nodes.add(argumentNode);
 
@@ -554,7 +568,7 @@ public final class CommandManager {
         return argumentNode;
     }
 
-    private byte getFlag(NodeType type, boolean executable, boolean redirect, boolean suggestionType) {
+    private byte getFlag(@NotNull NodeType type, boolean executable, boolean redirect, boolean suggestionType) {
         byte result = (byte) type.mask;
 
         if (executable) {
diff --git a/src/main/java/net/minestom/server/command/CommandProcessor.java b/src/main/java/net/minestom/server/command/CommandProcessor.java
index 08cecc69a..9b9cefebc 100644
--- a/src/main/java/net/minestom/server/command/CommandProcessor.java
+++ b/src/main/java/net/minestom/server/command/CommandProcessor.java
@@ -1,6 +1,8 @@
 package net.minestom.server.command;
 
 import net.minestom.server.entity.Player;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 /**
  * Represents a simple command which give you the whole string representation
@@ -18,6 +20,7 @@ public interface CommandProcessor {
      *
      * @return the main command's name
      */
+    @NotNull
     String getCommandName();
 
     /**
@@ -27,6 +30,7 @@ public interface CommandProcessor {
      *
      * @return the command aliases
      */
+    @Nullable
     String[] getAliases();
 
     /**
@@ -37,7 +41,7 @@ public interface CommandProcessor {
      * @param args    an array containing all the args (split by space char)
      * @return true when the command is successful, false otherwise
      */
-    boolean process(CommandSender sender, String command, String[] args);
+    boolean process(@NotNull CommandSender sender, @NotNull String command, @NotNull String[] args);
 
     /**
      * Called to know if a player has access to the command.
@@ -48,7 +52,7 @@ public interface CommandProcessor {
      * @param player the player to check the access
      * @return true if the player has access to the command, false otherwise
      */
-    boolean hasAccess(Player player);
+    boolean hasAccess(@NotNull Player player);
 
     /**
      * Needed to enable {@link #onWrite(String)} callback.
@@ -71,7 +75,8 @@ public interface CommandProcessor {
      * @return the array containing all the suggestion for the current arg (split " "), can be null
      * @see #enableWritingTracking()
      */
-    default String[] onWrite(String text) {
+    @Nullable
+    default String[] onWrite(@NotNull String text) {
         return null;
     }
 }
diff --git a/src/main/java/net/minestom/server/command/CommandSender.java b/src/main/java/net/minestom/server/command/CommandSender.java
index a5ab19a92..40c2807ec 100644
--- a/src/main/java/net/minestom/server/command/CommandSender.java
+++ b/src/main/java/net/minestom/server/command/CommandSender.java
@@ -2,6 +2,7 @@ package net.minestom.server.command;
 
 import net.minestom.server.entity.Player;
 import net.minestom.server.permission.Permission;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.Collection;
 
@@ -17,14 +18,14 @@ public interface CommandSender {
      *
      * @param message the message to send
      */
-    void sendMessage(String message);
+    void sendMessage(@NotNull String message);
 
     /**
      * Sends multiple raw string messages.
      *
      * @param messages the messages to send
      */
-    default void sendMessage(String[] messages) {
+    default void sendMessage(@NotNull String[] messages) {
         for (String message : messages) {
             sendMessage(message);
         }
@@ -36,6 +37,7 @@ public interface CommandSender {
      *
      * @return the permissions of this command sender.
      */
+    @NotNull
     Collection<Permission> getAllPermissions();
 
     /**
@@ -43,7 +45,7 @@ public interface CommandSender {
      *
      * @param permission the permission to add
      */
-    default void addPermission(Permission permission) {
+    default void addPermission(@NotNull Permission permission) {
         getAllPermissions().add(permission);
     }
 
@@ -52,7 +54,7 @@ public interface CommandSender {
      *
      * @param permission the permission to remove
      */
-    default void removePermission(Permission permission) {
+    default void removePermission(@NotNull Permission permission) {
         getAllPermissions().remove(permission);
     }
 
@@ -63,7 +65,7 @@ public interface CommandSender {
      * @param p permission to check against
      * @return true if the sender has the permission and validate {@link Permission#isValidFor(CommandSender)}
      */
-    default boolean hasPermission(Permission p) {
+    default boolean hasPermission(@NotNull Permission p) {
         return getAllPermissions().contains(p) && p.isValidFor(this);
     }
 
@@ -76,7 +78,7 @@ public interface CommandSender {
      * @return true if the sender has the permission and validate {@link Permission#isValidFor(CommandSender)}
      * @see #getAllPermissions()
      */
-    default boolean hasPermission(Class<? extends Permission> permissionClass) {
+    default boolean hasPermission(@NotNull Class<? extends Permission> permissionClass) {
         boolean result = true;
         boolean foundPerm = false;
         for (Permission p : getAllPermissions()) {
diff --git a/src/main/java/net/minestom/server/command/ConsoleSender.java b/src/main/java/net/minestom/server/command/ConsoleSender.java
index a171b4c43..b7bd6eac0 100644
--- a/src/main/java/net/minestom/server/command/ConsoleSender.java
+++ b/src/main/java/net/minestom/server/command/ConsoleSender.java
@@ -1,6 +1,7 @@
 package net.minestom.server.command;
 
 import net.minestom.server.permission.Permission;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.Collection;
 import java.util.LinkedList;
@@ -14,10 +15,11 @@ public class ConsoleSender implements CommandSender {
     private final List<Permission> permissions = new LinkedList<>();
 
     @Override
-    public void sendMessage(String message) {
+    public void sendMessage(@NotNull String message) {
         System.out.println(message);
     }
 
+    @NotNull
     @Override
     public Collection<Permission> getAllPermissions() {
         return permissions;
diff --git a/src/main/java/net/minestom/server/command/builder/ArgumentCallback.java b/src/main/java/net/minestom/server/command/builder/ArgumentCallback.java
index fc2623c72..0c0cf563b 100644
--- a/src/main/java/net/minestom/server/command/builder/ArgumentCallback.java
+++ b/src/main/java/net/minestom/server/command/builder/ArgumentCallback.java
@@ -2,6 +2,7 @@ package net.minestom.server.command.builder;
 
 import net.minestom.server.command.CommandSender;
 import net.minestom.server.command.builder.arguments.Argument;
+import org.jetbrains.annotations.NotNull;
 
 /**
  * Callback executed when an error is found within the {@link Argument}.
@@ -16,5 +17,5 @@ public interface ArgumentCallback {
      * @param value  the raw string argument which is responsible for the error
      * @param error  the error id (you can check its meaning in the specific argument class or ask the developer about it)
      */
-    void apply(CommandSender source, String value, int error);
+    void apply(@NotNull CommandSender source, @NotNull String value, int error);
 }
diff --git a/src/main/java/net/minestom/server/command/builder/Arguments.java b/src/main/java/net/minestom/server/command/builder/Arguments.java
index 499987fd9..787941a0b 100644
--- a/src/main/java/net/minestom/server/command/builder/Arguments.java
+++ b/src/main/java/net/minestom/server/command/builder/Arguments.java
@@ -10,6 +10,7 @@ import net.minestom.server.potion.PotionEffect;
 import net.minestom.server.utils.math.FloatRange;
 import net.minestom.server.utils.math.IntRange;
 import net.minestom.server.utils.time.UpdateOption;
+import org.jetbrains.annotations.NotNull;
 import org.jglrxavpok.hephaistos.nbt.NBT;
 import org.jglrxavpok.hephaistos.nbt.NBTCompound;
 
@@ -18,97 +19,97 @@ import java.util.List;
 import java.util.Map;
 
 /**
- * Class used to retrieve argument data
+ * Class used to retrieve argument data.
  */
 public class Arguments {
 
     private final Map<String, Object> args = new HashMap<>();
 
-    public boolean getBoolean(String id) {
+    public boolean getBoolean(@NotNull String id) {
         return (boolean) getObject(id);
     }
 
-    public long getLong(String id) {
+    public long getLong(@NotNull String id) {
         return (long) getObject(id);
     }
 
-    public int getInteger(String id) {
+    public int getInteger(@NotNull String id) {
         return (int) getObject(id);
     }
 
-    public double getDouble(String id) {
+    public double getDouble(@NotNull String id) {
         return (double) getObject(id);
     }
 
-    public float getFloat(String id) {
+    public float getFloat(@NotNull String id) {
         return (float) getObject(id);
     }
 
-    public String getString(String id) {
+    public String getString(@NotNull String id) {
         return (String) getObject(id);
     }
 
-    public String getWord(String id) {
+    public String getWord(@NotNull String id) {
         return getString(id);
     }
 
-    public String[] getStringArray(String id) {
+    public String[] getStringArray(@NotNull String id) {
         return (String[]) getObject(id);
     }
 
-    public ChatColor getColor(String id) {
+    public ChatColor getColor(@NotNull String id) {
         return (ChatColor) getObject(id);
     }
 
-    public UpdateOption getTime(String id) {
+    public UpdateOption getTime(@NotNull String id) {
         return (UpdateOption) getObject(id);
     }
 
-    public Enchantment getEnchantment(String id) {
+    public Enchantment getEnchantment(@NotNull String id) {
         return (Enchantment) getObject(id);
     }
 
-    public Particle getParticle(String id) {
+    public Particle getParticle(@NotNull String id) {
         return (Particle) getObject(id);
     }
 
-    public PotionEffect getPotionEffect(String id) {
+    public PotionEffect getPotionEffect(@NotNull String id) {
         return (PotionEffect) getObject(id);
     }
 
-    public EntityType getEntityType(String id) {
+    public EntityType getEntityType(@NotNull String id) {
         return (EntityType) getObject(id);
     }
 
-    public IntRange getIntRange(String id) {
+    public IntRange getIntRange(@NotNull String id) {
         return (IntRange) getObject(id);
     }
 
-    public FloatRange getFloatRange(String id) {
+    public FloatRange getFloatRange(@NotNull String id) {
         return (FloatRange) getObject(id);
     }
 
-    public List<Entity> getEntities(String id) {
+    public List<Entity> getEntities(@NotNull String id) {
         return (List<Entity>) getObject(id);
     }
 
-    public ItemStack getItemStack(String id) {
+    public ItemStack getItemStack(@NotNull String id) {
         return (ItemStack) getObject(id);
     }
 
-    public NBTCompound getNbtCompound(String id) {
+    public NBTCompound getNbtCompound(@NotNull String id) {
         return (NBTCompound) getObject(id);
     }
 
-    public NBT getNBT(String id) {
+    public NBT getNBT(@NotNull String id) {
         return (NBT) getObject(id);
     }
 
-    public Object getObject(String id) {
+    public Object getObject(@NotNull String id) {
         return args.getOrDefault(id, null);
     }
 
-    protected void setArg(String id, Object value) {
+    protected void setArg(@NotNull String id, Object value) {
         this.args.put(id, value);
     }
 
diff --git a/src/main/java/net/minestom/server/command/builder/Command.java b/src/main/java/net/minestom/server/command/builder/Command.java
index 3ce3782e3..69d032ea0 100644
--- a/src/main/java/net/minestom/server/command/builder/Command.java
+++ b/src/main/java/net/minestom/server/command/builder/Command.java
@@ -6,6 +6,8 @@ import net.minestom.server.command.builder.arguments.ArgumentDynamicStringArray;
 import net.minestom.server.command.builder.arguments.ArgumentDynamicWord;
 import net.minestom.server.command.builder.arguments.ArgumentType;
 import net.minestom.server.command.builder.condition.CommandCondition;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 import java.util.ArrayList;
 import java.util.Collection;
@@ -48,7 +50,7 @@ public class Command {
      * @param aliases the command aliases
      * @see #Command(String)
      */
-    public Command(String name, String... aliases) {
+    public Command(@NotNull String name, @Nullable String... aliases) {
         this.name = name;
         this.aliases = aliases;
 
@@ -61,7 +63,7 @@ public class Command {
      * @param name the name of the command
      * @see #Command(String, String...)
      */
-    public Command(String name) {
+    public Command(@NotNull String name) {
         this(name, new String[0]);
     }
 
@@ -94,7 +96,7 @@ public class Command {
      * @param callback the callback for the argument
      * @param argument the argument which get the callback
      */
-    public void setArgumentCallback(ArgumentCallback callback, Argument<?> argument) {
+    public void setArgumentCallback(@NotNull ArgumentCallback callback, @NotNull Argument<?> argument) {
         argument.setCallback(callback);
     }
 
@@ -106,9 +108,8 @@ public class Command {
      * @param executor the executor to call when the syntax is successfully received
      * @param args     all the arguments of the syntax
      */
-    public void addSyntax(CommandExecutor executor, Argument<?>... args) {
-        CommandSyntax syntax = new CommandSyntax(args);
-        syntax.setExecutor(executor);
+    public void addSyntax(@NotNull CommandExecutor executor, @NotNull Argument<?>... args) {
+        final CommandSyntax syntax = new CommandSyntax(executor, args);
         this.syntaxes.add(syntax);
     }
 
@@ -117,6 +118,7 @@ public class Command {
      *
      * @return the main command's name
      */
+    @NotNull
     public String getName() {
         return name;
     }
@@ -128,6 +130,7 @@ public class Command {
      *
      * @return the command aliases
      */
+    @Nullable
     public String[] getAliases() {
         return aliases;
     }
@@ -138,6 +141,7 @@ public class Command {
      *
      * @return the default executor
      */
+    @Nullable
     public CommandExecutor getDefaultExecutor() {
         return defaultExecutor;
     }
@@ -147,7 +151,7 @@ public class Command {
      *
      * @param executor the new default executor
      */
-    public void setDefaultExecutor(CommandExecutor executor) {
+    public void setDefaultExecutor(@Nullable CommandExecutor executor) {
         this.defaultExecutor = executor;
     }
 
@@ -156,6 +160,7 @@ public class Command {
      *
      * @return a collection containing all this command syntaxes
      */
+    @NotNull
     public Collection<CommandSyntax> getSyntaxes() {
         return syntaxes;
     }
@@ -167,7 +172,8 @@ public class Command {
      * @param text the whole player's text
      * @return the array containing all the suggestion for the current arg (split " "), can be null
      */
-    public String[] onDynamicWrite(String text) {
+    @Nullable
+    public String[] onDynamicWrite(@NotNull String text) {
         return null;
     }
 
@@ -183,7 +189,7 @@ public class Command {
      * @param arguments the UNCHECKED arguments of the command, some can be null even when unexpected
      * @param command   the raw UNCHECKED received command
      */
-    public void globalListener(CommandSender sender, Arguments arguments, String command) {
+    public void globalListener(@NotNull CommandSender sender, @NotNull Arguments arguments, @NotNull String command) {
     }
 
 }
diff --git a/src/main/java/net/minestom/server/command/builder/CommandExecutor.java b/src/main/java/net/minestom/server/command/builder/CommandExecutor.java
index 84fed0f62..ef6010964 100644
--- a/src/main/java/net/minestom/server/command/builder/CommandExecutor.java
+++ b/src/main/java/net/minestom/server/command/builder/CommandExecutor.java
@@ -1,6 +1,7 @@
 package net.minestom.server.command.builder;
 
 import net.minestom.server.command.CommandSender;
+import org.jetbrains.annotations.NotNull;
 
 /**
  * Callback executed once a syntax has been found for a {@link Command}.
@@ -9,5 +10,5 @@ import net.minestom.server.command.CommandSender;
  */
 @FunctionalInterface
 public interface CommandExecutor {
-    void apply(CommandSender source, Arguments args);
+    void apply(@NotNull CommandSender source, @NotNull Arguments args);
 }
\ No newline at end of file
diff --git a/src/main/java/net/minestom/server/command/builder/CommandSyntax.java b/src/main/java/net/minestom/server/command/builder/CommandSyntax.java
index 4cfab1b82..609367863 100644
--- a/src/main/java/net/minestom/server/command/builder/CommandSyntax.java
+++ b/src/main/java/net/minestom/server/command/builder/CommandSyntax.java
@@ -1,6 +1,7 @@
 package net.minestom.server.command.builder;
 
 import net.minestom.server.command.builder.arguments.Argument;
+import org.jetbrains.annotations.NotNull;
 
 /**
  * Represents a syntax in {@link Command}.
@@ -10,7 +11,8 @@ public class CommandSyntax {
     private final Argument[] args;
     private CommandExecutor executor;
 
-    protected CommandSyntax(Argument... args) {
+    protected CommandSyntax(@NotNull CommandExecutor commandExecutor, @NotNull Argument... args) {
+        this.executor = commandExecutor;
         this.args = args;
     }
 
@@ -19,6 +21,7 @@ public class CommandSyntax {
      *
      * @return the required arguments
      */
+    @NotNull
     public Argument[] getArguments() {
         return args;
     }
@@ -28,6 +31,7 @@ public class CommandSyntax {
      *
      * @return the executor of this syntax
      */
+    @NotNull
     public CommandExecutor getExecutor() {
         return executor;
     }
@@ -37,7 +41,7 @@ public class CommandSyntax {
      *
      * @param executor the new executor
      */
-    public void setExecutor(CommandExecutor executor) {
+    public void setExecutor(@NotNull CommandExecutor executor) {
         this.executor = executor;
     }
 
diff --git a/src/main/java/net/minestom/server/command/builder/arguments/Argument.java b/src/main/java/net/minestom/server/command/builder/arguments/Argument.java
index 2e27e02dc..e6827c52f 100644
--- a/src/main/java/net/minestom/server/command/builder/arguments/Argument.java
+++ b/src/main/java/net/minestom/server/command/builder/arguments/Argument.java
@@ -2,6 +2,8 @@ package net.minestom.server.command.builder.arguments;
 
 import net.minestom.server.command.builder.ArgumentCallback;
 import net.minestom.server.command.builder.Command;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 /**
  * An argument is meant to be parsed when added into a {@link Command} syntax.
@@ -26,17 +28,17 @@ public abstract class Argument<T> {
 
     private ArgumentCallback callback;
 
-    public Argument(String id, boolean allowSpace, boolean useRemaining) {
+    public Argument(@NotNull String id, boolean allowSpace, boolean useRemaining) {
         this.id = id;
         this.allowSpace = allowSpace;
         this.useRemaining = useRemaining;
     }
 
-    public Argument(String id, boolean allowSpace) {
+    public Argument(@NotNull String id, boolean allowSpace) {
         this(id, allowSpace, false);
     }
 
-    public Argument(String id) {
+    public Argument(@NotNull String id) {
         this(id, false, false);
     }
 
@@ -46,7 +48,7 @@ public abstract class Argument<T> {
      * @param value The received argument
      * @return The success/error code
      */
-    public abstract int getCorrectionResult(String value);
+    public abstract int getCorrectionResult(@NotNull String value);
 
     /**
      * The argument syntax is correct, parsed here to the correct type.
@@ -54,7 +56,8 @@ public abstract class Argument<T> {
      * @param value The correct argument
      * @return The parsed argument
      */
-    public abstract T parse(String value);
+    @NotNull
+    public abstract T parse(@NotNull String value);
 
     /**
      * The argument is at least partially correct (the syntax is good and the argument has been parsed)
@@ -63,7 +66,7 @@ public abstract class Argument<T> {
      * @param value The parsed argument
      * @return The success/error code
      */
-    public abstract int getConditionResult(T value);
+    public abstract int getConditionResult(@NotNull T value);
 
     /**
      * Gets the ID of the argument, showed in-game above the chat bar
@@ -71,6 +74,7 @@ public abstract class Argument<T> {
      *
      * @return the argument id
      */
+    @NotNull
     public String getId() {
         return id;
     }
@@ -101,6 +105,7 @@ public abstract class Argument<T> {
      *
      * @return the argument callback
      */
+    @Nullable
     public ArgumentCallback getCallback() {
         return callback;
     }
@@ -110,7 +115,7 @@ public abstract class Argument<T> {
      *
      * @param callback the argument callback
      */
-    public void setCallback(ArgumentCallback callback) {
+    public void setCallback(@Nullable ArgumentCallback callback) {
         this.callback = callback;
     }
 
diff --git a/src/main/java/net/minestom/server/command/builder/arguments/ArgumentBoolean.java b/src/main/java/net/minestom/server/command/builder/arguments/ArgumentBoolean.java
index 16bec43c9..7020bd490 100644
--- a/src/main/java/net/minestom/server/command/builder/arguments/ArgumentBoolean.java
+++ b/src/main/java/net/minestom/server/command/builder/arguments/ArgumentBoolean.java
@@ -1,5 +1,7 @@
 package net.minestom.server.command.builder.arguments;
 
+import org.jetbrains.annotations.NotNull;
+
 /**
  * Represents a boolean value.
  * <p>
@@ -14,18 +16,19 @@ public class ArgumentBoolean extends Argument<Boolean> {
     }
 
     @Override
-    public int getCorrectionResult(String value) {
+    public int getCorrectionResult(@NotNull String value) {
         return (value.equalsIgnoreCase("true")
                 || value.equalsIgnoreCase("false")) ? SUCCESS : NOT_BOOLEAN_ERROR;
     }
 
+    @NotNull
     @Override
-    public Boolean parse(String value) {
+    public Boolean parse(@NotNull String value) {
         return Boolean.parseBoolean(value);
     }
 
     @Override
-    public int getConditionResult(Boolean value) {
+    public int getConditionResult(@NotNull Boolean value) {
         return SUCCESS;
     }
 
diff --git a/src/main/java/net/minestom/server/command/builder/arguments/ArgumentDynamicStringArray.java b/src/main/java/net/minestom/server/command/builder/arguments/ArgumentDynamicStringArray.java
index d503f7f0c..e254e07f4 100644
--- a/src/main/java/net/minestom/server/command/builder/arguments/ArgumentDynamicStringArray.java
+++ b/src/main/java/net/minestom/server/command/builder/arguments/ArgumentDynamicStringArray.java
@@ -1,5 +1,7 @@
 package net.minestom.server.command.builder.arguments;
 
+import org.jetbrains.annotations.NotNull;
+
 import java.util.regex.Pattern;
 
 /**
@@ -13,17 +15,18 @@ public class ArgumentDynamicStringArray extends Argument<String[]> {
     }
 
     @Override
-    public int getCorrectionResult(String value) {
+    public int getCorrectionResult(@NotNull String value) {
         return SUCCESS;
     }
 
+    @NotNull
     @Override
-    public String[] parse(String value) {
+    public String[] parse(@NotNull String value) {
         return value.split(Pattern.quote(" "));
     }
 
     @Override
-    public int getConditionResult(String[] value) {
+    public int getConditionResult(@NotNull String[] value) {
         return SUCCESS;
     }
 }
diff --git a/src/main/java/net/minestom/server/command/builder/arguments/ArgumentDynamicWord.java b/src/main/java/net/minestom/server/command/builder/arguments/ArgumentDynamicWord.java
index 7e186c056..c4b88384e 100644
--- a/src/main/java/net/minestom/server/command/builder/arguments/ArgumentDynamicWord.java
+++ b/src/main/java/net/minestom/server/command/builder/arguments/ArgumentDynamicWord.java
@@ -1,5 +1,7 @@
 package net.minestom.server.command.builder.arguments;
 
+import org.jetbrains.annotations.NotNull;
+
 /**
  * Same as {@link ArgumentWord} with the exception
  * that this argument can trigger {@link net.minestom.server.command.builder.Command#onDynamicWrite(String)}.
@@ -11,17 +13,18 @@ public class ArgumentDynamicWord extends Argument<String> {
     }
 
     @Override
-    public int getCorrectionResult(String value) {
+    public int getCorrectionResult(@NotNull String value) {
         return SUCCESS;
     }
 
+    @NotNull
     @Override
-    public String parse(String value) {
+    public String parse(@NotNull String value) {
         return value;
     }
 
     @Override
-    public int getConditionResult(String value) {
+    public int getConditionResult(@NotNull String value) {
         return SUCCESS;
     }
 }
diff --git a/src/main/java/net/minestom/server/command/builder/arguments/ArgumentString.java b/src/main/java/net/minestom/server/command/builder/arguments/ArgumentString.java
index eb6bbfb08..b390aaf0e 100644
--- a/src/main/java/net/minestom/server/command/builder/arguments/ArgumentString.java
+++ b/src/main/java/net/minestom/server/command/builder/arguments/ArgumentString.java
@@ -1,5 +1,7 @@
 package net.minestom.server.command.builder.arguments;
 
+import org.jetbrains.annotations.NotNull;
+
 /**
  * Argument which will take a quoted string.
  * <p>
@@ -14,7 +16,7 @@ public class ArgumentString extends Argument<String> {
     }
 
     @Override
-    public int getCorrectionResult(String value) {
+    public int getCorrectionResult(@NotNull String value) {
         // Check if value start and end with quote
         final char first = value.charAt(0);
         final char last = value.charAt(value.length() - 1);
@@ -38,8 +40,9 @@ public class ArgumentString extends Argument<String> {
         return QUOTE_ERROR;
     }
 
+    @NotNull
     @Override
-    public String parse(String value) {
+    public String parse(@NotNull String value) {
         // Remove first and last characters (quote)
         value = value.substring(1, value.length() - 1);
 
@@ -50,7 +53,7 @@ public class ArgumentString extends Argument<String> {
     }
 
     @Override
-    public int getConditionResult(String value) {
+    public int getConditionResult(@NotNull String value) {
         return SUCCESS;
     }
 }
diff --git a/src/main/java/net/minestom/server/command/builder/arguments/ArgumentStringArray.java b/src/main/java/net/minestom/server/command/builder/arguments/ArgumentStringArray.java
index acc33076a..8f2a53bd2 100644
--- a/src/main/java/net/minestom/server/command/builder/arguments/ArgumentStringArray.java
+++ b/src/main/java/net/minestom/server/command/builder/arguments/ArgumentStringArray.java
@@ -1,5 +1,7 @@
 package net.minestom.server.command.builder.arguments;
 
+import org.jetbrains.annotations.NotNull;
+
 import java.util.regex.Pattern;
 
 /**
@@ -14,17 +16,18 @@ public class ArgumentStringArray extends Argument<String[]> {
     }
 
     @Override
-    public int getCorrectionResult(String value) {
+    public int getCorrectionResult(@NotNull String value) {
         return SUCCESS;
     }
 
+    @NotNull
     @Override
-    public String[] parse(String value) {
+    public String[] parse(@NotNull String value) {
         return value.split(Pattern.quote(" "));
     }
 
     @Override
-    public int getConditionResult(String[] value) {
+    public int getConditionResult(@NotNull String[] value) {
         return SUCCESS;
     }
 }
diff --git a/src/main/java/net/minestom/server/command/builder/arguments/ArgumentType.java b/src/main/java/net/minestom/server/command/builder/arguments/ArgumentType.java
index a2534179b..494340390 100644
--- a/src/main/java/net/minestom/server/command/builder/arguments/ArgumentType.java
+++ b/src/main/java/net/minestom/server/command/builder/arguments/ArgumentType.java
@@ -9,6 +9,7 @@ import net.minestom.server.command.builder.arguments.number.ArgumentDouble;
 import net.minestom.server.command.builder.arguments.number.ArgumentFloat;
 import net.minestom.server.command.builder.arguments.number.ArgumentInteger;
 import net.minestom.server.command.builder.arguments.number.ArgumentLong;
+import org.jetbrains.annotations.NotNull;
 
 /**
  * Convenient class listing all the basics {@link Argument}.
@@ -17,94 +18,94 @@ import net.minestom.server.command.builder.arguments.number.ArgumentLong;
  */
 public class ArgumentType {
 
-    public static ArgumentBoolean Boolean(String id) {
+    public static ArgumentBoolean Boolean(@NotNull String id) {
         return new ArgumentBoolean(id);
     }
 
-    public static ArgumentLong Long(String id) {
+    public static ArgumentLong Long(@NotNull String id) {
         return new ArgumentLong(id);
     }
 
-    public static ArgumentInteger Integer(String id) {
+    public static ArgumentInteger Integer(@NotNull String id) {
         return new ArgumentInteger(id);
     }
 
-    public static ArgumentDouble Double(String id) {
+    public static ArgumentDouble Double(@NotNull String id) {
         return new ArgumentDouble(id);
     }
 
-    public static ArgumentFloat Float(String id) {
+    public static ArgumentFloat Float(@NotNull String id) {
         return new ArgumentFloat(id);
     }
 
-    public static ArgumentString String(String id) {
+    public static ArgumentString String(@NotNull String id) {
         return new ArgumentString(id);
     }
 
-    public static ArgumentWord Word(String id) {
+    public static ArgumentWord Word(@NotNull String id) {
         return new ArgumentWord(id);
     }
 
-    public static ArgumentDynamicWord DynamicWord(String id) {
+    public static ArgumentDynamicWord DynamicWord(@NotNull String id) {
         return new ArgumentDynamicWord(id);
     }
 
-    public static ArgumentStringArray StringArray(String id) {
+    public static ArgumentStringArray StringArray(@NotNull String id) {
         return new ArgumentStringArray(id);
     }
 
-    public static ArgumentDynamicStringArray DynamicStringArray(String id) {
+    public static ArgumentDynamicStringArray DynamicStringArray(@NotNull String id) {
         return new ArgumentDynamicStringArray(id);
     }
 
     // Minecraft specific arguments
 
-    public static ArgumentColor Color(String id) {
+    public static ArgumentColor Color(@NotNull String id) {
         return new ArgumentColor(id);
     }
 
-    public static ArgumentTime Time(String id) {
+    public static ArgumentTime Time(@NotNull String id) {
         return new ArgumentTime(id);
     }
 
-    public static ArgumentEnchantment Enchantment(String id) {
+    public static ArgumentEnchantment Enchantment(@NotNull String id) {
         return new ArgumentEnchantment(id);
     }
 
-    public static ArgumentParticle Particle(String id) {
+    public static ArgumentParticle Particle(@NotNull String id) {
         return new ArgumentParticle(id);
     }
 
-    public static ArgumentPotionEffect Potion(String id) {
+    public static ArgumentPotionEffect Potion(@NotNull String id) {
         return new ArgumentPotionEffect(id);
     }
 
-    public static ArgumentEntityType EntityType(String id) {
+    public static ArgumentEntityType EntityType(@NotNull String id) {
         return new ArgumentEntityType(id);
     }
 
-    public static ArgumentIntRange IntRange(String id) {
+    public static ArgumentIntRange IntRange(@NotNull String id) {
         return new ArgumentIntRange(id);
     }
 
-    public static ArgumentFloatRange FloatRange(String id) {
+    public static ArgumentFloatRange FloatRange(@NotNull String id) {
         return new ArgumentFloatRange(id);
     }
 
     @Deprecated
-    public static ArgumentEntities Entities(String id) {
+    public static ArgumentEntities Entities(@NotNull String id) {
         return new ArgumentEntities(id);
     }
 
-    public static ArgumentItemStack ItemStack(String id) {
+    public static ArgumentItemStack ItemStack(@NotNull String id) {
         return new ArgumentItemStack(id);
     }
 
-    public static ArgumentNbtCompoundTag NbtCompound(String id) {
+    public static ArgumentNbtCompoundTag NbtCompound(@NotNull String id) {
         return new ArgumentNbtCompoundTag(id);
     }
 
-    public static ArgumentNbtTag NBT(String id) {
+    public static ArgumentNbtTag NBT(@NotNull String id) {
         return new ArgumentNbtTag(id);
     }
 
diff --git a/src/main/java/net/minestom/server/command/builder/arguments/ArgumentWord.java b/src/main/java/net/minestom/server/command/builder/arguments/ArgumentWord.java
index e9541688b..5c6460843 100644
--- a/src/main/java/net/minestom/server/command/builder/arguments/ArgumentWord.java
+++ b/src/main/java/net/minestom/server/command/builder/arguments/ArgumentWord.java
@@ -1,5 +1,7 @@
 package net.minestom.server.command.builder.arguments;
 
+import org.jetbrains.annotations.NotNull;
+
 /**
  * Represents a single word in the command.
  * <p>
@@ -24,20 +26,21 @@ public class ArgumentWord extends Argument<String> {
     }
 
     @Override
-    public int getCorrectionResult(String value) {
+    public int getCorrectionResult(@NotNull String value) {
         if (value.contains(" "))
             return SPACE_ERROR;
 
         return SUCCESS;
     }
 
+    @NotNull
     @Override
-    public String parse(String value) {
+    public String parse(@NotNull String value) {
         return value;
     }
 
     @Override
-    public int getConditionResult(String value) {
+    public int getConditionResult(@NotNull String value) {
         // Check restrictions
         if (restrictions != null && restrictions.length > 0) {
             for (String r : restrictions) {
diff --git a/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentColor.java b/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentColor.java
index 4d9196bad..0d8478a3a 100644
--- a/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentColor.java
+++ b/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentColor.java
@@ -2,6 +2,7 @@ package net.minestom.server.command.builder.arguments.minecraft;
 
 import net.minestom.server.chat.ChatColor;
 import net.minestom.server.command.builder.arguments.Argument;
+import org.jetbrains.annotations.NotNull;
 
 /**
  * Represents an argument which will give you a {@link ChatColor}.
@@ -17,18 +18,19 @@ public class ArgumentColor extends Argument<ChatColor> {
     }
 
     @Override
-    public int getCorrectionResult(String value) {
+    public int getCorrectionResult(@NotNull String value) {
         final ChatColor color = ChatColor.fromName(value);
         return color == ChatColor.NO_COLOR ? UNDEFINED_COLOR : SUCCESS;
     }
 
+    @NotNull
     @Override
-    public ChatColor parse(String value) {
+    public ChatColor parse(@NotNull String value) {
         return ChatColor.fromName(value);
     }
 
     @Override
-    public int getConditionResult(ChatColor value) {
+    public int getConditionResult(@NotNull ChatColor value) {
         return SUCCESS;
     }
 }
diff --git a/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentEntities.java b/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentEntities.java
index e97914db2..e6462d737 100644
--- a/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentEntities.java
+++ b/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentEntities.java
@@ -4,6 +4,7 @@ import net.minestom.server.MinecraftServer;
 import net.minestom.server.command.builder.arguments.Argument;
 import net.minestom.server.entity.Entity;
 import net.minestom.server.network.ConnectionManager;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.ArrayList;
 import java.util.Arrays;
@@ -46,7 +47,7 @@ public class ArgumentEntities extends Argument<ArrayList<Entity>> {
     }
 
     @Override
-    public int getCorrectionResult(String value) {
+    public int getCorrectionResult(@NotNull String value) {
         System.out.println("check: " + value);
 
         // Check for raw player name
@@ -112,13 +113,14 @@ public class ArgumentEntities extends Argument<ArrayList<Entity>> {
         return finalIndex;
     }
 
+    @NotNull
     @Override
-    public ArrayList<Entity> parse(String value) {
+    public ArrayList<Entity> parse(@NotNull String value) {
         return null;
     }
 
     @Override
-    public int getConditionResult(ArrayList<Entity> value) {
+    public int getConditionResult(@NotNull ArrayList<Entity> value) {
         return SUCCESS;
     }
 
diff --git a/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentFloatRange.java b/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentFloatRange.java
index 81e25c284..eb6b1433b 100644
--- a/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentFloatRange.java
+++ b/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentFloatRange.java
@@ -1,6 +1,7 @@
 package net.minestom.server.command.builder.arguments.minecraft;
 
 import net.minestom.server.utils.math.FloatRange;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.regex.Pattern;
 
@@ -16,7 +17,7 @@ public class ArgumentFloatRange extends ArgumentRange<FloatRange> {
     }
 
     @Override
-    public int getCorrectionResult(String value) {
+    public int getCorrectionResult(@NotNull String value) {
         try {
             Float.valueOf(value);
             return SUCCESS; // Is a single number
@@ -43,8 +44,9 @@ public class ArgumentFloatRange extends ArgumentRange<FloatRange> {
         }
     }
 
+    @NotNull
     @Override
-    public FloatRange parse(String value) {
+    public FloatRange parse(@NotNull String value) {
         if (value.contains("..")) {
             final int index = value.indexOf('.');
             final String[] split = value.split(Pattern.quote(".."));
diff --git a/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentIntRange.java b/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentIntRange.java
index 4db7e278d..1f4849ec5 100644
--- a/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentIntRange.java
+++ b/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentIntRange.java
@@ -1,6 +1,7 @@
 package net.minestom.server.command.builder.arguments.minecraft;
 
 import net.minestom.server.utils.math.IntRange;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.regex.Pattern;
 
@@ -16,7 +17,7 @@ public class ArgumentIntRange extends ArgumentRange<IntRange> {
     }
 
     @Override
-    public int getCorrectionResult(String value) {
+    public int getCorrectionResult(@NotNull String value) {
         try {
             Integer.valueOf(value);
             return SUCCESS; // Is a single number
@@ -43,8 +44,9 @@ public class ArgumentIntRange extends ArgumentRange<IntRange> {
         }
     }
 
+    @NotNull
     @Override
-    public IntRange parse(String value) {
+    public IntRange parse(@NotNull String value) {
         if (value.contains("..")) {
             final int index = value.indexOf('.');
             final String[] split = value.split(Pattern.quote(".."));
diff --git a/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentItemStack.java b/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentItemStack.java
index 799b368c7..023db8573 100644
--- a/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentItemStack.java
+++ b/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentItemStack.java
@@ -5,6 +5,7 @@ import net.minestom.server.item.ItemStack;
 import net.minestom.server.item.Material;
 import net.minestom.server.registry.Registries;
 import net.minestom.server.utils.NBTUtils;
+import org.jetbrains.annotations.NotNull;
 import org.jglrxavpok.hephaistos.nbt.NBT;
 import org.jglrxavpok.hephaistos.nbt.NBTCompound;
 import org.jglrxavpok.hephaistos.nbt.NBTException;
@@ -29,7 +30,7 @@ public class ArgumentItemStack extends Argument<ItemStack> {
     }
 
     @Override
-    public int getCorrectionResult(String value) {
+    public int getCorrectionResult(@NotNull String value) {
         if (value.startsWith("{")) {
             return NO_MATERIAL;
         }
@@ -51,8 +52,9 @@ public class ArgumentItemStack extends Argument<ItemStack> {
         }
     }
 
+    @NotNull
     @Override
-    public ItemStack parse(String value) {
+    public ItemStack parse(@NotNull String value) {
         final int nbtIndex = value.indexOf("{");
 
         if (nbtIndex == -1) {
@@ -83,7 +85,7 @@ public class ArgumentItemStack extends Argument<ItemStack> {
     }
 
     @Override
-    public int getConditionResult(ItemStack value) {
+    public int getConditionResult(@NotNull ItemStack value) {
         return SUCCESS;
     }
 }
diff --git a/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentNbtCompoundTag.java b/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentNbtCompoundTag.java
index d0dd2d6fe..67d5f6122 100644
--- a/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentNbtCompoundTag.java
+++ b/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentNbtCompoundTag.java
@@ -1,6 +1,7 @@
 package net.minestom.server.command.builder.arguments.minecraft;
 
 import net.minestom.server.command.builder.arguments.Argument;
+import org.jetbrains.annotations.NotNull;
 import org.jglrxavpok.hephaistos.nbt.NBT;
 import org.jglrxavpok.hephaistos.nbt.NBTCompound;
 import org.jglrxavpok.hephaistos.nbt.NBTException;
@@ -22,7 +23,7 @@ public class ArgumentNbtCompoundTag extends Argument<NBTCompound> {
     }
 
     @Override
-    public int getCorrectionResult(String value) {
+    public int getCorrectionResult(@NotNull String value) {
         try {
             NBT nbt = new SNBTParser(new StringReader(value)).parse();
             return nbt instanceof NBTCompound ? SUCCESS : INVALID_NBT;
@@ -31,8 +32,9 @@ public class ArgumentNbtCompoundTag extends Argument<NBTCompound> {
         }
     }
 
+    @NotNull
     @Override
-    public NBTCompound parse(String value) {
+    public NBTCompound parse(@NotNull String value) {
         try {
             NBT nbt = new SNBTParser(new StringReader(value)).parse();
             return (NBTCompound) nbt;
@@ -42,7 +44,7 @@ public class ArgumentNbtCompoundTag extends Argument<NBTCompound> {
     }
 
     @Override
-    public int getConditionResult(NBTCompound value) {
+    public int getConditionResult(@NotNull NBTCompound value) {
         return SUCCESS;
     }
 }
diff --git a/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentNbtTag.java b/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentNbtTag.java
index 981be0efe..c715b4dde 100644
--- a/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentNbtTag.java
+++ b/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentNbtTag.java
@@ -1,6 +1,7 @@
 package net.minestom.server.command.builder.arguments.minecraft;
 
 import net.minestom.server.command.builder.arguments.Argument;
+import org.jetbrains.annotations.NotNull;
 import org.jglrxavpok.hephaistos.nbt.NBT;
 import org.jglrxavpok.hephaistos.nbt.NBTException;
 import org.jglrxavpok.hephaistos.nbt.SNBTParser;
@@ -23,7 +24,7 @@ public class ArgumentNbtTag extends Argument<NBT> {
     }
 
     @Override
-    public int getCorrectionResult(String value) {
+    public int getCorrectionResult(@NotNull String value) {
         try {
             NBT nbt = new SNBTParser(new StringReader(value)).parse();
             return nbt != null ? SUCCESS : INVALID_NBT;
@@ -32,8 +33,9 @@ public class ArgumentNbtTag extends Argument<NBT> {
         }
     }
 
+    @NotNull
     @Override
-    public NBT parse(String value) {
+    public NBT parse(@NotNull String value) {
         try {
             NBT nbt = new SNBTParser(new StringReader(value)).parse();
             return nbt;
@@ -43,7 +45,7 @@ public class ArgumentNbtTag extends Argument<NBT> {
     }
 
     @Override
-    public int getConditionResult(NBT value) {
+    public int getConditionResult(@NotNull NBT value) {
         return SUCCESS;
     }
 }
diff --git a/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentRange.java b/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentRange.java
index 968b2f021..394d0c02d 100644
--- a/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentRange.java
+++ b/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentRange.java
@@ -1,6 +1,7 @@
 package net.minestom.server.command.builder.arguments.minecraft;
 
 import net.minestom.server.command.builder.arguments.Argument;
+import org.jetbrains.annotations.NotNull;
 
 /**
  * Abstract class used by {@link ArgumentIntRange} and {@link ArgumentFloatRange}.
@@ -16,7 +17,7 @@ public abstract class ArgumentRange<T> extends Argument<T> {
     }
 
     @Override
-    public int getConditionResult(T value) {
+    public int getConditionResult(@NotNull T value) {
         return SUCCESS;
     }
 }
diff --git a/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentTime.java b/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentTime.java
index 6a40188ff..e64bd36e6 100644
--- a/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentTime.java
+++ b/src/main/java/net/minestom/server/command/builder/arguments/minecraft/ArgumentTime.java
@@ -5,6 +5,7 @@ import it.unimi.dsi.fastutil.chars.CharList;
 import net.minestom.server.command.builder.arguments.Argument;
 import net.minestom.server.utils.time.TimeUnit;
 import net.minestom.server.utils.time.UpdateOption;
+import org.jetbrains.annotations.NotNull;
 
 /**
  * Represents an argument giving a time (day/second/tick).
@@ -23,7 +24,7 @@ public class ArgumentTime extends Argument<UpdateOption> {
     }
 
     @Override
-    public int getCorrectionResult(String value) {
+    public int getCorrectionResult(@NotNull String value) {
         final char lastChar = value.charAt(value.length() - 1);
         if (!SUFFIXES.contains(lastChar))
             return INVALID_TIME_FORMAT;
@@ -39,8 +40,9 @@ public class ArgumentTime extends Argument<UpdateOption> {
         }
     }
 
+    @NotNull
     @Override
-    public UpdateOption parse(String value) {
+    public UpdateOption parse(@NotNull String value) {
         final char lastChar = value.charAt(value.length() - 1);
         TimeUnit timeUnit = null;
         if (lastChar == 'd') {
@@ -57,7 +59,7 @@ public class ArgumentTime extends Argument<UpdateOption> {
     }
 
     @Override
-    public int getConditionResult(UpdateOption value) {
+    public int getConditionResult(@NotNull UpdateOption value) {
         return SUCCESS;
     }
 }
diff --git a/src/main/java/net/minestom/server/command/builder/arguments/minecraft/registry/ArgumentRegistry.java b/src/main/java/net/minestom/server/command/builder/arguments/minecraft/registry/ArgumentRegistry.java
index 24f87faba..02e3ef6ca 100644
--- a/src/main/java/net/minestom/server/command/builder/arguments/minecraft/registry/ArgumentRegistry.java
+++ b/src/main/java/net/minestom/server/command/builder/arguments/minecraft/registry/ArgumentRegistry.java
@@ -1,6 +1,7 @@
 package net.minestom.server.command.builder.arguments.minecraft.registry;
 
 import net.minestom.server.command.builder.arguments.Argument;
+import org.jetbrains.annotations.NotNull;
 
 public abstract class ArgumentRegistry<T> extends Argument<T> {
 
@@ -13,17 +14,18 @@ public abstract class ArgumentRegistry<T> extends Argument<T> {
     public abstract T getRegistry(String value);
 
     @Override
-    public int getCorrectionResult(String value) {
+    public int getCorrectionResult(@NotNull String value) {
         return getRegistry(value) == null ? INVALID_NAME : SUCCESS;
     }
 
+    @NotNull
     @Override
-    public T parse(String value) {
+    public T parse(@NotNull String value) {
         return getRegistry(value);
     }
 
     @Override
-    public int getConditionResult(T value) {
+    public int getConditionResult(@NotNull T value) {
         return SUCCESS;
     }
 }
diff --git a/src/main/java/net/minestom/server/command/builder/arguments/number/ArgumentDouble.java b/src/main/java/net/minestom/server/command/builder/arguments/number/ArgumentDouble.java
index aca7e987d..f60b35ee6 100644
--- a/src/main/java/net/minestom/server/command/builder/arguments/number/ArgumentDouble.java
+++ b/src/main/java/net/minestom/server/command/builder/arguments/number/ArgumentDouble.java
@@ -1,5 +1,7 @@
 package net.minestom.server.command.builder.arguments.number;
 
+import org.jetbrains.annotations.NotNull;
+
 public class ArgumentDouble extends ArgumentNumber<Double> {
 
     public ArgumentDouble(String id) {
@@ -9,7 +11,7 @@ public class ArgumentDouble extends ArgumentNumber<Double> {
     }
 
     @Override
-    public int getCorrectionResult(String value) {
+    public int getCorrectionResult(@NotNull String value) {
         try {
             String parsed = parseValue(value);
             int radix = getRadix(value);
@@ -24,8 +26,9 @@ public class ArgumentDouble extends ArgumentNumber<Double> {
         }
     }
 
+    @NotNull
     @Override
-    public Double parse(String value) {
+    public Double parse(@NotNull String value) {
         String parsed = parseValue(value);
         int radix = getRadix(value);
         if (radix != 10) {
@@ -35,7 +38,7 @@ public class ArgumentDouble extends ArgumentNumber<Double> {
     }
 
     @Override
-    public int getConditionResult(Double value) {
+    public int getConditionResult(@NotNull Double value) {
         // Check range
         if (hasMin && value < min) {
             return RANGE_ERROR;
diff --git a/src/main/java/net/minestom/server/command/builder/arguments/number/ArgumentFloat.java b/src/main/java/net/minestom/server/command/builder/arguments/number/ArgumentFloat.java
index 1e7a16f0e..c0c1a66a8 100644
--- a/src/main/java/net/minestom/server/command/builder/arguments/number/ArgumentFloat.java
+++ b/src/main/java/net/minestom/server/command/builder/arguments/number/ArgumentFloat.java
@@ -1,5 +1,7 @@
 package net.minestom.server.command.builder.arguments.number;
 
+import org.jetbrains.annotations.NotNull;
+
 public class ArgumentFloat extends ArgumentNumber<Float> {
 
     public ArgumentFloat(String id) {
@@ -9,7 +11,7 @@ public class ArgumentFloat extends ArgumentNumber<Float> {
     }
 
     @Override
-    public int getCorrectionResult(String value) {
+    public int getCorrectionResult(@NotNull String value) {
         try {
             String parsed = parseValue(value);
             int radix = getRadix(value);
@@ -24,8 +26,9 @@ public class ArgumentFloat extends ArgumentNumber<Float> {
         }
     }
 
+    @NotNull
     @Override
-    public Float parse(String value) {
+    public Float parse(@NotNull String value) {
         String parsed = parseValue(value);
         int radix = getRadix(value);
         if (radix != 10) {
@@ -35,7 +38,7 @@ public class ArgumentFloat extends ArgumentNumber<Float> {
     }
 
     @Override
-    public int getConditionResult(Float value) {
+    public int getConditionResult(@NotNull Float value) {
         // Check range
         if (hasMin && value < min) {
             return RANGE_ERROR;
diff --git a/src/main/java/net/minestom/server/command/builder/arguments/number/ArgumentInteger.java b/src/main/java/net/minestom/server/command/builder/arguments/number/ArgumentInteger.java
index b2e4d5e1d..2dff21b04 100644
--- a/src/main/java/net/minestom/server/command/builder/arguments/number/ArgumentInteger.java
+++ b/src/main/java/net/minestom/server/command/builder/arguments/number/ArgumentInteger.java
@@ -1,5 +1,7 @@
 package net.minestom.server.command.builder.arguments.number;
 
+import org.jetbrains.annotations.NotNull;
+
 public class ArgumentInteger extends ArgumentNumber<Integer> {
 
     public ArgumentInteger(String id) {
@@ -9,7 +11,7 @@ public class ArgumentInteger extends ArgumentNumber<Integer> {
     }
 
     @Override
-    public int getCorrectionResult(String value) {
+    public int getCorrectionResult(@NotNull String value) {
         try {
             Integer.parseInt(parseValue(value), getRadix(value));
             return SUCCESS;
@@ -18,13 +20,14 @@ public class ArgumentInteger extends ArgumentNumber<Integer> {
         }
     }
 
+    @NotNull
     @Override
-    public Integer parse(String value) {
+    public Integer parse(@NotNull String value) {
         return Integer.parseInt(parseValue(value), getRadix(value));
     }
 
     @Override
-    public int getConditionResult(Integer value) {
+    public int getConditionResult(@NotNull Integer value) {
         // Check range
         if (hasMin && value < min) {
             return RANGE_ERROR;
diff --git a/src/main/java/net/minestom/server/command/builder/arguments/number/ArgumentLong.java b/src/main/java/net/minestom/server/command/builder/arguments/number/ArgumentLong.java
index e955d0e36..b925443f6 100644
--- a/src/main/java/net/minestom/server/command/builder/arguments/number/ArgumentLong.java
+++ b/src/main/java/net/minestom/server/command/builder/arguments/number/ArgumentLong.java
@@ -1,5 +1,7 @@
 package net.minestom.server.command.builder.arguments.number;
 
+import org.jetbrains.annotations.NotNull;
+
 public class ArgumentLong extends ArgumentNumber<Long> {
 
     public ArgumentLong(String id) {
@@ -9,7 +11,7 @@ public class ArgumentLong extends ArgumentNumber<Long> {
     }
 
     @Override
-    public int getCorrectionResult(String value) {
+    public int getCorrectionResult(@NotNull String value) {
         try {
             Long.parseLong(parseValue(value), getRadix(value));
             return SUCCESS;
@@ -18,13 +20,14 @@ public class ArgumentLong extends ArgumentNumber<Long> {
         }
     }
 
+    @NotNull
     @Override
-    public Long parse(String value) {
+    public Long parse(@NotNull String value) {
         return Long.parseLong(parseValue(value), getRadix(value));
     }
 
     @Override
-    public int getConditionResult(Long value) {
+    public int getConditionResult(@NotNull Long value) {
         // Check range
         if (hasMin && value < min) {
             return RANGE_ERROR;
diff --git a/src/main/java/net/minestom/server/command/builder/arguments/number/ArgumentNumber.java b/src/main/java/net/minestom/server/command/builder/arguments/number/ArgumentNumber.java
index 62e252285..a51b94864 100644
--- a/src/main/java/net/minestom/server/command/builder/arguments/number/ArgumentNumber.java
+++ b/src/main/java/net/minestom/server/command/builder/arguments/number/ArgumentNumber.java
@@ -1,6 +1,8 @@
 package net.minestom.server.command.builder.arguments.number;
 
 import net.minestom.server.command.builder.arguments.Argument;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 import java.math.BigDecimal;
 import java.util.regex.Pattern;
@@ -17,20 +19,23 @@ public abstract class ArgumentNumber<T extends Number> extends Argument<T> {
         super(id, false);
     }
 
-    public ArgumentNumber<T> min(T value) {
+    @NotNull
+    public ArgumentNumber<T> min(@NotNull T value) {
         this.min = value;
         this.hasMin = true;
         return this;
     }
 
-    public ArgumentNumber<T> max(T value) {
+    @NotNull
+    public ArgumentNumber<T> max(@NotNull T value) {
         this.max = value;
         this.hasMax = true;
 
         return this;
     }
 
-    public ArgumentNumber<T> between(T min, T max) {
+    @NotNull
+    public ArgumentNumber<T> between(@NotNull T min, @NotNull T max) {
         this.min = min;
         this.max = max;
         this.hasMin = true;
@@ -52,6 +57,7 @@ public abstract class ArgumentNumber<T extends Number> extends Argument<T> {
      *
      * @return the minimum of this argument
      */
+    @NotNull
     public T getMin() {
         return min;
     }
@@ -70,11 +76,12 @@ public abstract class ArgumentNumber<T extends Number> extends Argument<T> {
      *
      * @return the maximum of this argument
      */
+    @NotNull
     public T getMax() {
         return max;
     }
 
-    protected String parseValue(String value) {
+    protected String parseValue(@NotNull String value) {
         if (value.startsWith("0b")) {
             value = value.replaceFirst(Pattern.quote("0b"), "");
         } else if (value.startsWith("0x")) {
@@ -86,7 +93,7 @@ public abstract class ArgumentNumber<T extends Number> extends Argument<T> {
         return value;
     }
 
-    protected int getRadix(String value) {
+    protected int getRadix(@NotNull String value) {
         if (value.startsWith("0b")) {
             return 2;
         } else if (value.startsWith("0x")) {
@@ -95,7 +102,8 @@ public abstract class ArgumentNumber<T extends Number> extends Argument<T> {
         return 10;
     }
 
-    protected String removeScientificNotation(String value) {
+    @Nullable
+    protected String removeScientificNotation(@NotNull String value) {
         try {
             return new BigDecimal(value).toPlainString();
         } catch (NumberFormatException e) {
diff --git a/src/main/java/net/minestom/server/command/builder/condition/CommandCondition.java b/src/main/java/net/minestom/server/command/builder/condition/CommandCondition.java
index 288d04454..243872ba9 100644
--- a/src/main/java/net/minestom/server/command/builder/condition/CommandCondition.java
+++ b/src/main/java/net/minestom/server/command/builder/condition/CommandCondition.java
@@ -1,10 +1,11 @@
 package net.minestom.server.command.builder.condition;
 
 import net.minestom.server.command.CommandSender;
+import org.jetbrains.annotations.NotNull;
 
 /**
- * Used to know if the {@link CommandSender} is allowed to run the command
+ * Used to know if the {@link CommandSender} is allowed to run the command.
  */
 public interface CommandCondition {
-    boolean apply(CommandSender source);
+    boolean apply(@NotNull CommandSender source);
 }
diff --git a/src/main/java/net/minestom/server/data/Data.java b/src/main/java/net/minestom/server/data/Data.java
index 9d35c56c6..aa830ea5f 100644
--- a/src/main/java/net/minestom/server/data/Data.java
+++ b/src/main/java/net/minestom/server/data/Data.java
@@ -39,6 +39,7 @@ public interface Data {
             return true;
         }
 
+        @NotNull
         @Override
         public Data clone() {
             return this;
@@ -79,7 +80,7 @@ public interface Data {
      * @return {@link #get(String)} if found, {@code defaultValue} otherwise
      */
     @Nullable
-    <T> T getOrDefault(@NotNull String key, T defaultValue);
+    <T> T getOrDefault(@NotNull String key, @Nullable T defaultValue);
 
     /**
      * Gets if the data has a key.
@@ -109,6 +110,7 @@ public interface Data {
      *
      * @return a cloned data object
      */
+    @NotNull
     Data clone();
 
 }
diff --git a/src/main/java/net/minestom/server/data/DataImpl.java b/src/main/java/net/minestom/server/data/DataImpl.java
index bac02e354..5c44b9eb1 100644
--- a/src/main/java/net/minestom/server/data/DataImpl.java
+++ b/src/main/java/net/minestom/server/data/DataImpl.java
@@ -44,6 +44,7 @@ public class DataImpl implements Data {
         return data.isEmpty();
     }
 
+    @NotNull
     @Override
     public Data clone() {
         DataImpl data = new DataImpl();
diff --git a/src/main/java/net/minestom/server/data/DataManager.java b/src/main/java/net/minestom/server/data/DataManager.java
index 517a44109..44758988d 100644
--- a/src/main/java/net/minestom/server/data/DataManager.java
+++ b/src/main/java/net/minestom/server/data/DataManager.java
@@ -7,6 +7,8 @@ import net.minestom.server.inventory.Inventory;
 import net.minestom.server.item.ItemStack;
 import net.minestom.server.utils.PrimitiveConversion;
 import net.minestom.server.utils.validate.Check;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 import java.util.HashMap;
 import java.util.Map;
@@ -62,7 +64,7 @@ public final class DataManager {
      * @param <T>      the data type
      * @throws IllegalStateException if the type {@code clazz} is already registered
      */
-    public <T> void registerType(Class<T> clazz, DataType<T> dataType) {
+    public <T> void registerType(@NotNull Class<T> clazz, @NotNull DataType<T> dataType) {
         clazz = PrimitiveConversion.getObjectClass(clazz);
         Check.stateCondition(dataTypeMap.containsKey(clazz),
                 "Type " + clazz.getName() + " has already been registered");
@@ -78,7 +80,8 @@ public final class DataManager {
      * @return the {@link DataType} associated to the class
      * @throws NullPointerException if none is found
      */
-    public <T> DataType<T> getDataType(Class<T> clazz) {
+    @Nullable
+    public <T> DataType<T> getDataType(@NotNull Class<T> clazz) {
         clazz = PrimitiveConversion.getObjectClass(clazz);
         return dataTypeMap.get(clazz);
     }
diff --git a/src/main/java/net/minestom/server/data/DataType.java b/src/main/java/net/minestom/server/data/DataType.java
index 6b15f52ec..3cee713dc 100644
--- a/src/main/java/net/minestom/server/data/DataType.java
+++ b/src/main/java/net/minestom/server/data/DataType.java
@@ -2,6 +2,7 @@ package net.minestom.server.data;
 
 import net.minestom.server.utils.binary.BinaryReader;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 /**
  * Represents an object which can be encoded and decoded back.
@@ -14,19 +15,20 @@ import net.minestom.server.utils.binary.BinaryWriter;
 public abstract class DataType<T> {
 
     /**
-     * Encode the data type
+     * Encodes the data type.
      *
      * @param writer the data writer
      * @param value  the value to encode
      */
-    public abstract void encode(BinaryWriter writer, T value);
+    public abstract void encode(@NotNull BinaryWriter writer, @NotNull T value);
 
     /**
-     * Decode the data type
+     * Decodes the data type.
      *
      * @param reader the data reader
      * @return the decoded value
      */
-    public abstract T decode(BinaryReader reader);
+    @NotNull
+    public abstract T decode(@NotNull BinaryReader reader);
 
 }
\ No newline at end of file
diff --git a/src/main/java/net/minestom/server/data/SerializableData.java b/src/main/java/net/minestom/server/data/SerializableData.java
index 778e5a87a..274e2d3ea 100644
--- a/src/main/java/net/minestom/server/data/SerializableData.java
+++ b/src/main/java/net/minestom/server/data/SerializableData.java
@@ -5,6 +5,7 @@ import it.unimi.dsi.fastutil.objects.Object2ShortOpenHashMap;
 import net.minestom.server.MinecraftServer;
 import net.minestom.server.utils.binary.BinaryReader;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 /**
  * Represents a {@link Data} object which can be serialized and read back.
@@ -16,63 +17,66 @@ public interface SerializableData extends Data {
     DataManager DATA_MANAGER = MinecraftServer.getDataManager();
 
     /**
-     * Serialize the data into an array of bytes
+     * Serializes the data into an array of bytes.
      * <p>
      * Use {@link #readIndexedSerializedData(BinaryReader)} if {@code indexed} is true,
      * {@link #readSerializedData(BinaryReader, Object2ShortMap)} otherwise with the index map
-     * to convert it back to a {@link SerializableData}
+     * to convert it back to a {@link SerializableData}.
      *
      * @param typeToIndexMap the type to index map, will create entries if new types are discovered.
      *                       The map is not thread-safe
      * @param indexed        true to add the types index in the header
      * @return the array representation of this data object
      */
-    byte[] getSerializedData(Object2ShortMap<String> typeToIndexMap, boolean indexed);
+    @NotNull
+    byte[] getSerializedData(@NotNull Object2ShortMap<String> typeToIndexMap, boolean indexed);
 
     /**
-     * Read the data of a {@link SerializableData} when you already have the index map
+     * Reads the data of a {@link SerializableData} when you already have the index map.
      * <p>
-     * WARNING: the data to read should not have any index to read and your index map should be COMPLETE
-     * Use {@link #readIndexedSerializedData(BinaryReader)} if you need to read the header
+     * WARNING: the data to read should not have any index to read and your index map should be COMPLETE.
+     * Use {@link #readIndexedSerializedData(BinaryReader)} if you need to read the header.
      *
      * @param reader         the binary reader
      * @param typeToIndexMap the index map
      */
-    void readSerializedData(BinaryReader reader, Object2ShortMap<String> typeToIndexMap);
+    void readSerializedData(@NotNull BinaryReader reader, @NotNull Object2ShortMap<String> typeToIndexMap);
 
     /**
-     * Serialize the data into an array of bytes
+     * Serializes the data into an array of bytes.
      * <p>
      * Use {@link #readIndexedSerializedData(BinaryReader)}
-     * to convert it back to a {@link SerializableData}
+     * to convert it back to a {@link SerializableData}.
      * <p>
-     * This will create a type index map which will be present in the header
+     * This will create a type index map which will be present in the header.
      *
      * @return the array representation of this data object
      */
+    @NotNull
     default byte[] getIndexedSerializedData() {
         return getSerializedData(new Object2ShortOpenHashMap<>(), true);
     }
 
     /**
-     * Read the index map and the data of a serialized {@link SerializableData}
-     * Got from {@link #getIndexedSerializedData()}
+     * Reads the index map and the data of a serialized {@link SerializableData}.
+     *
+     * Got from {@link #getIndexedSerializedData()}.
      *
      * @param reader the binary reader
      */
-    default void readIndexedSerializedData(BinaryReader reader) {
+    default void readIndexedSerializedData(@NotNull BinaryReader reader) {
         final Object2ShortMap<String> typeToIndexMap = SerializableData.readDataIndexes(reader);
         readSerializedData(reader, typeToIndexMap);
     }
 
     /**
-     * Write the index info (class name -&gt; class index), used to write the header for indexed serialized data
+     * Writes the index info (class name -&gt; class index), used to write the header for indexed serialized data.
      * <p>
-     * Sized by a var-int
+     * Sized by a var-int.
      *
      * @param typeToIndexMap the data index map
      */
-    static void writeDataIndexHeader(BinaryWriter indexWriter, Object2ShortMap<String> typeToIndexMap) {
+    static void writeDataIndexHeader(@NotNull BinaryWriter indexWriter, @NotNull Object2ShortMap<String> typeToIndexMap) {
         // Write the size of the following index list (class name-> class index)
         indexWriter.writeVarInt(typeToIndexMap.size());
 
@@ -88,14 +92,15 @@ public interface SerializableData extends Data {
     }
 
     /**
-     * Read a data index map (type name -&gt; type index)
+     * Reads a data index map (type name -&gt; type index).
      * <p>
-     * Can then be used with {@link SerializableData#readSerializedData(BinaryReader, Object2ShortMap)}
+     * Can then be used with {@link SerializableData#readSerializedData(BinaryReader, Object2ShortMap)}.
      *
      * @param binaryReader the reader
      * @return a map containing the indexes of your data
      */
-    static Object2ShortMap<String> readDataIndexes(BinaryReader binaryReader) {
+    @NotNull
+    static Object2ShortMap<String> readDataIndexes(@NotNull BinaryReader binaryReader) {
         Object2ShortMap<String> typeToIndexMap = new Object2ShortOpenHashMap<>();
         {
             final int dataIndexSize = binaryReader.readVarInt();
diff --git a/src/main/java/net/minestom/server/data/SerializableDataImpl.java b/src/main/java/net/minestom/server/data/SerializableDataImpl.java
index 8c5a82882..af5d4dda5 100644
--- a/src/main/java/net/minestom/server/data/SerializableDataImpl.java
+++ b/src/main/java/net/minestom/server/data/SerializableDataImpl.java
@@ -49,6 +49,7 @@ public class SerializableDataImpl extends DataImpl implements SerializableData {
         this.dataType.put(key, type);
     }
 
+    @NotNull
     @Override
     public Data clone() {
         SerializableDataImpl data = new SerializableDataImpl();
@@ -57,8 +58,9 @@ public class SerializableDataImpl extends DataImpl implements SerializableData {
         return data;
     }
 
+    @NotNull
     @Override
-    public byte[] getSerializedData(Object2ShortMap<String> typeToIndexMap, boolean indexed) {
+    public byte[] getSerializedData(@NotNull Object2ShortMap<String> typeToIndexMap, boolean indexed) {
         // Get the current max index, it supposes that the index keep being incremented by 1
         short lastIndex = (short) typeToIndexMap.size();
 
@@ -113,7 +115,7 @@ public class SerializableDataImpl extends DataImpl implements SerializableData {
     }
 
     @Override
-    public void readSerializedData(BinaryReader reader, Object2ShortMap<String> typeToIndexMap) {
+    public void readSerializedData(@NotNull BinaryReader reader, @NotNull Object2ShortMap<String> typeToIndexMap) {
         // Map used to convert an index to the class name (opposite of typeToIndexMap)
         final Short2ObjectMap<String> indexToTypeMap = new Short2ObjectOpenHashMap<>(typeToIndexMap.size());
         {
diff --git a/src/main/java/net/minestom/server/data/type/BooleanData.java b/src/main/java/net/minestom/server/data/type/BooleanData.java
index 440c7456d..38e0d7491 100644
--- a/src/main/java/net/minestom/server/data/type/BooleanData.java
+++ b/src/main/java/net/minestom/server/data/type/BooleanData.java
@@ -3,15 +3,17 @@ package net.minestom.server.data.type;
 import net.minestom.server.data.DataType;
 import net.minestom.server.utils.binary.BinaryReader;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 public class BooleanData extends DataType<Boolean> {
     @Override
-    public void encode(BinaryWriter writer, Boolean value) {
+    public void encode(@NotNull BinaryWriter writer, @NotNull Boolean value) {
         writer.writeBoolean(value);
     }
 
+    @NotNull
     @Override
-    public Boolean decode(BinaryReader reader) {
+    public Boolean decode(@NotNull BinaryReader reader) {
         return reader.readBoolean();
     }
 }
diff --git a/src/main/java/net/minestom/server/data/type/ByteData.java b/src/main/java/net/minestom/server/data/type/ByteData.java
index 27a176663..0fd3c7693 100644
--- a/src/main/java/net/minestom/server/data/type/ByteData.java
+++ b/src/main/java/net/minestom/server/data/type/ByteData.java
@@ -3,15 +3,17 @@ package net.minestom.server.data.type;
 import net.minestom.server.data.DataType;
 import net.minestom.server.utils.binary.BinaryReader;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 public class ByteData extends DataType<Byte> {
     @Override
-    public void encode(BinaryWriter writer, Byte value) {
+    public void encode(@NotNull BinaryWriter writer, @NotNull Byte value) {
         writer.writeByte(value);
     }
 
+    @NotNull
     @Override
-    public Byte decode(BinaryReader reader) {
+    public Byte decode(@NotNull BinaryReader reader) {
         return reader.readByte();
     }
 }
diff --git a/src/main/java/net/minestom/server/data/type/CharacterData.java b/src/main/java/net/minestom/server/data/type/CharacterData.java
index cbeee8d0f..48d54ac7c 100644
--- a/src/main/java/net/minestom/server/data/type/CharacterData.java
+++ b/src/main/java/net/minestom/server/data/type/CharacterData.java
@@ -3,16 +3,18 @@ package net.minestom.server.data.type;
 import net.minestom.server.data.DataType;
 import net.minestom.server.utils.binary.BinaryReader;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 public class CharacterData extends DataType<Character> {
 
     @Override
-    public void encode(BinaryWriter writer, Character value) {
+    public void encode(@NotNull BinaryWriter writer, @NotNull Character value) {
         writer.writeChar(value);
     }
 
+    @NotNull
     @Override
-    public Character decode(BinaryReader reader) {
+    public Character decode(@NotNull BinaryReader reader) {
         return reader.readChar();
     }
 }
diff --git a/src/main/java/net/minestom/server/data/type/DoubleData.java b/src/main/java/net/minestom/server/data/type/DoubleData.java
index 7c64b860c..f5b897589 100644
--- a/src/main/java/net/minestom/server/data/type/DoubleData.java
+++ b/src/main/java/net/minestom/server/data/type/DoubleData.java
@@ -3,16 +3,18 @@ package net.minestom.server.data.type;
 import net.minestom.server.data.DataType;
 import net.minestom.server.utils.binary.BinaryReader;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 public class DoubleData extends DataType<Double> {
 
     @Override
-    public void encode(BinaryWriter writer, Double value) {
+    public void encode(@NotNull BinaryWriter writer, @NotNull Double value) {
         writer.writeDouble(value);
     }
 
+    @NotNull
     @Override
-    public Double decode(BinaryReader reader) {
+    public Double decode(@NotNull BinaryReader reader) {
         return reader.readDouble();
     }
 }
diff --git a/src/main/java/net/minestom/server/data/type/FloatData.java b/src/main/java/net/minestom/server/data/type/FloatData.java
index 97cb85ce4..69d392ddc 100644
--- a/src/main/java/net/minestom/server/data/type/FloatData.java
+++ b/src/main/java/net/minestom/server/data/type/FloatData.java
@@ -3,16 +3,18 @@ package net.minestom.server.data.type;
 import net.minestom.server.data.DataType;
 import net.minestom.server.utils.binary.BinaryReader;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 public class FloatData extends DataType<Float> {
 
     @Override
-    public void encode(BinaryWriter writer, Float value) {
+    public void encode(@NotNull BinaryWriter writer, @NotNull Float value) {
         writer.writeFloat(value);
     }
 
+    @NotNull
     @Override
-    public Float decode(BinaryReader reader) {
+    public Float decode(@NotNull BinaryReader reader) {
         return reader.readFloat();
     }
 }
diff --git a/src/main/java/net/minestom/server/data/type/IntegerData.java b/src/main/java/net/minestom/server/data/type/IntegerData.java
index 7465f82a0..c0902b59c 100644
--- a/src/main/java/net/minestom/server/data/type/IntegerData.java
+++ b/src/main/java/net/minestom/server/data/type/IntegerData.java
@@ -3,16 +3,18 @@ package net.minestom.server.data.type;
 import net.minestom.server.data.DataType;
 import net.minestom.server.utils.binary.BinaryReader;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 public class IntegerData extends DataType<Integer> {
 
     @Override
-    public void encode(BinaryWriter writer, Integer value) {
+    public void encode(@NotNull BinaryWriter writer, @NotNull Integer value) {
         writer.writeVarInt(value);
     }
 
+    @NotNull
     @Override
-    public Integer decode(BinaryReader reader) {
+    public Integer decode(@NotNull BinaryReader reader) {
         return reader.readVarInt();
     }
 }
\ No newline at end of file
diff --git a/src/main/java/net/minestom/server/data/type/InventoryData.java b/src/main/java/net/minestom/server/data/type/InventoryData.java
index 293098b71..78e05ebfb 100644
--- a/src/main/java/net/minestom/server/data/type/InventoryData.java
+++ b/src/main/java/net/minestom/server/data/type/InventoryData.java
@@ -5,11 +5,12 @@ import net.minestom.server.inventory.Inventory;
 import net.minestom.server.inventory.InventoryType;
 import net.minestom.server.utils.binary.BinaryReader;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 public class InventoryData extends DataType<Inventory> {
 
     @Override
-    public void encode(BinaryWriter writer, Inventory value) {
+    public void encode(@NotNull BinaryWriter writer, @NotNull Inventory value) {
         final InventoryType inventoryType = value.getInventoryType();
         final int size = inventoryType.getAdditionalSlot();
 
@@ -23,8 +24,9 @@ public class InventoryData extends DataType<Inventory> {
         }
     }
 
+    @NotNull
     @Override
-    public Inventory decode(BinaryReader reader) {
+    public Inventory decode(@NotNull BinaryReader reader) {
         final String title = reader.readSizedString();
         final InventoryType inventoryType = InventoryType.valueOf(reader.readSizedString());
         final int size = inventoryType.getAdditionalSlot();
diff --git a/src/main/java/net/minestom/server/data/type/ItemStackData.java b/src/main/java/net/minestom/server/data/type/ItemStackData.java
index 8c28e5172..810be1e98 100644
--- a/src/main/java/net/minestom/server/data/type/ItemStackData.java
+++ b/src/main/java/net/minestom/server/data/type/ItemStackData.java
@@ -4,15 +4,17 @@ import net.minestom.server.data.DataType;
 import net.minestom.server.item.ItemStack;
 import net.minestom.server.utils.binary.BinaryReader;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 public class ItemStackData extends DataType<ItemStack> {
     @Override
-    public void encode(BinaryWriter writer, ItemStack value) {
+    public void encode(@NotNull BinaryWriter writer, @NotNull ItemStack value) {
         writer.writeItemStack(value);
     }
 
+    @NotNull
     @Override
-    public ItemStack decode(BinaryReader reader) {
+    public ItemStack decode(@NotNull BinaryReader reader) {
         return reader.readSlot();
     }
 }
diff --git a/src/main/java/net/minestom/server/data/type/LongData.java b/src/main/java/net/minestom/server/data/type/LongData.java
index 83dc0a459..ebd97be26 100644
--- a/src/main/java/net/minestom/server/data/type/LongData.java
+++ b/src/main/java/net/minestom/server/data/type/LongData.java
@@ -3,15 +3,17 @@ package net.minestom.server.data.type;
 import net.minestom.server.data.DataType;
 import net.minestom.server.utils.binary.BinaryReader;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 public class LongData extends DataType<Long> {
     @Override
-    public void encode(BinaryWriter writer, Long value) {
+    public void encode(@NotNull BinaryWriter writer, @NotNull Long value) {
         writer.writeLong(value);
     }
 
+    @NotNull
     @Override
-    public Long decode(BinaryReader reader) {
+    public Long decode(@NotNull BinaryReader reader) {
         return reader.readLong();
     }
 }
diff --git a/src/main/java/net/minestom/server/data/type/SerializableDataData.java b/src/main/java/net/minestom/server/data/type/SerializableDataData.java
index 3507e872e..85ceaefd3 100644
--- a/src/main/java/net/minestom/server/data/type/SerializableDataData.java
+++ b/src/main/java/net/minestom/server/data/type/SerializableDataData.java
@@ -5,17 +5,19 @@ import net.minestom.server.data.SerializableData;
 import net.minestom.server.data.SerializableDataImpl;
 import net.minestom.server.utils.binary.BinaryReader;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 // Pretty weird name huh?
 public class SerializableDataData extends DataType<SerializableData> {
 
     @Override
-    public void encode(BinaryWriter writer, SerializableData value) {
+    public void encode(@NotNull BinaryWriter writer, @NotNull SerializableData value) {
         writer.writeBytes(value.getIndexedSerializedData());
     }
 
+    @NotNull
     @Override
-    public SerializableData decode(BinaryReader reader) {
+    public SerializableData decode(@NotNull BinaryReader reader) {
         SerializableData serializableData = new SerializableDataImpl();
         serializableData.readIndexedSerializedData(reader);
         return serializableData;
diff --git a/src/main/java/net/minestom/server/data/type/ShortData.java b/src/main/java/net/minestom/server/data/type/ShortData.java
index 5800ede45..01c746370 100644
--- a/src/main/java/net/minestom/server/data/type/ShortData.java
+++ b/src/main/java/net/minestom/server/data/type/ShortData.java
@@ -3,16 +3,18 @@ package net.minestom.server.data.type;
 import net.minestom.server.data.DataType;
 import net.minestom.server.utils.binary.BinaryReader;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 public class ShortData extends DataType<Short> {
 
     @Override
-    public void encode(BinaryWriter writer, Short value) {
+    public void encode(@NotNull BinaryWriter writer, @NotNull Short value) {
         writer.writeShort(value);
     }
 
+    @NotNull
     @Override
-    public Short decode(BinaryReader reader) {
+    public Short decode(@NotNull BinaryReader reader) {
         return reader.readShort();
     }
 }
diff --git a/src/main/java/net/minestom/server/data/type/StringData.java b/src/main/java/net/minestom/server/data/type/StringData.java
index 6a604ed50..99d0a3a5f 100644
--- a/src/main/java/net/minestom/server/data/type/StringData.java
+++ b/src/main/java/net/minestom/server/data/type/StringData.java
@@ -3,16 +3,18 @@ package net.minestom.server.data.type;
 import net.minestom.server.data.DataType;
 import net.minestom.server.utils.binary.BinaryReader;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 public class StringData extends DataType<String> {
 
     @Override
-    public void encode(BinaryWriter writer, String value) {
+    public void encode(@NotNull BinaryWriter writer, @NotNull String value) {
         writer.writeSizedString(value);
     }
 
+    @NotNull
     @Override
-    public String decode(BinaryReader reader) {
+    public String decode(@NotNull BinaryReader reader) {
         return reader.readSizedString();
     }
 }
diff --git a/src/main/java/net/minestom/server/data/type/UuidType.java b/src/main/java/net/minestom/server/data/type/UuidType.java
index 8df3929f7..e4c026ed0 100644
--- a/src/main/java/net/minestom/server/data/type/UuidType.java
+++ b/src/main/java/net/minestom/server/data/type/UuidType.java
@@ -3,17 +3,19 @@ package net.minestom.server.data.type;
 import net.minestom.server.data.DataType;
 import net.minestom.server.utils.binary.BinaryReader;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.UUID;
 
 public class UuidType extends DataType<UUID> {
     @Override
-    public void encode(BinaryWriter writer, UUID value) {
+    public void encode(@NotNull BinaryWriter writer, @NotNull UUID value) {
         writer.writeUuid(value);
     }
 
+    @NotNull
     @Override
-    public UUID decode(BinaryReader reader) {
+    public UUID decode(@NotNull BinaryReader reader) {
         return reader.readUuid();
     }
 }
diff --git a/src/main/java/net/minestom/server/data/type/array/BooleanArrayData.java b/src/main/java/net/minestom/server/data/type/array/BooleanArrayData.java
index b74ac2c81..bbcbdd19c 100644
--- a/src/main/java/net/minestom/server/data/type/array/BooleanArrayData.java
+++ b/src/main/java/net/minestom/server/data/type/array/BooleanArrayData.java
@@ -3,19 +3,21 @@ package net.minestom.server.data.type.array;
 import net.minestom.server.data.DataType;
 import net.minestom.server.utils.binary.BinaryReader;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 public class BooleanArrayData extends DataType<boolean[]> {
 
     @Override
-    public void encode(BinaryWriter writer, boolean[] value) {
+    public void encode(@NotNull BinaryWriter writer, @NotNull boolean[] value) {
         writer.writeVarInt(value.length);
         for (boolean val : value) {
             writer.writeBoolean(val);
         }
     }
 
+    @NotNull
     @Override
-    public boolean[] decode(BinaryReader reader) {
+    public boolean[] decode(@NotNull BinaryReader reader) {
         boolean[] array = new boolean[reader.readVarInt()];
         for (int i = 0; i < array.length; i++) {
             array[i] = reader.readBoolean();
diff --git a/src/main/java/net/minestom/server/data/type/array/ByteArrayData.java b/src/main/java/net/minestom/server/data/type/array/ByteArrayData.java
index 7ef154681..eb69118de 100644
--- a/src/main/java/net/minestom/server/data/type/array/ByteArrayData.java
+++ b/src/main/java/net/minestom/server/data/type/array/ByteArrayData.java
@@ -3,15 +3,17 @@ package net.minestom.server.data.type.array;
 import net.minestom.server.data.DataType;
 import net.minestom.server.utils.binary.BinaryReader;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 public class ByteArrayData extends DataType<byte[]> {
     @Override
-    public void encode(BinaryWriter writer, byte[] value) {
+    public void encode(@NotNull BinaryWriter writer, @NotNull byte[] value) {
         encodeByteArray(writer, value);
     }
 
+    @NotNull
     @Override
-    public byte[] decode(BinaryReader reader) {
+    public byte[] decode(@NotNull BinaryReader reader) {
         return decodeByteArray(reader);
     }
 
diff --git a/src/main/java/net/minestom/server/data/type/array/CharacterArrayData.java b/src/main/java/net/minestom/server/data/type/array/CharacterArrayData.java
index 8e7e36097..4a69fc99f 100644
--- a/src/main/java/net/minestom/server/data/type/array/CharacterArrayData.java
+++ b/src/main/java/net/minestom/server/data/type/array/CharacterArrayData.java
@@ -3,18 +3,20 @@ package net.minestom.server.data.type.array;
 import net.minestom.server.data.DataType;
 import net.minestom.server.utils.binary.BinaryReader;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 public class CharacterArrayData extends DataType<char[]> {
     @Override
-    public void encode(BinaryWriter writer, char[] value) {
+    public void encode(@NotNull BinaryWriter writer, @NotNull char[] value) {
         writer.writeVarInt(value.length);
         for (char val : value) {
             writer.writeChar(val);
         }
     }
 
+    @NotNull
     @Override
-    public char[] decode(BinaryReader reader) {
+    public char[] decode(@NotNull BinaryReader reader) {
         char[] array = new char[reader.readVarInt()];
         for (int i = 0; i < array.length; i++) {
             array[i] = reader.readChar();
diff --git a/src/main/java/net/minestom/server/data/type/array/DoubleArrayData.java b/src/main/java/net/minestom/server/data/type/array/DoubleArrayData.java
index 0182c2c87..e5131602f 100644
--- a/src/main/java/net/minestom/server/data/type/array/DoubleArrayData.java
+++ b/src/main/java/net/minestom/server/data/type/array/DoubleArrayData.java
@@ -3,19 +3,21 @@ package net.minestom.server.data.type.array;
 import net.minestom.server.data.DataType;
 import net.minestom.server.utils.binary.BinaryReader;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 public class DoubleArrayData extends DataType<double[]> {
 
     @Override
-    public void encode(BinaryWriter writer, double[] value) {
+    public void encode(@NotNull BinaryWriter writer, @NotNull double[] value) {
         writer.writeVarInt(value.length);
         for (double val : value) {
             writer.writeDouble(val);
         }
     }
 
+    @NotNull
     @Override
-    public double[] decode(BinaryReader reader) {
+    public double[] decode(@NotNull BinaryReader reader) {
         double[] array = new double[reader.readVarInt()];
         for (int i = 0; i < array.length; i++) {
             array[i] = reader.readDouble();
diff --git a/src/main/java/net/minestom/server/data/type/array/FloatArrayData.java b/src/main/java/net/minestom/server/data/type/array/FloatArrayData.java
index 119ab001b..6a11fdd36 100644
--- a/src/main/java/net/minestom/server/data/type/array/FloatArrayData.java
+++ b/src/main/java/net/minestom/server/data/type/array/FloatArrayData.java
@@ -3,19 +3,21 @@ package net.minestom.server.data.type.array;
 import net.minestom.server.data.DataType;
 import net.minestom.server.utils.binary.BinaryReader;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 public class FloatArrayData extends DataType<float[]> {
 
     @Override
-    public void encode(BinaryWriter writer, float[] value) {
+    public void encode(@NotNull BinaryWriter writer, @NotNull float[] value) {
         writer.writeVarInt(value.length);
         for (float val : value) {
             writer.writeFloat(val);
         }
     }
 
+    @NotNull
     @Override
-    public float[] decode(BinaryReader reader) {
+    public float[] decode(@NotNull BinaryReader reader) {
         float[] array = new float[reader.readVarInt()];
         for (int i = 0; i < array.length; i++) {
             array[i] = reader.readFloat();
diff --git a/src/main/java/net/minestom/server/data/type/array/IntegerArrayData.java b/src/main/java/net/minestom/server/data/type/array/IntegerArrayData.java
index eca283cf7..9f754d7ca 100644
--- a/src/main/java/net/minestom/server/data/type/array/IntegerArrayData.java
+++ b/src/main/java/net/minestom/server/data/type/array/IntegerArrayData.java
@@ -3,19 +3,21 @@ package net.minestom.server.data.type.array;
 import net.minestom.server.data.DataType;
 import net.minestom.server.utils.binary.BinaryReader;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 public class IntegerArrayData extends DataType<int[]> {
 
     @Override
-    public void encode(BinaryWriter writer, int[] value) {
+    public void encode(@NotNull BinaryWriter writer, @NotNull int[] value) {
         writer.writeVarInt(value.length);
         for (int val : value) {
             writer.writeInt(val);
         }
     }
 
+    @NotNull
     @Override
-    public int[] decode(BinaryReader reader) {
+    public int[] decode(@NotNull BinaryReader reader) {
         int[] array = new int[reader.readVarInt()];
         for (int i = 0; i < array.length; i++) {
             array[i] = reader.readInteger();
diff --git a/src/main/java/net/minestom/server/data/type/array/ItemStackArrayData.java b/src/main/java/net/minestom/server/data/type/array/ItemStackArrayData.java
index e1baa8157..d88dd75a7 100644
--- a/src/main/java/net/minestom/server/data/type/array/ItemStackArrayData.java
+++ b/src/main/java/net/minestom/server/data/type/array/ItemStackArrayData.java
@@ -4,18 +4,20 @@ import net.minestom.server.data.DataType;
 import net.minestom.server.item.ItemStack;
 import net.minestom.server.utils.binary.BinaryReader;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 public class ItemStackArrayData extends DataType<ItemStack[]> {
     @Override
-    public void encode(BinaryWriter writer, ItemStack[] value) {
+    public void encode(@NotNull BinaryWriter writer, @NotNull ItemStack[] value) {
         writer.writeVarInt(value.length);
         for (ItemStack itemStack : value) {
             writer.writeItemStack(itemStack);
         }
     }
 
+    @NotNull
     @Override
-    public ItemStack[] decode(BinaryReader reader) {
+    public ItemStack[] decode(@NotNull BinaryReader reader) {
         ItemStack[] items = new ItemStack[reader.readVarInt()];
         for (int i = 0; i < items.length; i++) {
             items[i] = reader.readSlot();
diff --git a/src/main/java/net/minestom/server/data/type/array/LongArrayData.java b/src/main/java/net/minestom/server/data/type/array/LongArrayData.java
index 486847dbd..cb7e9df54 100644
--- a/src/main/java/net/minestom/server/data/type/array/LongArrayData.java
+++ b/src/main/java/net/minestom/server/data/type/array/LongArrayData.java
@@ -3,19 +3,21 @@ package net.minestom.server.data.type.array;
 import net.minestom.server.data.DataType;
 import net.minestom.server.utils.binary.BinaryReader;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 public class LongArrayData extends DataType<long[]> {
 
     @Override
-    public void encode(BinaryWriter writer, long[] value) {
+    public void encode(@NotNull BinaryWriter writer, @NotNull long[] value) {
         writer.writeVarInt(value.length);
         for (long val : value) {
             writer.writeLong(val);
         }
     }
 
+    @NotNull
     @Override
-    public long[] decode(BinaryReader reader) {
+    public long[] decode(@NotNull BinaryReader reader) {
         long[] array = new long[reader.readVarInt()];
         for (int i = 0; i < array.length; i++) {
             array[i] = reader.readLong();
diff --git a/src/main/java/net/minestom/server/data/type/array/ShortArrayData.java b/src/main/java/net/minestom/server/data/type/array/ShortArrayData.java
index 51835f463..c133a1478 100644
--- a/src/main/java/net/minestom/server/data/type/array/ShortArrayData.java
+++ b/src/main/java/net/minestom/server/data/type/array/ShortArrayData.java
@@ -3,19 +3,21 @@ package net.minestom.server.data.type.array;
 import net.minestom.server.data.DataType;
 import net.minestom.server.utils.binary.BinaryReader;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 public class ShortArrayData extends DataType<short[]> {
 
     @Override
-    public void encode(BinaryWriter writer, short[] value) {
+    public void encode(@NotNull BinaryWriter writer, @NotNull short[] value) {
         writer.writeVarInt(value.length);
         for (short val : value) {
             writer.writeShort(val);
         }
     }
 
+    @NotNull
     @Override
-    public short[] decode(BinaryReader reader) {
+    public short[] decode(@NotNull BinaryReader reader) {
         short[] array = new short[reader.readVarInt()];
         for (int i = 0; i < array.length; i++) {
             array[i] = reader.readShort();
diff --git a/src/main/java/net/minestom/server/data/type/array/StringArrayData.java b/src/main/java/net/minestom/server/data/type/array/StringArrayData.java
index d37fc4b19..cbdde7ab1 100644
--- a/src/main/java/net/minestom/server/data/type/array/StringArrayData.java
+++ b/src/main/java/net/minestom/server/data/type/array/StringArrayData.java
@@ -3,19 +3,21 @@ package net.minestom.server.data.type.array;
 import net.minestom.server.data.DataType;
 import net.minestom.server.utils.binary.BinaryReader;
 import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
 
 public class StringArrayData extends DataType<String[]> {
 
     @Override
-    public void encode(BinaryWriter writer, String[] value) {
+    public void encode(@NotNull BinaryWriter writer, @NotNull String[] value) {
         writer.writeVarInt(value.length);
         for (String val : value) {
             writer.writeSizedString(val);
         }
     }
 
+    @NotNull
     @Override
-    public String[] decode(BinaryReader reader) {
+    public String[] decode(@NotNull BinaryReader reader) {
         String[] array = new String[reader.readVarInt()];
         for (int i = 0; i < array.length; i++) {
             array[i] = reader.readSizedString();
diff --git a/src/main/java/net/minestom/server/entity/ItemEntity.java b/src/main/java/net/minestom/server/entity/ItemEntity.java
index bd86edf18..f9ecb43b0 100644
--- a/src/main/java/net/minestom/server/entity/ItemEntity.java
+++ b/src/main/java/net/minestom/server/entity/ItemEntity.java
@@ -39,7 +39,7 @@ public class ItemEntity extends ObjectEntity {
     private long spawnTime;
     private long pickupDelay;
 
-    public ItemEntity(ItemStack itemStack, Position spawnPosition) {
+    public ItemEntity(@NotNull ItemStack itemStack, @NotNull Position spawnPosition) {
         super(EntityType.ITEM, spawnPosition);
         this.itemStack = itemStack;
         setBoundingBox(0.25f, 0.25f, 0.25f);
diff --git a/src/main/java/net/minestom/server/entity/LivingEntity.java b/src/main/java/net/minestom/server/entity/LivingEntity.java
index 63ba2bbf8..1555f024b 100644
--- a/src/main/java/net/minestom/server/entity/LivingEntity.java
+++ b/src/main/java/net/minestom/server/entity/LivingEntity.java
@@ -22,6 +22,7 @@ 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 org.jetbrains.annotations.Nullable;
 
 import java.util.Set;
 import java.util.function.Consumer;
@@ -65,13 +66,13 @@ public abstract class LivingEntity extends Entity implements EquipmentHandler {
 
     private Team team;
 
-    public LivingEntity(EntityType entityType, Position spawnPosition) {
+    public LivingEntity(@NotNull EntityType entityType, @NotNull Position spawnPosition) {
         super(entityType, spawnPosition);
         setupAttributes();
         setGravity(0.02f);
     }
 
-    public LivingEntity(EntityType entityType) {
+    public LivingEntity(@NotNull EntityType entityType) {
         this(entityType, new Position());
     }
 
@@ -255,8 +256,8 @@ public abstract class LivingEntity extends Entity implements EquipmentHandler {
      * @param value the amount of damage
      * @return true if damage has been applied, false if it didn't
      */
-    public boolean damage(DamageType type, float value) {
-        Check.notNull(type, "The damage type cannot be null!o");
+    public boolean damage(@NotNull DamageType type, float value) {
+        Check.notNull(type, "The damage type cannot be null!");
         if (isDead())
             return false;
         if (isInvulnerable() || isImmune(type)) {
@@ -318,7 +319,7 @@ public abstract class LivingEntity extends Entity implements EquipmentHandler {
      * @param type the type of damage
      * @return true if this entity is immune to the given type of damage
      */
-    public boolean isImmune(DamageType type) {
+    public boolean isImmune(@NotNull DamageType type) {
         return false;
     }
 
@@ -351,6 +352,7 @@ public abstract class LivingEntity extends Entity implements EquipmentHandler {
      *
      * @return the last damage source, null if not any
      */
+    @Nullable
     public DamageType getLastDamageSource() {
         return lastDamageSource;
     }
@@ -379,7 +381,7 @@ public abstract class LivingEntity extends Entity implements EquipmentHandler {
      * @param attribute The attribute to change
      * @param value     the new value of the attribute
      */
-    public void setAttribute(Attribute attribute, float value) {
+    public void setAttribute(@NotNull Attribute attribute, float value) {
         this.attributeValues[attribute.ordinal()] = value;
     }
 
@@ -389,7 +391,7 @@ public abstract class LivingEntity extends Entity implements EquipmentHandler {
      * @param attribute the attribute value to get
      * @return the attribute value
      */
-    public float getAttributeValue(Attribute attribute) {
+    public float getAttributeValue(@NotNull Attribute attribute) {
         return this.attributeValues[attribute.ordinal()];
     }
 
@@ -470,6 +472,7 @@ public abstract class LivingEntity extends Entity implements EquipmentHandler {
      *
      * @return an {@link EntityPropertiesPacket} linked to this entity
      */
+    @NotNull
     protected EntityPropertiesPacket getPropertiesPacket() {
         EntityPropertiesPacket propertiesPacket = new EntityPropertiesPacket();
         propertiesPacket.entityId = getEntityId();
@@ -525,7 +528,7 @@ public abstract class LivingEntity extends Entity implements EquipmentHandler {
      * @param fireDamagePeriod the delay
      * @param timeUnit         the time unit
      */
-    public void setFireDamagePeriod(long fireDamagePeriod, TimeUnit timeUnit) {
+    public void setFireDamagePeriod(long fireDamagePeriod, @NotNull TimeUnit timeUnit) {
         fireDamagePeriod = timeUnit.toMilliseconds(fireDamagePeriod);
         this.fireDamagePeriod = fireDamagePeriod;
     }
diff --git a/src/main/java/net/minestom/server/entity/ObjectEntity.java b/src/main/java/net/minestom/server/entity/ObjectEntity.java
index 0e8273ec3..7f0459273 100644
--- a/src/main/java/net/minestom/server/entity/ObjectEntity.java
+++ b/src/main/java/net/minestom/server/entity/ObjectEntity.java
@@ -7,7 +7,7 @@ import org.jetbrains.annotations.NotNull;
 
 public abstract class ObjectEntity extends Entity {
 
-    public ObjectEntity(EntityType entityType, Position spawnPosition) {
+    public ObjectEntity(@NotNull EntityType entityType, @NotNull Position spawnPosition) {
         super(entityType, spawnPosition);
         setGravity(0.02f);
     }
diff --git a/src/main/java/net/minestom/server/entity/Player.java b/src/main/java/net/minestom/server/entity/Player.java
index b8dc36068..de46e73f7 100644
--- a/src/main/java/net/minestom/server/entity/Player.java
+++ b/src/main/java/net/minestom/server/entity/Player.java
@@ -280,7 +280,7 @@ public class Player extends LivingEntity implements CommandSender {
     }
 
     @Override
-    public float getAttributeValue(Attribute attribute) {
+    public float getAttributeValue(@NotNull Attribute attribute) {
         if (attribute == Attribute.MOVEMENT_SPEED) {
             return walkingSpeed;
         }
@@ -679,6 +679,7 @@ public class Player extends LivingEntity implements CommandSender {
         sendMessage(ColoredText.of(message));
     }
 
+    @NotNull
     @Override
     public Collection<Permission> getAllPermissions() {
         return permissions;
@@ -775,10 +776,10 @@ public class Player extends LivingEntity implements CommandSender {
     /**
      * Sets the header and footer of a player which will be displayed in his tab window.
      *
-     * @param header the header text
-     * @param footer the footer text
+     * @param header the header text, null to set empty
+     * @param footer the footer text, null to set empty
      */
-    public void sendHeaderFooter(@NotNull ColoredText header, @NotNull ColoredText footer) {
+    public void sendHeaderFooter(@Nullable ColoredText header, @Nullable ColoredText footer) {
         PlayerListHeaderAndFooterPacket playerListHeaderAndFooterPacket = new PlayerListHeaderAndFooterPacket();
         playerListHeaderAndFooterPacket.emptyHeader = header == null;
         playerListHeaderAndFooterPacket.emptyFooter = footer == null;
@@ -1624,7 +1625,7 @@ public class Player extends LivingEntity implements CommandSender {
 
             Inventory newInventory = inventoryOpenEvent.getInventory();
 
-            if(newInventory == null){
+            if (newInventory == null) {
                 // just close the inventory
                 return;
             }
diff --git a/src/main/java/net/minestom/server/entity/damage/DamageType.java b/src/main/java/net/minestom/server/entity/damage/DamageType.java
index f46ef2616..563ac9310 100644
--- a/src/main/java/net/minestom/server/entity/damage/DamageType.java
+++ b/src/main/java/net/minestom/server/entity/damage/DamageType.java
@@ -9,6 +9,8 @@ import net.minestom.server.entity.Entity;
 import net.minestom.server.entity.LivingEntity;
 import net.minestom.server.entity.Player;
 import net.minestom.server.sound.Sound;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 /**
  * Represents a type of damage, required when calling {@link LivingEntity#damage(DamageType, float)}
@@ -24,7 +26,7 @@ public class DamageType implements DataContainer {
     public static final DamageType GRAVITY = new DamageType("attack.fall");
     public static final DamageType ON_FIRE = new DamageType("attack.onFire") {
         @Override
-        protected Sound getPlayerSound(Player player) {
+        protected Sound getPlayerSound(@NotNull Player player) {
             return Sound.ENTITY_PLAYER_HURT_ON_FIRE;
         }
     };
@@ -37,7 +39,7 @@ public class DamageType implements DataContainer {
      * @param identifier the identifier of this damage type,
      *                   does not need to be unique
      */
-    public DamageType(String identifier) {
+    public DamageType(@NotNull String identifier) {
         this.identifier = identifier;
     }
 
@@ -48,6 +50,7 @@ public class DamageType implements DataContainer {
      *
      * @return the damage type identifier
      */
+    @NotNull
     public String getIdentifier() {
         return identifier;
     }
@@ -61,7 +64,8 @@ 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}.
      */
-    public JsonMessage buildDeathMessage(Player killed) {
+    @NotNull
+    public JsonMessage buildDeathMessage(@NotNull Player killed) {
         return ColoredText.of("{@death." + identifier + "," + killed.getUsername() + "}");
     }
 
@@ -72,7 +76,8 @@ public class DamageType implements DataContainer {
      * @param projectile the actual projectile
      * @return a new {@link EntityProjectileDamage}
      */
-    public static DamageType fromProjectile(Entity shooter, Entity projectile) {
+    @NotNull
+    public static DamageType fromProjectile(@Nullable Entity shooter, @NotNull Entity projectile) {
         return new EntityProjectileDamage(shooter, projectile);
     }
 
@@ -82,7 +87,8 @@ public class DamageType implements DataContainer {
      * @param player the player damager
      * @return a new {@link EntityDamage}
      */
-    public static EntityDamage fromPlayer(Player player) {
+    @NotNull
+    public static EntityDamage fromPlayer(@NotNull Player player) {
         return new EntityDamage(player);
     }
 
@@ -92,7 +98,8 @@ public class DamageType implements DataContainer {
      * @param entity the entity damager
      * @return a new {@link EntityDamage}
      */
-    public static EntityDamage fromEntity(Entity entity) {
+    @NotNull
+    public static EntityDamage fromEntity(@NotNull Entity entity) {
         return new EntityDamage(entity);
     }
 
@@ -102,7 +109,8 @@ public class DamageType implements DataContainer {
      * @param killed the player who has been killed
      * @return the death screen text, null to do not send anything
      */
-    public ColoredText buildDeathScreenText(Player killed) {
+    @NotNull
+    public ColoredText buildDeathScreenText(@NotNull Player killed) {
         return ColoredText.of("{@death." + identifier + "}");
     }
 
@@ -112,18 +120,19 @@ public class DamageType implements DataContainer {
      * @param entity the entity hit by this damage
      * @return the sound to play when the given entity is hurt by this damage type. Can be null if no sound should play
      */
-    public Sound getSound(LivingEntity entity) {
+    @Nullable
+    public Sound getSound(@NotNull LivingEntity entity) {
         if (entity instanceof Player) {
             return getPlayerSound((Player) entity);
         }
         return getGenericSound(entity);
     }
 
-    protected Sound getGenericSound(LivingEntity entity) {
+    protected Sound getGenericSound(@NotNull LivingEntity entity) {
         return Sound.ENTITY_GENERIC_HURT;
     }
 
-    protected Sound getPlayerSound(Player player) {
+    protected Sound getPlayerSound(@NotNull Player player) {
         return Sound.ENTITY_PLAYER_HURT;
     }
 
diff --git a/src/main/java/net/minestom/server/entity/damage/EntityDamage.java b/src/main/java/net/minestom/server/entity/damage/EntityDamage.java
index 4e00d0521..ff6bd2979 100644
--- a/src/main/java/net/minestom/server/entity/damage/EntityDamage.java
+++ b/src/main/java/net/minestom/server/entity/damage/EntityDamage.java
@@ -1,6 +1,7 @@
 package net.minestom.server.entity.damage;
 
 import net.minestom.server.entity.Entity;
+import org.jetbrains.annotations.NotNull;
 
 /**
  * Represents damage inflicted by an {@link Entity}.
@@ -9,7 +10,7 @@ public class EntityDamage extends DamageType {
 
     private final Entity source;
 
-    public EntityDamage(Entity source) {
+    public EntityDamage(@NotNull Entity source) {
         super("entity_source");
         this.source = source;
     }
@@ -19,6 +20,7 @@ public class EntityDamage extends DamageType {
      *
      * @return the source
      */
+    @NotNull
     public Entity getSource() {
         return source;
     }
diff --git a/src/main/java/net/minestom/server/entity/damage/EntityProjectileDamage.java b/src/main/java/net/minestom/server/entity/damage/EntityProjectileDamage.java
index 610f9f829..e7d6f5de9 100644
--- a/src/main/java/net/minestom/server/entity/damage/EntityProjectileDamage.java
+++ b/src/main/java/net/minestom/server/entity/damage/EntityProjectileDamage.java
@@ -1,6 +1,8 @@
 package net.minestom.server.entity.damage;
 
 import net.minestom.server.entity.Entity;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 /**
  * Represents damage inflicted by an entity, via a projectile.
@@ -10,7 +12,7 @@ public class EntityProjectileDamage extends DamageType {
     private final Entity shooter;
     private final Entity projectile;
 
-    public EntityProjectileDamage(Entity shooter, Entity projectile) {
+    public EntityProjectileDamage(@Nullable Entity shooter, @NotNull Entity projectile) {
         super("projectile_source");
         this.shooter = shooter;
         this.projectile = projectile;
@@ -21,6 +23,7 @@ public class EntityProjectileDamage extends DamageType {
      *
      * @return the projectile
      */
+    @NotNull
     public Entity getProjectile() {
         return projectile;
     }
@@ -30,6 +33,7 @@ public class EntityProjectileDamage extends DamageType {
      *
      * @return the shooter of the projectile, null if not any
      */
+    @Nullable
     public Entity getShooter() {
         return shooter;
     }
diff --git a/src/main/java/net/minestom/server/entity/fakeplayer/FakePlayer.java b/src/main/java/net/minestom/server/entity/fakeplayer/FakePlayer.java
index d6f530c1d..3fe4e6f57 100644
--- a/src/main/java/net/minestom/server/entity/fakeplayer/FakePlayer.java
+++ b/src/main/java/net/minestom/server/entity/fakeplayer/FakePlayer.java
@@ -6,6 +6,7 @@ import net.minestom.server.event.player.PlayerLoginEvent;
 import net.minestom.server.network.player.FakePlayerConnection;
 import net.minestom.server.network.player.PlayerConnection;
 import net.minestom.server.utils.time.TimeUnit;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.UUID;
 import java.util.function.Consumer;
@@ -15,7 +16,7 @@ public class FakePlayer extends Player {
     private final FakePlayerOption option;
     private final FakePlayerController fakePlayerController;
 
-    private FakePlayer(UUID uuid, String username, FakePlayerOption option) {
+    private FakePlayer(@NotNull UUID uuid, @NotNull String username, @NotNull FakePlayerOption option) {
         super(uuid, username, new FakePlayerConnection());
 
         this.option = option;
@@ -37,7 +38,8 @@ public class FakePlayer extends Player {
      *                          WARNING: it will be called in the
      *                          {@link net.minestom.server.timer.SchedulerManager} thread pool
      */
-    public static void initPlayer(UUID uuid, String username, FakePlayerOption option, Consumer<FakePlayer> scheduledCallback) {
+    public static void initPlayer(@NotNull UUID uuid, @NotNull String username,
+                                  @NotNull FakePlayerOption option, @NotNull Consumer<FakePlayer> scheduledCallback) {
         final FakePlayer fakePlayer = new FakePlayer(uuid, username, option);
 
         fakePlayer.addEventCallback(PlayerLoginEvent.class, event -> MinecraftServer.getSchedulerManager().buildTask(() -> scheduledCallback.accept(fakePlayer)).delay(1, TimeUnit.TICK).schedule());
@@ -52,7 +54,7 @@ public class FakePlayer extends Player {
      *                          WARNING: it will be called in the
      *                          {@link net.minestom.server.timer.SchedulerManager} thread pool
      */
-    public static void initPlayer(UUID uuid, String username, Consumer<FakePlayer> scheduledCallback) {
+    public static void initPlayer(@NotNull UUID uuid, @NotNull String username, @NotNull Consumer<FakePlayer> scheduledCallback) {
         initPlayer(uuid, username, new FakePlayerOption(), scheduledCallback);
     }
 
@@ -61,16 +63,18 @@ public class FakePlayer extends Player {
      *
      * @return the fake player option
      */
+    @NotNull
     public FakePlayerOption getOption() {
         return option;
     }
 
+    @NotNull
     public FakePlayerController getController() {
         return fakePlayerController;
     }
 
     @Override
-    protected void showPlayer(PlayerConnection connection) {
+    protected void showPlayer(@NotNull PlayerConnection connection) {
         super.showPlayer(connection);
         if (!option.isInTabList()) {
             // Remove from tab-list