Fix ExperienceOrb#setExperienceCount with a proper updating client-side

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

@@ -175,6 +175,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
     }
 
     public void teleport(Position position, Runnable callback) {
+        Check.notNull(position, "Teleport position cannot be null");
         Check.stateCondition(instance == null, "You need to use Entity#setInstance before teleporting an entity!");
 
         Runnable runnable = () -> {
@@ -230,6 +231,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
 
     @Override
     public boolean addViewer(Player player) {
+        Check.notNull(player, "Viewer cannot be null");
         boolean result = this.viewers.add(player);
         player.viewableEntities.add(this);
         return result;
@@ -237,6 +239,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
 
     @Override
     public boolean removeViewer(Player player) {
+        Check.notNull(player, "Viewer cannot be null");
         if (!viewers.remove(player))
             return false;
 
@@ -404,6 +407,8 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
 
     @Override
     public <E extends Event> void addEventCallback(Class<E> eventClass, EventCallback<E> eventCallback) {
+        Check.notNull(eventClass, "Event class cannot be null");
+        Check.notNull(eventCallback, "Event callback cannot be null");
         List<EventCallback> callbacks = getEventCallbacks(eventClass);
         callbacks.add(eventCallback);
         this.eventCallbacks.put(eventClass, callbacks);
@@ -411,6 +416,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
 
     @Override
     public <E extends Event> List<EventCallback> getEventCallbacks(Class<E> eventClass) {
+        Check.notNull(eventClass, "Event class cannot be null");
         return eventCallbacks.getOrDefault(eventClass, new CopyOnWriteArrayList<>());
     }
 
@@ -482,6 +488,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
     }
 
     public float getDistance(Entity entity) {
+        Check.notNull(entity, "Entity cannot be null");
         return getPosition().getDistance(entity.getPosition());
     }
 
@@ -738,6 +745,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
      * @return true if the entity is in the same chunk as {@code position}
      */
     public boolean sameChunk(Position position) {
+        Check.notNull(position, "Position cannot be null");
         Position pos = getPosition();
         int chunkX1 = ChunkUtils.getChunkCoordinate((int) Math.floor(pos.getX()));
         int chunkZ1 = ChunkUtils.getChunkCoordinate((int) Math.floor(pos.getZ()));

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

@@ -3,6 +3,9 @@ package net.minestom.server.entity;
 import net.minestom.server.network.packet.server.play.SpawnExperienceOrbPacket;
 import net.minestom.server.network.player.PlayerConnection;
 
+import java.util.HashSet;
+import java.util.Set;
+
 public class ExperienceOrb extends Entity {
 
     private short experienceCount;
@@ -32,15 +35,31 @@ public class ExperienceOrb extends Entity {
         experienceOrbPacket.entityId = getEntityId();
         experienceOrbPacket.position = getPosition();
         experienceOrbPacket.expCount = experienceCount;
+
         playerConnection.sendPacket(experienceOrbPacket);
-        return super.addViewer(player); // Add player to viewers list and send velocity packet
+        playerConnection.sendPacket(getVelocityPacket());
+
+        return super.addViewer(player); // Add player to viewers list
     }
 
+    /**
+     * @return the experience amount contained in the entity
+     */
     public short getExperienceCount() {
         return experienceCount;
     }
 
+    /**
+     * @param experienceCount the new experience amount
+     */
     public void setExperienceCount(short experienceCount) {
+        // Remove the entity in order to respawn it with the correct experience count
+        Set<Player> viewerCache = new HashSet<>(getViewers());
+
+        viewerCache.forEach(player -> removeViewer(player));
+
         this.experienceCount = experienceCount;
+
+        viewerCache.forEach(player -> addViewer(player));
     }
 }

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

@@ -887,6 +887,7 @@ public class Player extends LivingEntity {
      * @param gameMode the new player gamemode
      */
     public void setGameMode(GameMode gameMode) {
+        Check.notNull(gameMode, "GameMode cannot be null");
         ChangeGameStatePacket changeGameStatePacket = new ChangeGameStatePacket();
         changeGameStatePacket.reason = ChangeGameStatePacket.Reason.CHANGE_GAMEMODE;
         changeGameStatePacket.value = gameMode.getId();