Add ability to change player item in hand. Adds BUKKIT-3318

src/main/java/net/minecraft/server/PlayerConnection.java

@@ -760,6 +760,10 @@ public class PlayerConnection extends Connection {
         if (packet16blockitemswitch.itemInHandIndex >= 0 && packet16blockitemswitch.itemInHandIndex < PlayerInventory.getHotbarSize()) {
             PlayerItemHeldEvent event = new PlayerItemHeldEvent(this.getPlayer(), this.player.inventory.itemInHandIndex, packet16blockitemswitch.itemInHandIndex);
             this.server.getPluginManager().callEvent(event);
+            if (event.isCancelled()) {
+                this.sendPacket(new Packet16BlockItemSwitch(this.player.inventory.itemInHandIndex));
+                return;
+            }
             // CraftBukkit end
 
             this.player.inventory.itemInHandIndex = packet16blockitemswitch.itemInHandIndex;

src/main/java/org/bukkit/craftbukkit/inventory/CraftInventoryPlayer.java

@@ -1,7 +1,10 @@
 package org.bukkit.craftbukkit.inventory;
 
+import net.minecraft.server.Packet16BlockItemSwitch;
 import net.minecraft.server.PlayerInventory;
 
+import org.apache.commons.lang.Validate;
+import org.bukkit.craftbukkit.entity.CraftPlayer;
 import org.bukkit.entity.HumanEntity;
 import org.bukkit.inventory.EntityEquipment;
 import org.bukkit.inventory.ItemStack;
@@ -33,6 +36,12 @@ public class CraftInventoryPlayer extends CraftInventory implements org.bukkit.i
         return getInventory().itemInHandIndex;
     }
 
+    public void setHeldItemSlot(int slot) {
+        Validate.isTrue(slot >= 0 && slot < PlayerInventory.getHotbarSize(), "Slot is not between 0 and 8 inclusive");
+        this.getInventory().itemInHandIndex = slot;
+        ((CraftPlayer) this.getHolder()).getHandle().playerConnection.sendPacket(new Packet16BlockItemSwitch(slot));
+    }
+
     public ItemStack getHelmet() {
         return getItem(getSize() + 3);
     }