Added InventoryModifier#replaceItemStack

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

@@ -29,6 +29,7 @@ import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.concurrent.CopyOnWriteArraySet;
 import java.util.concurrent.atomic.AtomicInteger;
+import java.util.function.UnaryOperator;
 
 /**
  * Represents an inventory which can be viewed by a collection of {@link Player}.
@@ -159,7 +160,13 @@ public class Inventory implements InventoryModifier, InventoryClickHandler, View
     }
 
     @Override
-    public void clear() {
+    public synchronized void replaceItemStack(int slot, @NotNull UnaryOperator<@NotNull ItemStack> operator) {
+        // Make the method synchronized
+        InventoryModifier.super.replaceItemStack(slot, operator);
+    }
+
+    @Override
+    public synchronized void clear() {
         // Clear the item array
         Arrays.fill(itemStacks, ItemStack.AIR);
         // Send the cleared inventory to viewers

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

@@ -9,6 +9,7 @@ import net.minestom.server.utils.validate.Check;
 import org.jetbrains.annotations.NotNull;
 
 import java.util.List;
+import java.util.function.UnaryOperator;
 
 /**
  * Represents an inventory where items can be modified/retrieved.
@@ -75,6 +76,11 @@ public interface InventoryModifier {
         }
     }
 
+    default void replaceItemStack(int slot, @NotNull UnaryOperator<@NotNull ItemStack> operator) {
+        var currentItem = getItemStack(slot);
+        setItemStack(slot, operator.apply(currentItem));
+    }
+
     /**
      * Clears the inventory and send relevant update to the viewer(s).
      */

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

@@ -23,6 +23,7 @@ import org.jetbrains.annotations.Nullable;
 import java.util.Arrays;
 import java.util.List;
 import java.util.concurrent.CopyOnWriteArrayList;
+import java.util.function.UnaryOperator;
 
 import static net.minestom.server.utils.inventory.PlayerInventoryUtils.*;
 
@@ -73,7 +74,7 @@ public class PlayerInventory implements InventoryModifier, InventoryClickHandler
     }
 
     @Override
-    public void setItemStack(int slot, @NotNull ItemStack itemStack) {
+    public synchronized void setItemStack(int slot, @NotNull ItemStack itemStack) {
         PlayerSetItemStackEvent setItemStackEvent = new PlayerSetItemStackEvent(player, slot, itemStack);
         player.callEvent(PlayerSetItemStackEvent.class, setItemStackEvent);
         if (setItemStackEvent.isCancelled())
@@ -107,7 +108,13 @@ public class PlayerInventory implements InventoryModifier, InventoryClickHandler
     }
 
     @Override
-    public void clear() {
+    public synchronized void replaceItemStack(int slot, @NotNull UnaryOperator<@NotNull ItemStack> operator) {
+        // Make the method synchronized
+        InventoryModifier.super.replaceItemStack(slot, operator);
+    }
+
+    @Override
+    public synchronized void clear() {
         // Clear the item array
         Arrays.fill(itemStacks, ItemStack.AIR);
 

src/test/java/demo/PlayerInit.java

@@ -225,7 +225,8 @@ public class PlayerInit {
 
             PlayerInventory inventory = player.getInventory();
             ItemStack itemStack = ItemStack.of(Material.STONE, 64);
-            //inventory.addItemStack(itemStack);
+            inventory.addItemStack(itemStack);
+            inventory.replaceItemStack(0, i -> i.withAmount(32));
 
             {
                 ItemStack item = ItemStack.builder(Material.DIAMOND_CHESTPLATE)