diff --git a/InvUI/src/main/java/de/studiocode/invui/item/Item.java b/InvUI/src/main/java/de/studiocode/invui/item/Item.java
index 3b2d562..4748103 100644
--- a/InvUI/src/main/java/de/studiocode/invui/item/Item.java
+++ b/InvUI/src/main/java/de/studiocode/invui/item/Item.java
@@ -6,6 +6,7 @@ import org.bukkit.event.inventory.ClickType;
 import org.bukkit.event.inventory.InventoryClickEvent;
 import org.bukkit.inventory.Inventory;
 import org.bukkit.inventory.ItemStack;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.Set;
 
@@ -60,6 +61,6 @@ public interface Item {
      * @param player    The {@link Player} who clicked on the {@link ItemStack}
      * @param event     The {@link InventoryClickEvent} associated with this click.
      */
-    void handleClick(ClickType clickType, Player player, InventoryClickEvent event);
+    void handleClick(@NotNull ClickType clickType, @NotNull Player player, @NotNull InventoryClickEvent event);
     
 }
diff --git a/InvUI/src/main/java/de/studiocode/invui/item/impl/AsyncItem.java b/InvUI/src/main/java/de/studiocode/invui/item/impl/AsyncItem.java
index 5e57e27..0b6d237 100644
--- a/InvUI/src/main/java/de/studiocode/invui/item/impl/AsyncItem.java
+++ b/InvUI/src/main/java/de/studiocode/invui/item/impl/AsyncItem.java
@@ -42,7 +42,7 @@ public class AsyncItem extends BaseItem {
     }
     
     @Override
-    public void handleClick(ClickType clickType, Player player, InventoryClickEvent event) {
+    public void handleClick(@NotNull ClickType clickType, @NotNull Player player, @NotNull InventoryClickEvent event) {
         // empty
     }
     
diff --git a/InvUI/src/main/java/de/studiocode/invui/item/impl/AutoCycleItem.java b/InvUI/src/main/java/de/studiocode/invui/item/impl/AutoCycleItem.java
index 6cdd73a..198042e 100644
--- a/InvUI/src/main/java/de/studiocode/invui/item/impl/AutoCycleItem.java
+++ b/InvUI/src/main/java/de/studiocode/invui/item/impl/AutoCycleItem.java
@@ -9,6 +9,7 @@ import org.bukkit.entity.Player;
 import org.bukkit.event.inventory.ClickType;
 import org.bukkit.event.inventory.InventoryClickEvent;
 import org.bukkit.scheduler.BukkitTask;
+import org.jetbrains.annotations.NotNull;
 
 /**
  * An {@link Item} that automatically cycles through a predefined array of
@@ -61,7 +62,7 @@ public class AutoCycleItem extends BaseItem {
     }
     
     @Override
-    public void handleClick(ClickType clickType, Player player, InventoryClickEvent event) {
+    public void handleClick(@NotNull ClickType clickType, @NotNull Player player, @NotNull InventoryClickEvent event) {
         // empty
     }
     
diff --git a/InvUI/src/main/java/de/studiocode/invui/item/impl/CommandItem.java b/InvUI/src/main/java/de/studiocode/invui/item/impl/CommandItem.java
index c62251b..d400784 100644
--- a/InvUI/src/main/java/de/studiocode/invui/item/impl/CommandItem.java
+++ b/InvUI/src/main/java/de/studiocode/invui/item/impl/CommandItem.java
@@ -20,7 +20,7 @@ public class CommandItem extends SimpleItem {
     }
     
     @Override
-    public void handleClick(ClickType clickType, Player player, InventoryClickEvent event) {
+    public void handleClick(@NotNull ClickType clickType, @NotNull Player player, @NotNull InventoryClickEvent event) {
         player.chat(command);
     }
     
diff --git a/InvUI/src/main/java/de/studiocode/invui/item/impl/CycleItem.java b/InvUI/src/main/java/de/studiocode/invui/item/impl/CycleItem.java
index 39b4366..358fc66 100644
--- a/InvUI/src/main/java/de/studiocode/invui/item/impl/CycleItem.java
+++ b/InvUI/src/main/java/de/studiocode/invui/item/impl/CycleItem.java
@@ -47,7 +47,7 @@ public class CycleItem extends BaseItem {
     }
     
     @Override
-    public void handleClick(ClickType clickType, Player player, InventoryClickEvent event) {
+    public void handleClick(@NotNull ClickType clickType, @NotNull Player player, @NotNull InventoryClickEvent event) {
         if (clickType.isLeftClick()) cycle(player, true);
         else if (clickType.isRightClick()) cycle(player, false);
     }
diff --git a/InvUI/src/main/java/de/studiocode/invui/item/impl/SimpleItem.java b/InvUI/src/main/java/de/studiocode/invui/item/impl/SimpleItem.java
index f782f4b..be28477 100644
--- a/InvUI/src/main/java/de/studiocode/invui/item/impl/SimpleItem.java
+++ b/InvUI/src/main/java/de/studiocode/invui/item/impl/SimpleItem.java
@@ -34,7 +34,7 @@ public class SimpleItem extends BaseItem {
     }
     
     @Override
-    public void handleClick(ClickType clickType, Player player, InventoryClickEvent event) {
+    public void handleClick(@NotNull ClickType clickType, @NotNull Player player, @NotNull InventoryClickEvent event) {
         if (clickHandler != null) clickHandler.accept(new Click(event));
     }
     
diff --git a/InvUI/src/main/java/de/studiocode/invui/item/impl/SuppliedItem.java b/InvUI/src/main/java/de/studiocode/invui/item/impl/SuppliedItem.java
index 972712a..539e61b 100644
--- a/InvUI/src/main/java/de/studiocode/invui/item/impl/SuppliedItem.java
+++ b/InvUI/src/main/java/de/studiocode/invui/item/impl/SuppliedItem.java
@@ -27,7 +27,7 @@ public class SuppliedItem extends BaseItem {
     }
     
     @Override
-    public void handleClick(ClickType clickType, Player player, InventoryClickEvent event) {
+    public void handleClick(@NotNull ClickType clickType, @NotNull Player player, @NotNull InventoryClickEvent event) {
         if (clickHandler != null && clickHandler.apply(new Click(event))) notifyWindows();
     }
     
diff --git a/InvUI/src/main/java/de/studiocode/invui/item/impl/controlitem/PageItem.java b/InvUI/src/main/java/de/studiocode/invui/item/impl/controlitem/PageItem.java
index 88368f9..d16f035 100644
--- a/InvUI/src/main/java/de/studiocode/invui/item/impl/controlitem/PageItem.java
+++ b/InvUI/src/main/java/de/studiocode/invui/item/impl/controlitem/PageItem.java
@@ -4,6 +4,7 @@ import de.studiocode.invui.gui.impl.PagedGUI;
 import org.bukkit.entity.Player;
 import org.bukkit.event.inventory.ClickType;
 import org.bukkit.event.inventory.InventoryClickEvent;
+import org.jetbrains.annotations.NotNull;
 
 /**
  * Switches between pages in a {@link PagedGUI}
@@ -17,7 +18,7 @@ public abstract class PageItem extends ControlItem<PagedGUI> {
     }
     
     @Override
-    public void handleClick(ClickType clickType, Player player, InventoryClickEvent event) {
+    public void handleClick(@NotNull ClickType clickType, @NotNull Player player, @NotNull InventoryClickEvent event) {
         if (clickType == ClickType.LEFT) {
             if (forward) getGui().goForward();
             else getGui().goBack();
diff --git a/InvUI/src/main/java/de/studiocode/invui/item/impl/controlitem/ScrollItem.java b/InvUI/src/main/java/de/studiocode/invui/item/impl/controlitem/ScrollItem.java
index db34a56..6542cbf 100644
--- a/InvUI/src/main/java/de/studiocode/invui/item/impl/controlitem/ScrollItem.java
+++ b/InvUI/src/main/java/de/studiocode/invui/item/impl/controlitem/ScrollItem.java
@@ -4,6 +4,7 @@ import de.studiocode.invui.gui.impl.ScrollGUI;
 import org.bukkit.entity.Player;
 import org.bukkit.event.inventory.ClickType;
 import org.bukkit.event.inventory.InventoryClickEvent;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.HashMap;
 
@@ -24,7 +25,7 @@ public abstract class ScrollItem extends ControlItem<ScrollGUI> {
     }
     
     @Override
-    public void handleClick(ClickType clickType, Player player, InventoryClickEvent event) {
+    public void handleClick(@NotNull ClickType clickType, @NotNull Player player, @NotNull InventoryClickEvent event) {
         if (scroll.containsKey(clickType)) getGui().scroll(scroll.get(clickType));
     }
     
diff --git a/InvUI/src/main/java/de/studiocode/invui/item/impl/controlitem/TabItem.java b/InvUI/src/main/java/de/studiocode/invui/item/impl/controlitem/TabItem.java
index 0a85ac2..b392d97 100644
--- a/InvUI/src/main/java/de/studiocode/invui/item/impl/controlitem/TabItem.java
+++ b/InvUI/src/main/java/de/studiocode/invui/item/impl/controlitem/TabItem.java
@@ -4,6 +4,7 @@ import de.studiocode.invui.gui.impl.TabGUI;
 import org.bukkit.entity.Player;
 import org.bukkit.event.inventory.ClickType;
 import org.bukkit.event.inventory.InventoryClickEvent;
+import org.jetbrains.annotations.NotNull;
 
 /**
  * Switches between tabs in a {@link TabGUI}
@@ -17,7 +18,7 @@ public abstract class TabItem extends ControlItem<TabGUI> {
     }
     
     @Override
-    public void handleClick(ClickType clickType, Player player, InventoryClickEvent event) {
+    public void handleClick(@NotNull ClickType clickType, @NotNull Player player, @NotNull InventoryClickEvent event) {
         if (clickType == ClickType.LEFT) getGui().showTab(tab);
     }