diff --git a/src/main/java/de/studiocode/invui/gui/GUI.java b/src/main/java/de/studiocode/invui/gui/GUI.java
index f786bc2..871daf3 100644
--- a/src/main/java/de/studiocode/invui/gui/GUI.java
+++ b/src/main/java/de/studiocode/invui/gui/GUI.java
@@ -68,7 +68,7 @@ public interface GUI extends GUIParent {
      * @param y           The y coordinate
      * @param slotElement The {@link SlotElement} to be placed there.
      */
-    void setSlotElement(int x, int y, SlotElement slotElement);
+    void setSlotElement(int x, int y, @Nullable SlotElement slotElement);
     
     /**
      * Sets the {@link SlotElement} on these coordinates.
@@ -77,7 +77,7 @@ public interface GUI extends GUIParent {
      * @param index       The slot index
      * @param slotElement The {@link SlotElement} to be placed there.
      */
-    void setSlotElement(int index, SlotElement slotElement);
+    void setSlotElement(int index, @Nullable SlotElement slotElement);
     
     /**
      * Gets the {@link SlotElement} on these coordinates.
@@ -271,7 +271,7 @@ public interface GUI extends GUIParent {
      * @param item            The {@link Item} that should be used or null to remove an existing item.
      * @param replaceExisting If existing {@link Item}s should be replaced.
      */
-    void fill(int start, int end, Item item, boolean replaceExisting);
+    void fill(int start, int end, @Nullable Item item, boolean replaceExisting);
     
     /**
      * Fills the entire {@link GUI} with {@link Item}s.
@@ -279,7 +279,7 @@ public interface GUI extends GUIParent {
      * @param item            The {@link Item} that should be used or null to remove an existing item.
      * @param replaceExisting If existing {@link Item}s should be replaced.
      */
-    void fill(Item item, boolean replaceExisting);
+    void fill(@Nullable Item item, boolean replaceExisting);
     
     /**
      * Fills one row with an specific {@link Item}
@@ -288,7 +288,7 @@ public interface GUI extends GUIParent {
      * @param item            The {@link Item} that should be used or null to remove an existing item.
      * @param replaceExisting If existing {@link Item}s should be replaced.
      */
-    void fillRow(int row, Item item, boolean replaceExisting);
+    void fillRow(int row, @Nullable Item item, boolean replaceExisting);
     
     /**
      * Fills one column with an specific {@link Item}
@@ -297,7 +297,7 @@ public interface GUI extends GUIParent {
      * @param item            The {@link Item} that should be used or null to remove an existing item.
      * @param replaceExisting If existing {@link Item}s should be replaced.
      */
-    void fillColumn(int column, Item item, boolean replaceExisting);
+    void fillColumn(int column, @Nullable Item item, boolean replaceExisting);
     
     /**
      * Fills the borders of this {@link GUI} with an specific {@link Item}
@@ -305,7 +305,7 @@ public interface GUI extends GUIParent {
      * @param item            The {@link Item} that should be used or null to remove an existing item.
      * @param replaceExisting If existing {@link Item}s should be replaced.
      */
-    void fillBorders(Item item, boolean replaceExisting);
+    void fillBorders(@Nullable Item item, boolean replaceExisting);
     
     /**
      * Fills a rectangle in this {@link GUI} with an specific {@link Item}
@@ -317,7 +317,7 @@ public interface GUI extends GUIParent {
      * @param item            The {@link Item} that should be used or null to remove an existing item.
      * @param replaceExisting If existing {@link Item}s should be replaced.
      */
-    void fillRectangle(int x, int y, int width, int height, Item item, boolean replaceExisting);
+    void fillRectangle(int x, int y, int width, int height, @Nullable Item item, boolean replaceExisting);
     
     /**
      * Fills a rectangle of another {@link GUI} in this {@link GUI}.
@@ -327,7 +327,7 @@ public interface GUI extends GUIParent {
      * @param gui             The {@link GUI} to be put into this {@link GUI}
      * @param replaceExisting If existing {@link SlotElement}s should be replaced.
      */
-    void fillRectangle(int x, int y, GUI gui, boolean replaceExisting);
+    void fillRectangle(int x, int y, @NotNull GUI gui, boolean replaceExisting);
     
     /**
      * Fills a rectangle of a {@link VirtualInventory} in this {@link GUI}.
@@ -338,6 +338,6 @@ public interface GUI extends GUIParent {
      * @param virtualInventory The {@link VirtualInventory} to be put into this {@link GUI}.
      * @param replaceExisting  If existing {@link SlotElement}s should be replaced.
      */
-    void fillRectangle(int x, int y, int width, VirtualInventory virtualInventory, boolean replaceExisting);
+    void fillRectangle(int x, int y, int width, @NotNull VirtualInventory virtualInventory, boolean replaceExisting);
     
 }
diff --git a/src/main/java/de/studiocode/invui/gui/impl/BaseGUI.java b/src/main/java/de/studiocode/invui/gui/impl/BaseGUI.java
index 0b3b8e9..c7480ee 100644
--- a/src/main/java/de/studiocode/invui/gui/impl/BaseGUI.java
+++ b/src/main/java/de/studiocode/invui/gui/impl/BaseGUI.java
@@ -9,6 +9,7 @@ import de.studiocode.invui.item.Item;
 import de.studiocode.invui.util.SlotUtils;
 import de.studiocode.invui.virtualinventory.VirtualInventory;
 import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 import java.util.Set;
 
@@ -79,7 +80,7 @@ public abstract class BaseGUI extends IndexedGUI {
     
     // filling methods
     
-    public void fill(@NotNull Set<Integer> slots, Item item, boolean replaceExisting) {
+    public void fill(@NotNull Set<Integer> slots, @Nullable Item item, boolean replaceExisting) {
         for (int slot : slots) {
             if (!replaceExisting && hasSlotElement(slot)) continue;
             setItem(slot, item);
@@ -87,7 +88,7 @@ public abstract class BaseGUI extends IndexedGUI {
     }
     
     @Override
-    public void fill(int start, int end, Item item, boolean replaceExisting) {
+    public void fill(int start, int end, @Nullable Item item, boolean replaceExisting) {
         for (int i = start; i < end; i++) {
             if (!replaceExisting && hasSlotElement(i)) continue;
             setItem(i, item);
@@ -95,34 +96,34 @@ public abstract class BaseGUI extends IndexedGUI {
     }
     
     @Override
-    public void fill(Item item, boolean replaceExisting) {
+    public void fill(@Nullable Item item, boolean replaceExisting) {
         fill(0, getSize(), item, replaceExisting);
     }
     
     @Override
-    public void fillRow(int row, Item item, boolean replaceExisting) {
+    public void fillRow(int row, @Nullable Item item, boolean replaceExisting) {
         if (row >= height) throw new IllegalArgumentException("Row out of bounds");
         fill(SlotUtils.getSlotsRow(row, width), item, replaceExisting);
     }
     
     @Override
-    public void fillColumn(int column, Item item, boolean replaceExisting) {
+    public void fillColumn(int column, @Nullable Item item, boolean replaceExisting) {
         if (column >= width) throw new IllegalArgumentException("Column out of bounds");
         fill(SlotUtils.getSlotsColumn(column, width, height), item, replaceExisting);
     }
     
     @Override
-    public void fillBorders(Item item, boolean replaceExisting) {
+    public void fillBorders(@Nullable Item item, boolean replaceExisting) {
         fill(SlotUtils.getSlotsBorders(width, height), item, replaceExisting);
     }
     
     @Override
-    public void fillRectangle(int x, int y, int width, int height, Item item, boolean replaceExisting) {
+    public void fillRectangle(int x, int y, int width, int height, @Nullable Item item, boolean replaceExisting) {
         fill(SlotUtils.getSlotsRect(x, y, width, height, this.width), item, replaceExisting);
     }
     
     @Override
-    public void fillRectangle(int x, int y, GUI gui, boolean replaceExisting) {
+    public void fillRectangle(int x, int y, @NotNull GUI gui, boolean replaceExisting) {
         int slotIndex = 0;
         for (int slot : SlotUtils.getSlotsRect(x, y, gui.getWidth(), gui.getHeight(), this.width)) {
             if (hasSlotElement(slot) && !replaceExisting) continue;
@@ -132,7 +133,7 @@ public abstract class BaseGUI extends IndexedGUI {
     }
     
     @Override
-    public void fillRectangle(int x, int y, int width, VirtualInventory virtualInventory, boolean replaceExisting) {
+    public void fillRectangle(int x, int y, int width, @NotNull VirtualInventory virtualInventory, boolean replaceExisting) {
         int height = (int) Math.ceil((double) virtualInventory.getSize() / (double) width);
         
         int slotIndex = 0;
diff --git a/src/main/java/de/studiocode/invui/util/SlotUtils.java b/src/main/java/de/studiocode/invui/util/SlotUtils.java
index a3530ed..2d9c7b3 100644
--- a/src/main/java/de/studiocode/invui/util/SlotUtils.java
+++ b/src/main/java/de/studiocode/invui/util/SlotUtils.java
@@ -1,5 +1,7 @@
 package de.studiocode.invui.util;
 
+import org.jetbrains.annotations.NotNull;
+
 import java.util.Arrays;
 import java.util.LinkedHashSet;
 import java.util.Set;
@@ -7,18 +9,21 @@ import java.util.stream.Collectors;
 
 public class SlotUtils {
     
+    @NotNull
     public static Set<Integer> getSlotsRow(int row, int width) {
         Set<Integer> slots = new LinkedHashSet<>();
         for (int x = 0; x < width; x++) slots.add(convertToIndex(x, row, width));
         return slots;
     }
     
+    @NotNull
     public static Set<Integer> getSlotsColumn(int column, int width, int height) {
         Set<Integer> slots = new LinkedHashSet<>();
         for (int y = 0; y < height; y++) slots.add(convertToIndex(column, y, width));
         return slots;
     }
     
+    @NotNull
     public static Set<Integer> getSlotsBorders(int width, int height) {
         Set<Integer> slots = new LinkedHashSet<>();
         if (height > 0) slots.addAll(getSlotsRow(0, width));
@@ -28,11 +33,13 @@ public class SlotUtils {
         return slots;
     }
     
+    @NotNull
     public static Set<Integer> getSlotsRect(int x, int y, int width, int height, int frameWidth) {
         return getSlotsRect(Order.HORIZONTAL, x, y, width, height, frameWidth);
     }
     
-    public static Set<Integer> getSlotsRect(Order order, int x, int y, int width, int height, int frameWidth) {
+    @NotNull
+    public static Set<Integer> getSlotsRect(@NotNull Order order, int x, int y, int width, int height, int frameWidth) {
         Set<Integer> slots = new LinkedHashSet<>();
         
         switch (order) {
@@ -50,12 +57,13 @@ public class SlotUtils {
                         slots.add(convertToIndex(x1, y1, frameWidth));
                     }
                 }
+                break;
         }
         
         return slots;
     }
     
-    public static int convertToIndex(Point2D point, int width) {
+    public static int convertToIndex(@NotNull Point2D point, int width) {
         return convertToIndex(point.getX(), point.getY(), width);
     }
     
@@ -63,6 +71,7 @@ public class SlotUtils {
         return y * width + x;
     }
     
+    @NotNull
     public static Point2D convertFromIndex(int index, int width) {
         return new Point2D(index % width, index / width);
     }
@@ -97,7 +106,7 @@ public class SlotUtils {
         return longestLength;
     }
     
-    public static boolean isNeighbor(Point2D point1, Point2D point2) {
+    public static boolean isNeighbor(@NotNull Point2D point1, @NotNull Point2D point2) {
         return Math.abs(point1.getX() - point2.getX()) + Math.abs(point1.getY() - point2.getY()) == 1;
     }
     
diff --git a/src/main/java/de/studiocode/invui/window/Window.java b/src/main/java/de/studiocode/invui/window/Window.java
index f67cc54..bb13555 100644
--- a/src/main/java/de/studiocode/invui/window/Window.java
+++ b/src/main/java/de/studiocode/invui/window/Window.java
@@ -5,8 +5,8 @@ import de.studiocode.invui.gui.GUIParent;
 import de.studiocode.invui.item.Item;
 import de.studiocode.invui.item.itembuilder.ItemBuilder;
 import de.studiocode.invui.virtualinventory.VirtualInventory;
-import de.studiocode.invui.window.impl.combined.combinedgui.SimpleCombinedWindow;
-import de.studiocode.invui.window.impl.combined.splitgui.SimpleSplitWindow;
+import de.studiocode.invui.window.impl.merged.combined.SimpleCombinedWindow;
+import de.studiocode.invui.window.impl.merged.split.SimpleSplitWindow;
 import de.studiocode.invui.window.impl.single.SimpleWindow;
 import org.bukkit.entity.Player;
 import org.bukkit.event.entity.PlayerDeathEvent;
diff --git a/src/main/java/de/studiocode/invui/window/WindowManager.java b/src/main/java/de/studiocode/invui/window/WindowManager.java
index 75113cc..73941b8 100644
--- a/src/main/java/de/studiocode/invui/window/WindowManager.java
+++ b/src/main/java/de/studiocode/invui/window/WindowManager.java
@@ -1,7 +1,7 @@
 package de.studiocode.invui.window;
 
 import de.studiocode.invui.InvUI;
-import de.studiocode.invui.window.impl.combined.BaseCombinedWindow;
+import de.studiocode.invui.window.impl.merged.MergedWindow;
 import org.bukkit.Bukkit;
 import org.bukkit.entity.Entity;
 import org.bukkit.entity.Player;
@@ -153,7 +153,7 @@ public class WindowManager implements Listener {
         Entity entity = event.getEntity();
         if (entity instanceof Player) {
             Optional<Window> window = findOpenWindow(((Player) entity));
-            if (window.isPresent() && window.get() instanceof BaseCombinedWindow)
+            if (window.isPresent() && window.get() instanceof MergedWindow)
                 event.setCancelled(true);
         }
     }
diff --git a/src/main/java/de/studiocode/invui/window/impl/combined/BaseCombinedWindow.java b/src/main/java/de/studiocode/invui/window/impl/merged/MergedWindow.java
similarity index 94%
rename from src/main/java/de/studiocode/invui/window/impl/combined/BaseCombinedWindow.java
rename to src/main/java/de/studiocode/invui/window/impl/merged/MergedWindow.java
index 5ee4efb..d5795f1 100644
--- a/src/main/java/de/studiocode/invui/window/impl/combined/BaseCombinedWindow.java
+++ b/src/main/java/de/studiocode/invui/window/impl/merged/MergedWindow.java
@@ -1,4 +1,4 @@
-package de.studiocode.invui.window.impl.combined;
+package de.studiocode.invui.window.impl.merged;
 
 import de.studiocode.invui.gui.SlotElement.ItemStackHolder;
 import de.studiocode.invui.gui.SlotElement.VISlotElement;
@@ -18,14 +18,14 @@ import java.util.Objects;
 /**
  * A {@link Window} that uses both top and player {@link Inventory}.
  */
-public abstract class BaseCombinedWindow extends BaseWindow {
+public abstract class MergedWindow extends BaseWindow {
     
     private final Inventory playerInventory;
     private final ItemStack[] playerItems = new ItemStack[36];
     protected Inventory upperInventory;
     private boolean isCurrentlyOpened;
     
-    public BaseCombinedWindow(Player player, int size, Inventory upperInventory, boolean closeable, boolean closeOnEvent) {
+    public MergedWindow(Player player, int size, Inventory upperInventory, boolean closeable, boolean closeOnEvent) {
         super(player.getUniqueId(), size, closeable, closeOnEvent);
         this.upperInventory = upperInventory;
         this.playerInventory = player.getInventory();
diff --git a/src/main/java/de/studiocode/invui/window/impl/combined/combinedgui/CombinedWindow.java b/src/main/java/de/studiocode/invui/window/impl/merged/combined/CombinedWindow.java
similarity index 88%
rename from src/main/java/de/studiocode/invui/window/impl/combined/combinedgui/CombinedWindow.java
rename to src/main/java/de/studiocode/invui/window/impl/merged/combined/CombinedWindow.java
index 0cf1e5d..a844c87 100644
--- a/src/main/java/de/studiocode/invui/window/impl/combined/combinedgui/CombinedWindow.java
+++ b/src/main/java/de/studiocode/invui/window/impl/merged/combined/CombinedWindow.java
@@ -1,10 +1,10 @@
-package de.studiocode.invui.window.impl.combined.combinedgui;
+package de.studiocode.invui.window.impl.merged.combined;
 
 import de.studiocode.invui.gui.GUI;
 import de.studiocode.invui.gui.SlotElement.ItemStackHolder;
 import de.studiocode.invui.util.SlotUtils;
 import de.studiocode.invui.window.Window;
-import de.studiocode.invui.window.impl.combined.BaseCombinedWindow;
+import de.studiocode.invui.window.impl.merged.MergedWindow;
 import org.bukkit.entity.Player;
 import org.bukkit.event.inventory.InventoryClickEvent;
 import org.bukkit.inventory.Inventory;
@@ -12,7 +12,7 @@ import org.bukkit.inventory.Inventory;
 /**
  * A {@link Window} where top and player {@link Inventory} are affected by the same {@link GUI}.
  */
-public abstract class CombinedWindow extends BaseCombinedWindow {
+public abstract class CombinedWindow extends MergedWindow {
     
     private final GUI gui;
     
diff --git a/src/main/java/de/studiocode/invui/window/impl/combined/combinedgui/SimpleCombinedWindow.java b/src/main/java/de/studiocode/invui/window/impl/merged/combined/SimpleCombinedWindow.java
similarity index 81%
rename from src/main/java/de/studiocode/invui/window/impl/combined/combinedgui/SimpleCombinedWindow.java
rename to src/main/java/de/studiocode/invui/window/impl/merged/combined/SimpleCombinedWindow.java
index 8649969..6191472 100644
--- a/src/main/java/de/studiocode/invui/window/impl/combined/combinedgui/SimpleCombinedWindow.java
+++ b/src/main/java/de/studiocode/invui/window/impl/merged/combined/SimpleCombinedWindow.java
@@ -1,4 +1,4 @@
-package de.studiocode.invui.window.impl.combined.combinedgui;
+package de.studiocode.invui.window.impl.merged.combined;
 
 import de.studiocode.invui.gui.GUI;
 import org.bukkit.Bukkit;
@@ -11,6 +11,10 @@ public class SimpleCombinedWindow extends CombinedWindow {
         super(player, gui, createInventory(gui, title), closeable, closeOnEvent);
     }
     
+    public SimpleCombinedWindow(Player player, String title, GUI gui) {
+        this(player, title, gui, true, true);
+    }
+    
     private static Inventory createInventory(GUI gui, String title) {
         if (gui.getWidth() != 9)
             throw new IllegalArgumentException("GUI width has to be 9");
diff --git a/src/main/java/de/studiocode/invui/window/impl/combined/splitgui/AnvilSplitWindow.java b/src/main/java/de/studiocode/invui/window/impl/merged/split/AnvilSplitWindow.java
similarity index 84%
rename from src/main/java/de/studiocode/invui/window/impl/combined/splitgui/AnvilSplitWindow.java
rename to src/main/java/de/studiocode/invui/window/impl/merged/split/AnvilSplitWindow.java
index ea965e1..a4d9519 100644
--- a/src/main/java/de/studiocode/invui/window/impl/combined/splitgui/AnvilSplitWindow.java
+++ b/src/main/java/de/studiocode/invui/window/impl/merged/split/AnvilSplitWindow.java
@@ -1,4 +1,4 @@
-package de.studiocode.invui.window.impl.combined.splitgui;
+package de.studiocode.invui.window.impl.merged.split;
 
 import de.studiocode.inventoryaccess.api.abstraction.inventory.AnvilInventory;
 import de.studiocode.inventoryaccess.api.version.InventoryAccess;
@@ -23,6 +23,10 @@ public class AnvilSplitWindow extends SplitWindow {
         initUpperItems();
     }
     
+    public AnvilSplitWindow(Player player, String title, GUI upperGui, GUI lowerGui, Consumer<String> renameHandler) {
+        this(player, title, upperGui, lowerGui, true, renameHandler);
+    }
+    
     @Override
     protected void setUpperInvItem(int slot, ItemStack itemStack) {
         anvilInventory.setItem(slot, itemStack);
diff --git a/src/main/java/de/studiocode/invui/window/impl/combined/splitgui/SimpleSplitWindow.java b/src/main/java/de/studiocode/invui/window/impl/merged/split/SimpleSplitWindow.java
similarity index 67%
rename from src/main/java/de/studiocode/invui/window/impl/combined/splitgui/SimpleSplitWindow.java
rename to src/main/java/de/studiocode/invui/window/impl/merged/split/SimpleSplitWindow.java
index 3ed3cca..a512b58 100644
--- a/src/main/java/de/studiocode/invui/window/impl/combined/splitgui/SimpleSplitWindow.java
+++ b/src/main/java/de/studiocode/invui/window/impl/merged/split/SimpleSplitWindow.java
@@ -1,4 +1,4 @@
-package de.studiocode.invui.window.impl.combined.splitgui;
+package de.studiocode.invui.window.impl.merged.split;
 
 import de.studiocode.invui.gui.GUI;
 import de.studiocode.invui.util.InventoryUtils;
@@ -10,4 +10,8 @@ public class SimpleSplitWindow extends SplitWindow {
         super(player, upperGui, lowerGui, InventoryUtils.createMatchingInventory(upperGui, title), true, closeable, closeOnEvent);
     }
     
+    public SimpleSplitWindow(Player player, String title, GUI upperGui, GUI lowerGui) {
+        this(player, title, upperGui, lowerGui, true, true);
+    }
+    
 }
diff --git a/src/main/java/de/studiocode/invui/window/impl/combined/splitgui/SplitWindow.java b/src/main/java/de/studiocode/invui/window/impl/merged/split/SplitWindow.java
similarity index 91%
rename from src/main/java/de/studiocode/invui/window/impl/combined/splitgui/SplitWindow.java
rename to src/main/java/de/studiocode/invui/window/impl/merged/split/SplitWindow.java
index 46db1f1..c16d911 100644
--- a/src/main/java/de/studiocode/invui/window/impl/combined/splitgui/SplitWindow.java
+++ b/src/main/java/de/studiocode/invui/window/impl/merged/split/SplitWindow.java
@@ -1,10 +1,10 @@
-package de.studiocode.invui.window.impl.combined.splitgui;
+package de.studiocode.invui.window.impl.merged.split;
 
 import de.studiocode.invui.gui.GUI;
 import de.studiocode.invui.gui.SlotElement.ItemStackHolder;
 import de.studiocode.invui.util.SlotUtils;
 import de.studiocode.invui.window.Window;
-import de.studiocode.invui.window.impl.combined.BaseCombinedWindow;
+import de.studiocode.invui.window.impl.merged.MergedWindow;
 import org.bukkit.entity.Player;
 import org.bukkit.event.inventory.InventoryClickEvent;
 import org.bukkit.inventory.Inventory;
@@ -12,7 +12,7 @@ import org.bukkit.inventory.Inventory;
 /**
  * A {@link Window} where top and player {@link Inventory} are affected by different {@link GUI}s.
  */
-public abstract class SplitWindow extends BaseCombinedWindow {
+public abstract class SplitWindow extends MergedWindow {
     
     private final GUI upperGui;
     private final GUI lowerGui;
diff --git a/src/main/java/de/studiocode/invui/window/impl/single/AnvilWindow.java b/src/main/java/de/studiocode/invui/window/impl/single/AnvilWindow.java
index a1c74f0..a4763fe 100644
--- a/src/main/java/de/studiocode/invui/window/impl/single/AnvilWindow.java
+++ b/src/main/java/de/studiocode/invui/window/impl/single/AnvilWindow.java
@@ -20,6 +20,10 @@ public class AnvilWindow extends SingleWindow {
         initItems();
     }
     
+    public AnvilWindow(Player player, String title, GUI gui, Consumer<String> renameHandler) {
+        this(player, title, gui, true, renameHandler);
+    }
+    
     @Override
     protected void setInvItem(int slot, ItemStack itemStack) {
         anvilInventory.setItem(slot, itemStack);