diff --git a/invui/src/main/java/xyz/xenondevs/invui/gui/AbstractGui.java b/invui/src/main/java/xyz/xenondevs/invui/gui/AbstractGui.java
index 0316828..e1d9dff 100644
--- a/invui/src/main/java/xyz/xenondevs/invui/gui/AbstractGui.java
+++ b/invui/src/main/java/xyz/xenondevs/invui/gui/AbstractGui.java
@@ -694,91 +694,91 @@ public abstract class AbstractGui implements Gui, GuiParent {
         protected List<Consumer<Gui>> modifiers;
         
         @Override
-        public S setStructure(int width, int height, @NotNull String structureData) {
+        public @NotNull S setStructure(int width, int height, @NotNull String structureData) {
             structure = new Structure(width, height, structureData);
             return (S) this;
         }
         
         @Override
-        public S setStructure(@NotNull String... structureData) {
+        public @NotNull S setStructure(@NotNull String... structureData) {
             structure = new Structure(structureData);
             return (S) this;
         }
         
         @Override
-        public S setStructure(@NotNull Structure structure) {
+        public @NotNull S setStructure(@NotNull Structure structure) {
             this.structure = structure;
             return (S) this;
         }
         
         @Override
-        public S addIngredient(char key, @NotNull ItemStack itemStack) {
+        public @NotNull S addIngredient(char key, @NotNull ItemStack itemStack) {
             structure.addIngredient(key, itemStack);
             return (S) this;
         }
         
         @Override
-        public S addIngredient(char key, @NotNull ItemProvider itemProvider) {
+        public @NotNull S addIngredient(char key, @NotNull ItemProvider itemProvider) {
             structure.addIngredient(key, itemProvider);
             return (S) this;
         }
         
         @Override
-        public S addIngredient(char key, @NotNull Item item) {
+        public @NotNull S addIngredient(char key, @NotNull Item item) {
             structure.addIngredient(key, item);
             return (S) this;
         }
         
         @Override
-        public S addIngredient(char key, @NotNull VirtualInventory inventory) {
+        public @NotNull S addIngredient(char key, @NotNull VirtualInventory inventory) {
             structure.addIngredient(key, inventory);
             return (S) this;
         }
         
         @Override
-        public S addIngredient(char key, @NotNull VirtualInventory inventory, @Nullable ItemProvider background) {
+        public @NotNull S addIngredient(char key, @NotNull VirtualInventory inventory, @Nullable ItemProvider background) {
             structure.addIngredient(key, inventory, background);
             return (S) this;
         }
         
         @Override
-        public S addIngredient(char key, @NotNull SlotElement element) {
+        public @NotNull S addIngredient(char key, @NotNull SlotElement element) {
             structure.addIngredient(key, element);
             return (S) this;
         }
         
         @Override
-        public S addIngredient(char key, @NotNull Marker marker) {
+        public @NotNull S addIngredient(char key, @NotNull Marker marker) {
             structure.addIngredient(key, marker);
             return (S) this;
         }
         
         @Override
-        public S addIngredient(char key, @NotNull Supplier<? extends Item> itemSupplier) {
+        public @NotNull S addIngredient(char key, @NotNull Supplier<? extends Item> itemSupplier) {
             structure.addIngredient(key, itemSupplier);
             return (S) this;
         }
         
         @Override
-        public S addIngredientElementSupplier(char key, @NotNull Supplier<? extends SlotElement> elementSupplier) {
+        public @NotNull S addIngredientElementSupplier(char key, @NotNull Supplier<? extends SlotElement> elementSupplier) {
             structure.addIngredientElementSupplier(key, elementSupplier);
             return (S) this;
         }
         
         @Override
-        public S setBackground(@NotNull ItemProvider itemProvider) {
+        public @NotNull S setBackground(@NotNull ItemProvider itemProvider) {
             background = itemProvider;
             return (S) this;
         }
         
         @Override
-        public S setBackground(@NotNull ItemStack itemStack) {
+        public @NotNull S setBackground(@NotNull ItemStack itemStack) {
             background = new ItemWrapper(itemStack);
             return (S) this;
         }
         
         @Override
-        public S addModifier(@NotNull Consumer<@NotNull Gui> modifier) {
+        public @NotNull S addModifier(@NotNull Consumer<@NotNull Gui> modifier) {
             if (modifiers == null)
                 modifiers = new ArrayList<>();
             
@@ -787,7 +787,7 @@ public abstract class AbstractGui implements Gui, GuiParent {
         }
         
         @Override
-        public S setModifiers(@NotNull List<@NotNull Consumer<@NotNull Gui>> modifiers) {
+        public @NotNull S setModifiers(@NotNull List<@NotNull Consumer<@NotNull Gui>> modifiers) {
             this.modifiers = modifiers;
             return (S) this;
         }
diff --git a/invui/src/main/java/xyz/xenondevs/invui/gui/AbstractPagedGui.java b/invui/src/main/java/xyz/xenondevs/invui/gui/AbstractPagedGui.java
index 3619c76..e6aa991 100644
--- a/invui/src/main/java/xyz/xenondevs/invui/gui/AbstractPagedGui.java
+++ b/invui/src/main/java/xyz/xenondevs/invui/gui/AbstractPagedGui.java
@@ -140,13 +140,13 @@ public abstract class AbstractPagedGui<C> extends AbstractGui implements PagedGu
         protected List<BiConsumer<Integer, Integer>> pageChangeHandlers;
         
         @Override
-        public PagedGui.Builder<C> setContent(@NotNull List<@NotNull C> content) {
+        public PagedGui.@NotNull Builder<C> setContent(@NotNull List<@NotNull C> content) {
             this.content = content;
             return this;
         }
         
         @Override
-        public PagedGui.Builder<C> addContent(@NotNull C content) {
+        public PagedGui.@NotNull Builder<C> addContent(@NotNull C content) {
             if (this.content == null)
                 this.content = new ArrayList<>();
             
@@ -155,13 +155,13 @@ public abstract class AbstractPagedGui<C> extends AbstractGui implements PagedGu
         }
         
         @Override
-        public PagedGui.Builder<C> setPageChangeHandlers(@NotNull List<@NotNull BiConsumer<Integer, Integer>> handlers) {
+        public PagedGui.@NotNull Builder<C> setPageChangeHandlers(@NotNull List<@NotNull BiConsumer<Integer, Integer>> handlers) {
             pageChangeHandlers = handlers;
             return this;
         }
         
         @Override
-        public PagedGui.Builder<C> addPageChangeHandler(@NotNull BiConsumer<Integer, Integer> handler) {
+        public PagedGui.@NotNull Builder<C> addPageChangeHandler(@NotNull BiConsumer<Integer, Integer> handler) {
             if (pageChangeHandlers == null)
                 pageChangeHandlers = new ArrayList<>(1);
             
diff --git a/invui/src/main/java/xyz/xenondevs/invui/gui/AbstractScrollGui.java b/invui/src/main/java/xyz/xenondevs/invui/gui/AbstractScrollGui.java
index 4f3373e..db220a7 100644
--- a/invui/src/main/java/xyz/xenondevs/invui/gui/AbstractScrollGui.java
+++ b/invui/src/main/java/xyz/xenondevs/invui/gui/AbstractScrollGui.java
@@ -141,13 +141,13 @@ public abstract class AbstractScrollGui<C> extends AbstractGui implements Scroll
         protected List<BiConsumer<Integer, Integer>> scrollHandlers;
         
         @Override
-        public ScrollGui.Builder<C> setContent(@NotNull List<@NotNull C> content) {
+        public ScrollGui.@NotNull Builder<C> setContent(@NotNull List<@NotNull C> content) {
             this.content = content;
             return this;
         }
         
         @Override
-        public ScrollGui.Builder<C> addContent(@NotNull C content) {
+        public ScrollGui.@NotNull Builder<C> addContent(@NotNull C content) {
             if (this.content == null)
                 this.content = new ArrayList<>();
             
@@ -156,13 +156,13 @@ public abstract class AbstractScrollGui<C> extends AbstractGui implements Scroll
         }
         
         @Override
-        public ScrollGui.Builder<C> setScrollHandlers(@NotNull List<@NotNull BiConsumer<Integer, Integer>> handlers) {
+        public ScrollGui.@NotNull Builder<C> setScrollHandlers(@NotNull List<@NotNull BiConsumer<Integer, Integer>> handlers) {
             scrollHandlers = handlers;
             return this;
         }
         
         @Override
-        public ScrollGui.Builder<C> addScrollHandler(@NotNull BiConsumer<Integer, Integer> handler) {
+        public ScrollGui.@NotNull Builder<C> addScrollHandler(@NotNull BiConsumer<Integer, Integer> handler) {
             if (scrollHandlers == null)
                 scrollHandlers = new ArrayList<>(1);
             
diff --git a/invui/src/main/java/xyz/xenondevs/invui/gui/AbstractTabGui.java b/invui/src/main/java/xyz/xenondevs/invui/gui/AbstractTabGui.java
index b762b4a..6086053 100644
--- a/invui/src/main/java/xyz/xenondevs/invui/gui/AbstractTabGui.java
+++ b/invui/src/main/java/xyz/xenondevs/invui/gui/AbstractTabGui.java
@@ -105,13 +105,13 @@ public abstract class AbstractTabGui extends AbstractGui implements TabGui {
         protected List<BiConsumer<Integer, Integer>> tabChangeHandlers;
         
         @Override
-        public TabGui.Builder setTabs(@NotNull List<@Nullable Gui> tabs) {
+        public TabGui.@NotNull Builder setTabs(@NotNull List<@Nullable Gui> tabs) {
             this.tabs = tabs;
             return this;
         }
         
         @Override
-        public TabGui.Builder addTab(@Nullable Gui tab) {
+        public TabGui.@NotNull Builder addTab(@Nullable Gui tab) {
             if (this.tabs == null)
                 this.tabs = new ArrayList<>();
             
@@ -120,7 +120,7 @@ public abstract class AbstractTabGui extends AbstractGui implements TabGui {
         }
         
         @Override
-        public TabGui.Builder addTabChangeHandler(@NotNull BiConsumer<Integer, Integer> handler) {
+        public TabGui.@NotNull Builder addTabChangeHandler(@NotNull BiConsumer<Integer, Integer> handler) {
             if (tabChangeHandlers == null)
                 tabChangeHandlers = new ArrayList<>(1);
             
@@ -129,7 +129,7 @@ public abstract class AbstractTabGui extends AbstractGui implements TabGui {
         }
         
         @Override
-        public TabGui.Builder setTabChangeHandlers(@NotNull List<@NotNull BiConsumer<Integer, Integer>> handlers) {
+        public TabGui.@NotNull Builder setTabChangeHandlers(@NotNull List<@NotNull BiConsumer<Integer, Integer>> handlers) {
             tabChangeHandlers = handlers;
             return this;
         }
diff --git a/invui/src/main/java/xyz/xenondevs/invui/gui/Gui.java b/invui/src/main/java/xyz/xenondevs/invui/gui/Gui.java
index b4a51da..01ebb39 100644
--- a/invui/src/main/java/xyz/xenondevs/invui/gui/Gui.java
+++ b/invui/src/main/java/xyz/xenondevs/invui/gui/Gui.java
@@ -392,7 +392,7 @@ public interface Gui {
          * @return This {@link Builder}
          */
         @Contract("_ -> this")
-        S setStructure(@NotNull Structure structure);
+        @NotNull S setStructure(@NotNull Structure structure);
         
         /**
          * Sets the {@link Structure} of the {@link Gui} using the given structure data Strings.
@@ -402,7 +402,7 @@ public interface Gui {
          * @return This {@link Builder Gui Builder}
          */
         @Contract("_ -> this")
-        S setStructure(@NotNull String... structureData);
+        @NotNull S setStructure(@NotNull String... structureData);
         
         /**
          * Sets the {@link Structure} of the {@link Gui} using the given structure data, width and height.
@@ -413,7 +413,7 @@ public interface Gui {
          * @return This {@link Builder Gui Builder}
          */
         @Contract("_, _, _, -> this")
-        S setStructure(int width, int height, @NotNull String structureData);
+        @NotNull S setStructure(int width, int height, @NotNull String structureData);
         
         /**
          * Adds an {@link ItemStack} ingredient under the given key.
@@ -423,7 +423,7 @@ public interface Gui {
          * @return This {@link Builder Gui Builder}
          */
         @Contract("_, _ -> this")
-        S addIngredient(char key, @NotNull ItemStack itemStack);
+        @NotNull S addIngredient(char key, @NotNull ItemStack itemStack);
         
         /**
          * Adds an {@link ItemProvider} ingredient under the given key.
@@ -433,7 +433,7 @@ public interface Gui {
          * @return This {@link Builder Gui Builder}
          */
         @Contract("_, _ -> this")
-        S addIngredient(char key, @NotNull ItemProvider itemProvider);
+        @NotNull S addIngredient(char key, @NotNull ItemProvider itemProvider);
         
         /**
          * Adds an {@link Item} ingredient under the given key.
@@ -443,7 +443,7 @@ public interface Gui {
          * @return This {@link Builder Gui Builder}
          */
         @Contract("_, _ -> this")
-        S addIngredient(char key, @NotNull Item item);
+        @NotNull S addIngredient(char key, @NotNull Item item);
         
         /**
          * Adds an {@link VirtualInventory} ingredient under the given key.
@@ -453,7 +453,7 @@ public interface Gui {
          * @return This {@link Builder Gui Builder}
          */
         @Contract("_, _ -> this")
-        S addIngredient(char key, @NotNull VirtualInventory inventory);
+        @NotNull S addIngredient(char key, @NotNull VirtualInventory inventory);
         
         /**
          * Adds an {@link VirtualInventory} ingredient under the given key.
@@ -464,7 +464,7 @@ public interface Gui {
          * @return This {@link Builder Gui Builder}
          */
         @Contract("_, _, _ -> this")
-        S addIngredient(char key, @NotNull VirtualInventory inventory, @Nullable ItemProvider background);
+        @NotNull S addIngredient(char key, @NotNull VirtualInventory inventory, @Nullable ItemProvider background);
         
         /**
          * Adds a {@link SlotElement} ingredient under the given key.
@@ -474,7 +474,7 @@ public interface Gui {
          * @return This {@link Builder Gui Builder}
          */
         @Contract("_, _ -> this")
-        S addIngredient(char key, @NotNull SlotElement element);
+        @NotNull S addIngredient(char key, @NotNull SlotElement element);
         
         /**
          * Adds a {@link Marker} ingredient under the given key.
@@ -484,7 +484,7 @@ public interface Gui {
          * @return This {@link Builder Gui Builder}
          */
         @Contract("_, _ -> this")
-        S addIngredient(char key, @NotNull Marker marker);
+        @NotNull S addIngredient(char key, @NotNull Marker marker);
         
         /**
          * Adds a {@link Supplier} of {@link Item Items} ingredient under the given key.
@@ -494,7 +494,7 @@ public interface Gui {
          * @return This {@link Builder Gui Builder}
          */
         @Contract("_, _ -> this")
-        S addIngredient(char key, @NotNull Supplier<? extends Item> itemSupplier);
+        @NotNull S addIngredient(char key, @NotNull Supplier<? extends Item> itemSupplier);
         
         /**
          * Adds a {@link Supplier} of {@link SlotElement SlotElements} ingredient under the given key.
@@ -504,7 +504,7 @@ public interface Gui {
          * @return This {@link Builder Gui Builder}
          */
         @Contract("_, _ -> this")
-        S addIngredientElementSupplier(char key, @NotNull Supplier<? extends SlotElement> elementSupplier);
+        @NotNull S addIngredientElementSupplier(char key, @NotNull Supplier<? extends SlotElement> elementSupplier);
         
         /**
          * Sets the background of the {@link Gui}.
@@ -513,7 +513,7 @@ public interface Gui {
          * @return This {@link Builder Gui Builder}
          */
         @Contract("_ -> this")
-        S setBackground(@NotNull ItemProvider itemProvider);
+        @NotNull S setBackground(@NotNull ItemProvider itemProvider);
         
         /**
          * Sets the background of the {@link Gui}.
@@ -522,7 +522,7 @@ public interface Gui {
          * @return This {@link Builder Gui Builder}
          */
         @Contract("_ -> this")
-        S setBackground(@NotNull ItemStack itemStack);
+        @NotNull S setBackground(@NotNull ItemStack itemStack);
         
         /**
          * Sets the background of the {@link Gui}.
@@ -531,7 +531,7 @@ public interface Gui {
          * @return This {@link Builder Gui Builder}
          */
         @Contract("_ -> this")
-        S addModifier(@NotNull Consumer<@NotNull Gui> modifier);
+        @NotNull S addModifier(@NotNull Consumer<@NotNull Gui> modifier);
         
         /**
          * Sets the background of the {@link Gui}.
@@ -540,7 +540,7 @@ public interface Gui {
          * @return This {@link Builder Gui Builder}
          */
         @Contract("_ -> this")
-        S setModifiers(@NotNull List<@NotNull Consumer<@NotNull Gui>> modifiers);
+        @NotNull S setModifiers(@NotNull List<@NotNull Consumer<@NotNull Gui>> modifiers);
         
         /**
          * Builds the {@link Gui}.
@@ -557,10 +557,10 @@ public interface Gui {
          */
         @Contract("-> new")
         @NotNull S clone();
-    
+        
         /**
          * A normal {@link Gui} builder.
-         * 
+         *
          * @see PagedGui.Builder
          * @see ScrollGui.Builder
          * @see TabGui.Builder
diff --git a/invui/src/main/java/xyz/xenondevs/invui/gui/PagedGui.java b/invui/src/main/java/xyz/xenondevs/invui/gui/PagedGui.java
index bc39496..77f4057 100644
--- a/invui/src/main/java/xyz/xenondevs/invui/gui/PagedGui.java
+++ b/invui/src/main/java/xyz/xenondevs/invui/gui/PagedGui.java
@@ -215,7 +215,7 @@ public interface PagedGui<C> extends Gui {
          * @return This {@link Builder Gui Builder}.
          */
         @Contract("_ -> this")
-        Builder<C> setContent(@NotNull List<@NotNull C> content);
+        @NotNull Builder<C> setContent(@NotNull List<@NotNull C> content);
         
         /**
          * Adds content to the {@link PagedGui}.
@@ -224,7 +224,7 @@ public interface PagedGui<C> extends Gui {
          * @return This {@link Builder Gui Builder}.
          */
         @Contract("_ -> this")
-        Builder<C> addContent(@NotNull C content);
+        @NotNull Builder<C> addContent(@NotNull C content);
         
         /**
          * Sets the page change handlers of the {@link PagedGui}.
@@ -233,7 +233,7 @@ public interface PagedGui<C> extends Gui {
          * @return This {@link Builder Gui Builder}.
          */
         @Contract("_ -> this")
-        Builder<C> setPageChangeHandlers(@NotNull List<@NotNull BiConsumer<Integer, Integer>> handlers);
+        @NotNull Builder<C> setPageChangeHandlers(@NotNull List<@NotNull BiConsumer<Integer, Integer>> handlers);
         
         /**
          * Adds a page change handler to the {@link PagedGui}.
@@ -242,7 +242,7 @@ public interface PagedGui<C> extends Gui {
          * @return This {@link Builder Gui Builder}.
          */
         @Contract("_ -> this")
-        Builder<C> addPageChangeHandler(@NotNull BiConsumer<Integer, Integer> handler);
+        @NotNull Builder<C> addPageChangeHandler(@NotNull BiConsumer<Integer, Integer> handler);
         
     }
     
diff --git a/invui/src/main/java/xyz/xenondevs/invui/gui/ScrollGui.java b/invui/src/main/java/xyz/xenondevs/invui/gui/ScrollGui.java
index 1a192d9..d953205 100644
--- a/invui/src/main/java/xyz/xenondevs/invui/gui/ScrollGui.java
+++ b/invui/src/main/java/xyz/xenondevs/invui/gui/ScrollGui.java
@@ -228,7 +228,7 @@ public interface ScrollGui<C> extends Gui {
          * @return This {@link Builder Gui Builder}.
          */
         @Contract("_ -> this")
-        Builder<C> setContent(@NotNull List<@NotNull C> content);
+        @NotNull Builder<C> setContent(@NotNull List<@NotNull C> content);
         
         /**
          * Adds content to the {@link ScrollGui}.
@@ -237,7 +237,7 @@ public interface ScrollGui<C> extends Gui {
          * @return This {@link Builder Gui Builder}.
          */
         @Contract("_ -> this")
-        Builder<C> addContent(@NotNull C content);
+        @NotNull Builder<C> addContent(@NotNull C content);
         
         /**
          * Adds content to the {@link ScrollGui}.
@@ -246,7 +246,7 @@ public interface ScrollGui<C> extends Gui {
          * @return This {@link Builder Gui Builder}.
          */
         @Contract("_ -> this")
-        Builder<C> setScrollHandlers(@NotNull List<@NotNull BiConsumer<Integer, Integer>> handlers);
+        @NotNull Builder<C> setScrollHandlers(@NotNull List<@NotNull BiConsumer<Integer, Integer>> handlers);
         
         /**
          * Adds a scroll handler to the {@link ScrollGui}.
@@ -255,7 +255,7 @@ public interface ScrollGui<C> extends Gui {
          * @return This {@link Builder Gui Builder}.
          */
         @Contract("_ -> this")
-        Builder<C> addScrollHandler(@NotNull BiConsumer<Integer, Integer> handler);
+        @NotNull Builder<C> addScrollHandler(@NotNull BiConsumer<Integer, Integer> handler);
         
     }
     
diff --git a/invui/src/main/java/xyz/xenondevs/invui/gui/TabGui.java b/invui/src/main/java/xyz/xenondevs/invui/gui/TabGui.java
index f2720d2..7327f9b 100644
--- a/invui/src/main/java/xyz/xenondevs/invui/gui/TabGui.java
+++ b/invui/src/main/java/xyz/xenondevs/invui/gui/TabGui.java
@@ -126,7 +126,7 @@ public interface TabGui extends Gui {
          * @return This {@link Builder Gui Builder}.
          */
         @Contract("_ -> this")
-        Builder setTabs(@NotNull List<@Nullable Gui> tabs);
+        @NotNull Builder setTabs(@NotNull List<@Nullable Gui> tabs);
         
         /**
          * Adds a tab to the {@link TabGui}.
@@ -136,7 +136,7 @@ public interface TabGui extends Gui {
          * @return This {@link Builder Gui Builder}.
          */
         @Contract("_ -> this")
-        Builder addTab(@Nullable Gui tab);
+        @NotNull Builder addTab(@Nullable Gui tab);
         
         /**
          * Sets the tab change handlers of the {@link TabGui}.
@@ -145,7 +145,7 @@ public interface TabGui extends Gui {
          * @return This {@link Builder Gui Builder}.
          */
         @Contract("_ -> this")
-        Builder setTabChangeHandlers(@NotNull List<@NotNull BiConsumer<Integer, Integer>> handlers);
+        @NotNull Builder setTabChangeHandlers(@NotNull List<@NotNull BiConsumer<Integer, Integer>> handlers);
         
         /**
          * Adds a tab change handler to the {@link TabGui}.
@@ -154,7 +154,7 @@ public interface TabGui extends Gui {
          * @return This {@link Builder Gui Builder}.
          */
         @Contract("_ -> this")
-        Builder addTabChangeHandler(@NotNull BiConsumer<Integer, Integer> handler);
+        @NotNull Builder addTabChangeHandler(@NotNull BiConsumer<Integer, Integer> handler);
         
     }
     
diff --git a/invui/src/main/java/xyz/xenondevs/invui/item/builder/AbstractItemBuilder.java b/invui/src/main/java/xyz/xenondevs/invui/item/builder/AbstractItemBuilder.java
index 3858f7d..c3f7fef 100644
--- a/invui/src/main/java/xyz/xenondevs/invui/item/builder/AbstractItemBuilder.java
+++ b/invui/src/main/java/xyz/xenondevs/invui/item/builder/AbstractItemBuilder.java
@@ -7,6 +7,7 @@ import org.bukkit.inventory.ItemFlag;
 import org.bukkit.inventory.ItemStack;
 import org.bukkit.inventory.meta.Damageable;
 import org.bukkit.inventory.meta.ItemMeta;
+import org.jetbrains.annotations.Contract;
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 import xyz.xenondevs.inventoryaccess.InventoryAccess;
@@ -23,7 +24,8 @@ import java.util.List;
 import java.util.function.Function;
 import java.util.stream.Collectors;
 
-public abstract class AbstractItemBuilder<T> implements ItemProvider {
+@SuppressWarnings("unchecked")
+public abstract class AbstractItemBuilder<S> implements ItemProvider {
     
     protected ItemStack base;
     protected Material material;
@@ -72,8 +74,9 @@ public abstract class AbstractItemBuilder<T> implements ItemProvider {
      *
      * @return The {@link ItemStack}
      */
+    @Contract(value = "_ -> new", pure = true)
     @Override
-    public ItemStack get(@Nullable String lang) {
+    public @NotNull ItemStack get(@Nullable String lang) {
         ItemStack itemStack;
         if (base != null) {
             itemStack = base;
@@ -142,101 +145,114 @@ public abstract class AbstractItemBuilder<T> implements ItemProvider {
         return itemStack;
     }
     
-    public T removeLoreLine(int index) {
+    @Contract("_ -> this")
+    public @NotNull S removeLoreLine(int index) {
         if (lore != null) lore.remove(index);
-        return getThis();
+        return (S) this;
     }
     
-    public T clearLore() {
+    @Contract("-> this")
+    public @NotNull S clearLore() {
         if (lore != null) lore.clear();
-        return getThis();
+        return (S) this;
     }
     
-    public ItemStack getBase() {
+    public @Nullable ItemStack getBase() {
         return base;
     }
     
-    public Material getMaterial() {
+    public @Nullable Material getMaterial() {
         return material;
     }
     
-    public T setMaterial(@NotNull Material material) {
+    @Contract("_ -> this")
+    public @NotNull S setMaterial(@NotNull Material material) {
         this.material = material;
-        return getThis();
+        return (S) this;
     }
     
     public int getAmount() {
         return amount;
     }
     
-    public T setAmount(int amount) {
+    @Contract("_ -> this")
+    public @NotNull S setAmount(int amount) {
         this.amount = amount;
-        return getThis();
+        return (S) this;
     }
     
     public int getDamage() {
         return damage;
     }
     
-    public T setDamage(int damage) {
+    @Contract("_ -> this")
+    public @NotNull S setDamage(int damage) {
         this.damage = damage;
-        return getThis();
+        return (S) this;
     }
     
     public int getCustomModelData() {
         return customModelData;
     }
     
-    public T setCustomModelData(int customModelData) {
+    @Contract("_ -> this")
+    public @NotNull S setCustomModelData(int customModelData) {
         this.customModelData = customModelData;
-        return getThis();
+        return (S) this;
     }
     
-    public ComponentWrapper getDisplayName() {
+    public @Nullable ComponentWrapper getDisplayName() {
         return displayName;
     }
     
-    public T setDisplayName(String displayName) {
+    @Contract("_ -> this")
+    public @NotNull S setDisplayName(String displayName) {
         this.displayName = new BaseComponentWrapper(ComponentUtils.withoutPreFormatting(displayName));
-        return getThis();
+        return (S) this;
     }
     
-    public T setDisplayName(BaseComponent... displayName) {
+    @Contract("_ -> this")
+    public @NotNull S setDisplayName(BaseComponent... displayName) {
         this.displayName = new BaseComponentWrapper(ComponentUtils.withoutPreFormatting(displayName));
-        return getThis();
+        return (S) this;
     }
     
-    public T setDisplayName(ComponentWrapper component) {
+    @Contract("_ -> this")
+    public @NotNull S setDisplayName(ComponentWrapper component) {
         this.displayName = component;
-        return getThis();
+        return (S) this;
     }
     
     //<editor-fold desc="lore">
-    public List<ComponentWrapper> getLore() {
+    public @Nullable List<ComponentWrapper> getLore() {
         return lore;
     }
     
-    public T setLore(List<ComponentWrapper> lore) {
+    @Contract("_ -> this")
+    public @NotNull S setLore(List<ComponentWrapper> lore) {
         this.lore = lore;
-        return getThis();
+        return (S) this;
     }
     
-    public T setLegacyLore(@NotNull List<String> lore) {
+    @Contract("_ -> this")
+    public @NotNull S setLegacyLore(@NotNull List<String> lore) {
         this.lore = lore.stream()
             .map(line -> new BaseComponentWrapper(ComponentUtils.withoutPreFormatting(line)))
             .collect(Collectors.toList());
-        return getThis();
+        return (S) this;
     }
     
-    public T addLoreLines(@NotNull String... lines) {
+    @Contract("_ -> this")
+    public @NotNull S addLoreLines(@NotNull String... lines) {
         if (lore == null) lore = new ArrayList<>();
         
         for (String line : lines)
             lore.add(new BaseComponentWrapper(ComponentUtils.withoutPreFormatting(line)));
-        return getThis();
+        return (S) this;
     }
     
-    public T addLoreLines(@NotNull BaseComponent[]... lines) {
+    @Contract("_ -> this")
+    public @NotNull S addLoreLines(@NotNull BaseComponent[]... lines) {
         if (lore == null) lore = new ArrayList<>();
         
         lore.addAll(
@@ -245,108 +261,118 @@ public abstract class AbstractItemBuilder<T> implements ItemProvider {
                 .collect(Collectors.toList())
         );
         
-        return getThis();
+        return (S) this;
     }
     
-    public T addLoreLines(@NotNull ComponentWrapper... lines) {
+    @Contract("_ -> this")
+    public @NotNull S addLoreLines(@NotNull ComponentWrapper... lines) {
         if (lore == null) lore = new ArrayList<>();
         
         lore.addAll(Arrays.asList(lines));
         
-        return getThis();
+        return (S) this;
     }
     //</editor-fold>
     
     //<editor-fold desc="item flags">
-    public List<ItemFlag> getItemFlags() {
+    public @Nullable List<ItemFlag> getItemFlags() {
         return itemFlags;
     }
     
-    public T setItemFlags(@NotNull List<ItemFlag> itemFlags) {
+    @Contract("_ -> this")
+    public @NotNull S setItemFlags(@NotNull List<ItemFlag> itemFlags) {
         this.itemFlags = itemFlags;
-        return getThis();
+        return (S) this;
     }
     
-    public T addItemFlags(@NotNull ItemFlag... itemFlags) {
+    @Contract("_ -> this")
+    public @NotNull S addItemFlags(@NotNull ItemFlag... itemFlags) {
         if (this.itemFlags == null) this.itemFlags = new ArrayList<>();
         this.itemFlags.addAll(Arrays.asList(itemFlags));
-        return getThis();
+        return (S) this;
     }
     
-    public T removeItemFlags(@NotNull ItemFlag... itemFlags) {
+    @Contract("_ -> this")
+    public @NotNull S removeItemFlags(@NotNull ItemFlag... itemFlags) {
         if (this.itemFlags != null)
             this.itemFlags.removeAll(Arrays.asList(itemFlags));
-        return getThis();
+        return (S) this;
     }
     
-    public T clearItemFlags() {
+    @Contract("-> this")
+    public @NotNull S clearItemFlags() {
         if (itemFlags != null) itemFlags.clear();
-        return getThis();
+        return (S) this;
     }
     //</editor-fold>
     
     //<editor-fold desc="enchantments">
-    public HashMap<Enchantment, Pair<Integer, Boolean>> getEnchantments() {
+    public @Nullable HashMap<Enchantment, Pair<Integer, Boolean>> getEnchantments() {
         return enchantments;
     }
     
-    public T setEnchantments(@NotNull HashMap<Enchantment, Pair<Integer, Boolean>> enchantments) {
+    @Contract("_ -> this")
+    public @NotNull S setEnchantments(@NotNull HashMap<Enchantment, Pair<Integer, Boolean>> enchantments) {
         this.enchantments = enchantments;
-        return getThis();
+        return (S) this;
     }
     
-    public T addEnchantment(Enchantment enchantment, int level, boolean ignoreLevelRestriction) {
+    @Contract("_, _, _ -> this")
+    public @NotNull S addEnchantment(Enchantment enchantment, int level, boolean ignoreLevelRestriction) {
         if (enchantments == null) enchantments = new HashMap<>();
         enchantments.put(enchantment, new Pair<>(level, ignoreLevelRestriction));
-        return getThis();
+        return (S) this;
     }
     
-    public T removeEnchantment(Enchantment enchantment) {
+    @Contract("_ -> this")
+    public @NotNull S removeEnchantment(Enchantment enchantment) {
         if (enchantments == null) enchantments = new HashMap<>();
         enchantments.remove(enchantment);
-        return getThis();
+        return (S) this;
     }
     
-    public T clearEnchantments() {
+    @Contract("-> this")
+    public @NotNull S clearEnchantments() {
         if (enchantments != null) enchantments.clear();
-        return getThis();
+        return (S) this;
     }
     //</editor-fold>
     
     //<editor-fold desc="modifiers">
-    public List<Function<ItemStack, ItemStack>> getModifiers() {
+    public @Nullable List<Function<ItemStack, ItemStack>> getModifiers() {
         return modifiers;
     }
     
-    public T addModifier(Function<ItemStack, ItemStack> modifier) {
+    @Contract("_ -> this")
+    public @NotNull S addModifier(Function<ItemStack, ItemStack> modifier) {
         if (modifiers == null) modifiers = new ArrayList<>();
         modifiers.add(modifier);
-        return getThis();
+        return (S) this;
     }
     
-    public T clearModifiers() {
+    @Contract("-> this")
+    public @NotNull S clearModifiers() {
         if (modifiers != null) modifiers.clear();
-        return getThis();
+        return (S) this;
     }
     //</editor-fold>
     
     @SuppressWarnings("unchecked")
+    @Contract(value = "-> new", pure = true)
     @Override
-    public T clone() {
+    public @NotNull S clone() {
         try {
-            AbstractItemBuilder<T> clone = ((AbstractItemBuilder<T>) super.clone());
+            AbstractItemBuilder<S> clone = ((AbstractItemBuilder<S>) super.clone());
             if (base != null) clone.base = base.clone();
             if (lore != null) clone.lore = new ArrayList<>(lore);
             if (itemFlags != null) clone.itemFlags = new ArrayList<>(itemFlags);
             if (enchantments != null) clone.enchantments = new HashMap<>(enchantments);
             if (modifiers != null) clone.modifiers = new ArrayList<>(modifiers);
             
-            return (T) clone;
+            return (S) clone;
         } catch (CloneNotSupportedException e) {
             throw new AssertionError(e);
         }
     }
     
-    protected abstract T getThis();
-    
 }
diff --git a/invui/src/main/java/xyz/xenondevs/invui/item/builder/ItemBuilder.java b/invui/src/main/java/xyz/xenondevs/invui/item/builder/ItemBuilder.java
index ca928b2..a15c902 100644
--- a/invui/src/main/java/xyz/xenondevs/invui/item/builder/ItemBuilder.java
+++ b/invui/src/main/java/xyz/xenondevs/invui/item/builder/ItemBuilder.java
@@ -36,9 +36,4 @@ public final class ItemBuilder extends AbstractItemBuilder<ItemBuilder> {
         super(base);
     }
     
-    @Override
-    protected ItemBuilder getThis() {
-        return this;
-    }
-    
 }
diff --git a/invui/src/main/java/xyz/xenondevs/invui/item/builder/PotionBuilder.java b/invui/src/main/java/xyz/xenondevs/invui/item/builder/PotionBuilder.java
index 3f3e058..3449e04 100644
--- a/invui/src/main/java/xyz/xenondevs/invui/item/builder/PotionBuilder.java
+++ b/invui/src/main/java/xyz/xenondevs/invui/item/builder/PotionBuilder.java
@@ -6,6 +6,8 @@ import org.bukkit.inventory.ItemStack;
 import org.bukkit.inventory.meta.PotionMeta;
 import org.bukkit.potion.PotionData;
 import org.bukkit.potion.PotionEffect;
+import org.jetbrains.annotations.Contract;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.ArrayList;
 import java.util.List;
@@ -16,36 +18,41 @@ public final class PotionBuilder extends AbstractItemBuilder<PotionBuilder> {
     private Color color;
     private PotionData basePotionData;
     
-    public PotionBuilder(PotionType type) {
+    public PotionBuilder(@NotNull PotionType type) {
         super(type.getMaterial());
     }
     
-    public PotionBuilder(ItemStack base) {
+    public PotionBuilder(@NotNull ItemStack base) {
         super(base);
     }
     
-    public PotionBuilder setColor(Color color) {
+    @Contract("_ -> this")
+    public @NotNull PotionBuilder setColor(@NotNull Color color) {
         this.color = color;
         return this;
     }
     
-    public PotionBuilder setColor(java.awt.Color color) {
+    @Contract("_ -> this")
+    public @NotNull PotionBuilder setColor(@NotNull java.awt.Color color) {
         this.color = Color.fromRGB(color.getRed(), color.getGreen(), color.getBlue());
         return this;
     }
     
-    public PotionBuilder setBasePotionData(PotionData basePotionData) {
+    @Contract("_ -> this")
+    public @NotNull PotionBuilder setBasePotionData(@NotNull PotionData basePotionData) {
         this.basePotionData = basePotionData;
         return this;
     }
     
-    public PotionBuilder addEffect(PotionEffect effect) {
+    @Contract("_ -> this")
+    public @NotNull PotionBuilder addEffect(@NotNull PotionEffect effect) {
         effects.add(effect);
         return this;
     }
     
+    @Contract(value = "-> new", pure = true)
     @Override
-    public ItemStack get() {
+    public @NotNull ItemStack get() {
         ItemStack item = super.get();
         PotionMeta meta = (PotionMeta) item.getItemMeta();
         
@@ -59,30 +66,25 @@ public final class PotionBuilder extends AbstractItemBuilder<PotionBuilder> {
     }
     
     @Override
-    public PotionBuilder clone() {
+    public @NotNull PotionBuilder clone() {
         PotionBuilder builder = super.clone();
         builder.effects = new ArrayList<>(effects);
         return builder;
     }
     
-    @Override
-    protected PotionBuilder getThis() {
-        return this;
-    }
-    
     public enum PotionType {
         
         NORMAL(Material.POTION),
         SPLASH(Material.SPLASH_POTION),
         LINGERING(Material.LINGERING_POTION);
         
-        private final Material material;
+        private final @NotNull Material material;
         
-        PotionType(Material material) {
+        PotionType(@NotNull Material material) {
             this.material = material;
         }
         
-        public Material getMaterial() {
+        public @NotNull Material getMaterial() {
             return material;
         }
         
diff --git a/invui/src/main/java/xyz/xenondevs/invui/item/builder/SkullBuilder.java b/invui/src/main/java/xyz/xenondevs/invui/item/builder/SkullBuilder.java
index a46c044..b577e8a 100644
--- a/invui/src/main/java/xyz/xenondevs/invui/item/builder/SkullBuilder.java
+++ b/invui/src/main/java/xyz/xenondevs/invui/item/builder/SkullBuilder.java
@@ -11,6 +11,7 @@ import org.bukkit.OfflinePlayer;
 import org.bukkit.entity.Player;
 import org.bukkit.inventory.ItemStack;
 import org.bukkit.inventory.meta.ItemMeta;
+import org.jetbrains.annotations.Contract;
 import org.jetbrains.annotations.NotNull;
 import xyz.xenondevs.inventoryaccess.util.ReflectionRegistry;
 import xyz.xenondevs.inventoryaccess.util.ReflectionUtils;
@@ -30,7 +31,7 @@ public final class SkullBuilder extends AbstractItemBuilder<SkullBuilder> {
      *
      * @param uuid The {@link UUID} of the skull owner.
      */
-    public SkullBuilder(UUID uuid) {
+    public SkullBuilder(@NotNull UUID uuid) {
         this(HeadTexture.of(uuid));
     }
     
@@ -39,7 +40,7 @@ public final class SkullBuilder extends AbstractItemBuilder<SkullBuilder> {
      *
      * @param username The username of the skull owner.
      */
-    public SkullBuilder(String username) {
+    public SkullBuilder(@NotNull String username) {
         this(HeadTexture.of(username));
     }
     
@@ -48,19 +49,20 @@ public final class SkullBuilder extends AbstractItemBuilder<SkullBuilder> {
      *
      * @param headTexture The {@link HeadTexture} to be applied to the skull.
      */
-    public SkullBuilder(HeadTexture headTexture) {
+    public SkullBuilder(@NotNull HeadTexture headTexture) {
         super(Material.PLAYER_HEAD);
         setGameProfile(headTexture);
     }
     
-    private void setGameProfile(HeadTexture texture) {
+    private void setGameProfile(@NotNull HeadTexture texture) {
         gameProfile = new GameProfile(UUID.randomUUID(), null);
         PropertyMap propertyMap = gameProfile.getProperties();
         propertyMap.put("textures", new Property("textures", texture.getTextureValue()));
     }
     
+    @Contract(value = "-> new", pure = true)
     @Override
-    public ItemStack get() {
+    public @NotNull ItemStack get() {
         ItemStack item = super.get();
         ItemMeta meta = item.getItemMeta();
         
@@ -72,14 +74,16 @@ public final class SkullBuilder extends AbstractItemBuilder<SkullBuilder> {
         return item;
     }
     
+    @Contract("_ -> this")
     @Override
-    public SkullBuilder setMaterial(@NotNull Material material) {
+    public @NotNull SkullBuilder setMaterial(@NotNull Material material) {
         return this;
     }
     
+    @Contract(value = "-> new", pure = true)
     @Override
-    protected SkullBuilder getThis() {
-        return this;
+    public @NotNull SkullBuilder clone() {
+        return super.clone();
     }
     
     /**
@@ -185,7 +189,7 @@ public final class SkullBuilder extends AbstractItemBuilder<SkullBuilder> {
          *
          * @return The stored texture value.
          */
-        public String getTextureValue() {
+        public @NotNull String getTextureValue() {
             return textureValue;
         }
         
diff --git a/invui/src/main/java/xyz/xenondevs/invui/window/AnvilSingleWindowImpl.java b/invui/src/main/java/xyz/xenondevs/invui/window/AnvilSingleWindowImpl.java
index 4cbe4c5..65cdd47 100644
--- a/invui/src/main/java/xyz/xenondevs/invui/window/AnvilSingleWindowImpl.java
+++ b/invui/src/main/java/xyz/xenondevs/invui/window/AnvilSingleWindowImpl.java
@@ -60,13 +60,13 @@ final class AnvilSingleWindowImpl extends AbstractSingleWindow implements AnvilW
         private List<Consumer<String>> renameHandlers;
         
         @Override
-        public BuilderImpl setRenameHandlers(@NotNull List<@NotNull Consumer<String>> renameHandlers) {
+        public @NotNull BuilderImpl setRenameHandlers(@NotNull List<@NotNull Consumer<String>> renameHandlers) {
             this.renameHandlers = renameHandlers;
             return this;
         }
         
         @Override
-        public BuilderImpl addRenameHandler(@NotNull Consumer<String> renameHandler) {
+        public @NotNull BuilderImpl addRenameHandler(@NotNull Consumer<String> renameHandler) {
             if (renameHandlers == null)
                 renameHandlers = new ArrayList<>();
             
diff --git a/invui/src/main/java/xyz/xenondevs/invui/window/AnvilSplitWindowImpl.java b/invui/src/main/java/xyz/xenondevs/invui/window/AnvilSplitWindowImpl.java
index 181d423..cac68e5 100644
--- a/invui/src/main/java/xyz/xenondevs/invui/window/AnvilSplitWindowImpl.java
+++ b/invui/src/main/java/xyz/xenondevs/invui/window/AnvilSplitWindowImpl.java
@@ -62,13 +62,13 @@ final class AnvilSplitWindowImpl extends AbstractSplitWindow implements AnvilWin
         private List<Consumer<String>> renameHandlers;
         
         @Override
-        public BuilderImpl setRenameHandlers(@NotNull List<@NotNull Consumer<String>> renameHandlers) {
+        public @NotNull BuilderImpl setRenameHandlers(@NotNull List<@NotNull Consumer<String>> renameHandlers) {
             this.renameHandlers = renameHandlers;
             return this;
         }
         
         @Override
-        public BuilderImpl addRenameHandler(@NotNull Consumer<String> renameHandler) {
+        public @NotNull BuilderImpl addRenameHandler(@NotNull Consumer<String> renameHandler) {
             if (renameHandlers == null)
                 renameHandlers = new ArrayList<>();
             
diff --git a/invui/src/main/java/xyz/xenondevs/invui/window/AnvilWindow.java b/invui/src/main/java/xyz/xenondevs/invui/window/AnvilWindow.java
index a8b2e24..3a1d1af 100644
--- a/invui/src/main/java/xyz/xenondevs/invui/window/AnvilWindow.java
+++ b/invui/src/main/java/xyz/xenondevs/invui/window/AnvilWindow.java
@@ -35,10 +35,10 @@ public interface AnvilWindow extends Window {
     interface Builder<S extends Builder<S>> extends Window.Builder<AnvilWindow, Player, S> {
         
         @Contract("_ -> this")
-        S setRenameHandlers(@NotNull List<@NotNull Consumer<String>> renameHandlers);
+        @NotNull S setRenameHandlers(@NotNull List<@NotNull Consumer<String>> renameHandlers);
         
         @Contract("_ -> this")
-        S addRenameHandler(@NotNull Consumer<String> renameHandler);
+        @NotNull S addRenameHandler(@NotNull Consumer<String> renameHandler);
         
         interface Single extends Builder<Single>, Window.Builder.Single<AnvilWindow, Player, Single> {}