add a user-customizable gui

src/main/java/com/songoda/core/configuration/SimpleDataStore.java

@@ -2,6 +2,7 @@ package com.songoda.core.configuration;
 
 import java.io.File;
 import java.io.IOException;
+import java.util.Collection;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Timer;
@@ -18,17 +19,16 @@ import org.jetbrains.annotations.Nullable;
 /**
  * Used to easily store a set of one data value
  *
- * @param <K> Key value that DataObject class uses to uniquely identify values
  * @param <T> DataObject class that is used to store the data
  * @since 2019-09-06
  * @author jascotty2
  */
-public class SimpleDataStore<K, T extends DataStoreObject<K>> {
+public class SimpleDataStore<T extends DataStoreObject> {
 
     protected final Plugin plugin;
     protected final String filename, dirName;
     private final Function<ConfigurationSection, T> getFromSection;
-    protected final HashMap<K, T> data = new HashMap();
+    protected final HashMap<Object, T> data = new HashMap();
     private File file;
     private final Object lock = new Object();
     SaveTask saveTask;
@@ -38,7 +38,7 @@ public class SimpleDataStore<K, T extends DataStoreObject<K>> {
      */
     int autosaveInterval = 60;
 
-    public SimpleDataStore(@NotNull Plugin plugin, @NotNull String filename, Function<ConfigurationSection, T> loadFunction) {
+    public SimpleDataStore(@NotNull Plugin plugin, @NotNull String filename, @NotNull Function<ConfigurationSection, T> loadFunction) {
         this.plugin = plugin;
         this.filename = filename;
         dirName = null;
@@ -65,22 +65,23 @@ public class SimpleDataStore<K, T extends DataStoreObject<K>> {
     }
 
     /**
-     * @return a directly-modifiable instance of the data mapping for this storage
+     * @return a directly-modifiable instance of the data mapping for this
+     * storage
      */
-    public Map<K, T> getData() {
+    public Map<Object, T> getData() {
         return data;
     }
 
     /**
-     * Returns the value to which the specified key is mapped,
-     * or {@code null} if this map contains no mapping for the key.
+     * Returns the value to which the specified key is mapped, or {@code null}
+     * if this map contains no mapping for the key.
      *
      * @param key key whose mapping is to be retrieved from this storage
      * @return the value associated with <tt>key</tt>, or
      * <tt>null</tt> if there was no mapping for <tt>key</tt>.
      */
     @Nullable
-    public T get(K key) {
+    public T get(Object key) {
         return data.get(key);
     }
 
@@ -92,7 +93,7 @@ public class SimpleDataStore<K, T extends DataStoreObject<K>> {
      * <tt>null</tt> if there was no mapping for <tt>key</tt>.
      */
     @Nullable
-    public T remove(@NotNull K key) {
+    public T remove(@NotNull Object key) {
         T temp;
         synchronized (lock) {
             temp = data.remove(key);
@@ -142,6 +143,48 @@ public class SimpleDataStore<K, T extends DataStoreObject<K>> {
         return temp;
     }
 
+    /**
+     * Adds the specified value in this storage. If the map previously contained
+     * a mapping for the key, the old value is replaced.
+     *
+     * @param value values to be added
+     */
+    @Nullable
+    public void addAll(@NotNull T[] value) {
+        if (value == null) {
+            return;
+        }
+        synchronized (lock) {
+            for (int i = 0; i < value.length; ++i) {
+                if (value[i] != null) {
+                    data.put(value[i].getKey(), value[i]);
+                }
+            }
+        }
+        save();
+    }
+
+    /**
+     * Adds the specified value in this storage. If the map previously contained
+     * a mapping for the key, the old value is replaced.
+     *
+     * @param value values to be added
+     */
+    @Nullable
+    public void addAll(@NotNull Collection<T> value) {
+        if (value == null) {
+            return;
+        }
+        synchronized (lock) {
+            for (T v : value) {
+                if (v != null) {
+                    data.put(v.getKey(), v);
+                }
+            }
+        }
+        save();
+    }
+
     /**
      * Load data from the associated file
      */
@@ -168,7 +211,8 @@ public class SimpleDataStore<K, T extends DataStoreObject<K>> {
     }
 
     /**
-     * Optionally save this storage's data to file if there have been changes made
+     * Optionally save this storage's data to file if there have been changes
+     * made
      */
     public void saveChanges() {
         if (saveTask != null || data.values().stream().anyMatch(v -> v.hasChanged())) {

src/main/java/com/songoda/core/gui/CustomizableGui.java

@@ -0,0 +1,337 @@
+package com.songoda.core.gui;
+
+import com.songoda.core.compatibility.LegacyMaterials;
+import com.songoda.core.configuration.DataStoreObject;
+import com.songoda.core.configuration.SimpleDataStore;
+import com.songoda.core.gui.methods.Clickable;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import org.bukkit.configuration.ConfigurationSection;
+import org.bukkit.event.inventory.ClickType;
+import org.bukkit.inventory.ItemStack;
+import org.bukkit.inventory.meta.ItemMeta;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
+/**
+ * Represents a GUI screen that can be user-configured
+ *
+ * @since 2019-09-06
+ * @author jascotty2
+ */
+public class CustomizableGui extends Gui {
+
+    final Map<String, CustomButton> buttons;
+
+    public CustomizableGui(SimpleDataStore<CustomButton> buttons) {
+        this((Map<String, CustomButton>) (Map) buttons.getData());
+    }
+
+    public CustomizableGui(SimpleDataStore<CustomButton> buttons, Gui parent) {
+        this((Map<String, CustomButton>) (Map) buttons.getData(), parent);
+    }
+
+    public CustomizableGui(@NotNull Map<String, CustomButton> buttons) {
+        this(buttons, null);
+    }
+
+    public CustomizableGui(@NotNull Map<String, CustomButton> buttons, @Nullable Gui parent) {
+        super(parent);
+        this.buttons = buttons;
+        if (buttons.containsKey("__DEFAULT__")) {
+            blankItem = GuiUtils.getBorderItem(buttons.get("__DEFAULT__").icon);
+        }
+    }
+
+    @NotNull
+    public CustomButton[] getButtons() {
+        return buttons.values().toArray(new CustomButton[buttons.size()]);
+    }
+
+    @NotNull
+    @Override
+    public CustomizableGui setDefaultItem(ItemStack item) {
+        if ((blankItem = item) != null) {
+            buttons.put("__DEFAULT__", (new CustomButton("__DEFAULT__")).setIcon(LegacyMaterials.getMaterial(item)));
+        }
+        return this;
+    }
+
+    @Nullable
+    public CustomButton getButton(@NotNull String key) {
+        return key == null ? null : buttons.get(key.toLowerCase());
+    }
+
+    @NotNull
+    public CustomizableGui setItem(int defaultRow, int defaultCol, @NotNull String key, @NotNull ItemStack item) {
+        final int cell = defaultCol + defaultRow * 9;
+        return setItem(cell, key, item);
+    }
+
+    @NotNull
+    public CustomizableGui setItem(int defaultCell, @NotNull String key, @NotNull ItemStack item) {
+        CustomButton btn = key == null ? null : buttons.get(key = key.toLowerCase());
+        if (btn == null) {
+            buttons.put(key, btn = (new CustomButton(key, defaultCell)).setIcon(LegacyMaterials.getMaterial(item)));
+        } else {
+            ItemStack btnItem = btn.icon.getItem();
+            ItemMeta itemMeta = item.getItemMeta();
+            ItemMeta btnItemMeta = btnItem.getItemMeta();
+            if (itemMeta != null && btnItemMeta != null) {
+                btnItemMeta.setDisplayName(itemMeta.getDisplayName());
+                btnItemMeta.setLore(itemMeta.getLore());
+                btnItem.setItemMeta(itemMeta);
+            }
+            item = btnItem;
+        }
+        cellItems.put(btn.position, item);
+        if (inventory != null && btn.position >= 0 && btn.position < inventory.getSize()) {
+            inventory.setItem(btn.position, item);
+        }
+        return this;
+    }
+
+    @NotNull
+    public CustomizableGui setItem(int defaultRow, int defaultCol, @NotNull String key, @NotNull LegacyMaterials defaultItem, @NotNull String title, @NotNull String... lore) {
+        final int cell = defaultCol + defaultRow * 9;
+        return setItem(cell, key, defaultItem, title, lore);
+    }
+
+    @NotNull
+    public CustomizableGui setItem(int defaultCell, @NotNull String key, @NotNull LegacyMaterials defaultItem, @NotNull String title, @NotNull String... lore) {
+        CustomButton btn = key == null ? null : buttons.get(key = key.toLowerCase());
+        if (btn == null) {
+            buttons.put(key, btn = (new CustomButton(key, defaultCell)).setIcon(defaultItem));
+        }
+        ItemStack item = GuiUtils.createButtonItem(btn.icon, title, lore);
+        cellItems.put(btn.position, item);
+        if (inventory != null && btn.position >= 0 && btn.position < inventory.getSize()) {
+            inventory.setItem(btn.position, item);
+        }
+        return this;
+    }
+
+    @NotNull
+    public CustomizableGui highlightItem(@NotNull String key) {
+        CustomButton btn = key == null ? null : buttons.get(key.toLowerCase());
+        if (btn != null) {
+            this.highlightItem(btn.position);
+        }
+        return this;
+    }
+
+    @NotNull
+    public CustomizableGui removeHighlight(@NotNull String key) {
+        CustomButton btn = key == null ? null : buttons.get(key.toLowerCase());
+        if (btn != null) {
+            this.removeHighlight(btn.position);
+        }
+        return this;
+    }
+
+    @NotNull
+    public CustomizableGui updateItem(@NotNull String key, @Nullable String title, @NotNull String... lore) {
+        CustomButton btn = key == null ? null : buttons.get(key.toLowerCase());
+        if (btn != null) {
+            this.updateItem(btn.position, title, lore);
+        }
+        return this;
+    }
+
+    @NotNull
+    public CustomizableGui updateItem(@NotNull String key, @Nullable String title, @Nullable List<String> lore) {
+        CustomButton btn = key == null ? null : buttons.get(key.toLowerCase());
+        if (btn != null) {
+            this.updateItem(btn.position, title, lore);
+        }
+        return this;
+    }
+
+    @NotNull
+    public CustomizableGui updateItem(@NotNull String key, @NotNull LegacyMaterials itemTo, @NotNull String title, @NotNull String... lore) {
+        CustomButton btn = key == null ? null : buttons.get(key.toLowerCase());
+        if (btn != null) {
+            this.updateItem(btn.position, itemTo, title, lore);
+        }
+        return this;
+    }
+
+    @NotNull
+    public CustomizableGui updateItem(@NotNull String key, @NotNull LegacyMaterials itemTo, @NotNull String title, @Nullable List<String> lore) {
+        CustomButton btn = key == null ? null : buttons.get(key.toLowerCase());
+        if (btn != null) {
+            this.updateItem(btn.position, itemTo, title, lore);
+        }
+        return this;
+    }
+
+    @NotNull
+    public CustomizableGui setAction(@NotNull String key, Clickable action) {
+        CustomButton btn = key == null ? null : buttons.get(key = key.toLowerCase());
+        if (btn != null) {
+            setConditional(btn.position, null, action);
+        }
+        return this;
+    }
+
+    @NotNull
+    public CustomizableGui setAction(@NotNull String key, @Nullable ClickType type, @Nullable Clickable action) {
+        CustomButton btn = key == null ? null : buttons.get(key = key.toLowerCase());
+        if (btn != null) {
+            setConditional(btn.position, type, action);
+        }
+        return this;
+    }
+
+    @NotNull
+    public CustomizableGui setButton(int defaultCell, @NotNull String key, ItemStack item, @Nullable Clickable action) {
+        setItem(defaultCell, key, item);
+        setAction(key, null, action);
+        return this;
+    }
+
+    @NotNull
+    public CustomizableGui setButton(int defaultCell, @NotNull String key, ItemStack item, @Nullable ClickType type, @Nullable Clickable action) {
+        setItem(defaultCell, key, item);
+        setAction(key, type, action);
+        return this;
+    }
+
+    @NotNull
+    public CustomizableGui setButton(int defaultRow, int defaultCol, @NotNull String key, ItemStack item, @Nullable Clickable action) {
+        final int defaultCell = defaultCol + defaultRow * 9;
+        setItem(defaultCell, key, item);
+        setAction(key, null, action);
+        return this;
+    }
+
+    @NotNull
+    public CustomizableGui setButton(int defaultRow, int defaultCol, @NotNull String key, ItemStack item, @Nullable ClickType type, @Nullable Clickable action) {
+        final int defaultCell = defaultCol + defaultRow * 9;
+        setItem(defaultCell, key, item);
+        setAction(key, type, action);
+        return this;
+    }
+
+    @NotNull
+    @Override
+    public CustomizableGui setNextPage(int row, int col, @NotNull ItemStack item) {
+        return this.setNextPage(col + row * 9, item);
+    }
+
+    @NotNull
+    @Override
+    public CustomizableGui setNextPage(int cell, @NotNull ItemStack item) {
+        CustomButton btn = buttons.get("__NEXT__");
+        if (btn == null) {
+            buttons.put("__NEXT__", btn = (new CustomButton("__NEXT__", cell)).setIcon(LegacyMaterials.getMaterial(item)));
+        } else {
+            ItemStack btnItem = btn.icon.getItem();
+            ItemMeta itemMeta = item.getItemMeta();
+            ItemMeta btnItemMeta = btnItem.getItemMeta();
+            if (itemMeta != null && btnItemMeta != null) {
+                btnItemMeta.setDisplayName(itemMeta.getDisplayName());
+                btnItemMeta.setLore(itemMeta.getLore());
+                btnItem.setItemMeta(itemMeta);
+            }
+            item = btnItem;
+        }
+        return (CustomizableGui) super.setNextPage(btn.position, item);
+    }
+
+    @NotNull
+    @Override
+    public CustomizableGui setPrevPage(int row, int col, @NotNull ItemStack item) {
+        return this.setPrevPage(col + row * 9, item);
+    }
+
+    @NotNull
+    @Override
+    public CustomizableGui setPrevPage(int cell, @NotNull ItemStack item) {
+        CustomButton btn = buttons.get("__PREV__");
+        if (btn == null) {
+            buttons.put("__PREV__", btn = (new CustomButton("__PREV__", cell)).setIcon(LegacyMaterials.getMaterial(item)));
+        } else {
+            ItemStack btnItem = btn.icon.getItem();
+            ItemMeta itemMeta = item.getItemMeta();
+            ItemMeta btnItemMeta = btnItem.getItemMeta();
+            if (itemMeta != null && btnItemMeta != null) {
+                btnItemMeta.setDisplayName(itemMeta.getDisplayName());
+                btnItemMeta.setLore(itemMeta.getLore());
+                btnItem.setItemMeta(itemMeta);
+            }
+            item = btnItem;
+        }
+        return (CustomizableGui) super.setPrevPage(btn.position, item);
+    }
+
+    public CustomizableGui clearActions(@NotNull String key) {
+        CustomButton btn = key == null ? null : buttons.get(key = key.toLowerCase());
+        if (btn != null) {
+            this.clearActions(btn.position);
+        }
+        return this;
+    }
+
+    public static class CustomButton implements DataStoreObject<String> {
+
+        boolean _changed = false;
+        final String key;
+        int position = -1;
+        LegacyMaterials icon = LegacyMaterials.STONE;
+
+        public CustomButton(String key) {
+            this.key = key;
+        }
+
+        public CustomButton(String key, int position) {
+            this.key = key;
+            this.position = position;
+        }
+
+        public static CustomButton loadFromSection(ConfigurationSection sec) {
+            CustomButton dat = new CustomButton(sec.getName());
+            dat.icon = sec.contains("icon") ? LegacyMaterials.getMaterial(sec.getString("icon"), LegacyMaterials.STONE) : LegacyMaterials.STONE;
+            dat.position = sec.getInt("position");
+            return dat;
+        }
+
+        @Override
+        public void saveToSection(ConfigurationSection sec) {
+            sec.set("icon", icon.name());
+            sec.set("position", position);
+        }
+
+        @Override
+        public String getKey() {
+            return key;
+        }
+
+        @Override
+        public String getConfigKey() {
+            return key;
+        }
+
+        @Override
+        public boolean hasChanged() {
+            return _changed;
+        }
+
+        @Override
+        public void setChanged(boolean isChanged) {
+            _changed = isChanged;
+        }
+
+        public LegacyMaterials getIcon() {
+            return icon;
+        }
+
+        public CustomButton setIcon(LegacyMaterials icon) {
+            this.icon = icon != null ? icon : LegacyMaterials.STONE;
+            _changed = true;
+            return this;
+        }
+    }
+}

src/main/java/com/songoda/core/gui/Gui.java

@@ -26,6 +26,8 @@ import org.bukkit.event.inventory.InventoryClickEvent;
 import org.bukkit.event.inventory.InventoryType;
 import org.bukkit.inventory.Inventory;
 import org.bukkit.inventory.ItemStack;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 /**
  * TODO: animated buttons
@@ -62,8 +64,8 @@ public class Gui {
         this.rows = 3;
     }
 
-    public Gui(GuiType type) {
-        this.inventoryType = type;
+    public Gui(@NotNull GuiType type) {
+        this.inventoryType = type != null ? type : GuiType.STANDARD;
         switch (type) {
             case HOPPER:
             case DISPENSER:
@@ -74,7 +76,7 @@ public class Gui {
         }
     }
 
-    public Gui(Gui parent) {
+    public Gui(@Nullable Gui parent) {
         this.parent = parent;
     }
 
@@ -82,17 +84,18 @@ public class Gui {
         this.rows = Math.max(1, Math.min(6, rows));
     }
 
-    public Gui(int rows, Gui parent) {
+    public Gui(int rows, @Nullable Gui parent) {
         this.parent = parent;
         this.rows = Math.max(1, Math.min(6, rows));
     }
 
+    @NotNull
     public List<Player> getPlayers() {
         return inventory == null ? Collections.EMPTY_LIST
                 : inventory.getViewers().stream()
-                        .filter(e -> e instanceof Player)
-                        .map(e -> (Player) e)
-                        .collect(Collectors.toList());
+                .filter(e -> e instanceof Player)
+                .map(e -> (Player) e)
+                .collect(Collectors.toList());
     }
 
     public boolean isOpen() {
@@ -139,7 +142,8 @@ public class Gui {
     }
 
     /**
-     * Close the GUI without calling onClose() and without opening any parent GUIs
+     * Close the GUI without calling onClose() and without opening any parent
+     * GUIs
      */
     public void exit() {
         allowClose = true;
@@ -151,21 +155,25 @@ public class Gui {
                 .forEach(Player::closeInventory);
     }
 
+    @NotNull
     public GuiType getType() {
         return inventoryType;
     }
 
+    @NotNull
     public Gui setUnlocked(int cell) {
         unlockedCells.put(cell, true);
         return this;
     }
 
+    @NotNull
     public Gui setUnlocked(int row, int col) {
         final int cell = col + row * 9;
         unlockedCells.put(cell, true);
         return this;
     }
 
+    @NotNull
     public Gui setUnlockedRange(int cellFirst, int cellLast) {
         for (int cell = cellFirst; cell <= cellLast; ++cell) {
             unlockedCells.put(cell, true);
@@ -173,6 +181,7 @@ public class Gui {
         return this;
     }
 
+    @NotNull
     public Gui setUnlockedRange(int cellRowFirst, int cellColFirst, int cellRowLast, int cellColLast) {
         final int last = cellColLast + cellRowLast * 9;
         for (int cell = cellColFirst + cellRowFirst * 9; cell <= last; ++cell) {
@@ -181,17 +190,20 @@ public class Gui {
         return this;
     }
 
+    @NotNull
     public Gui setUnlocked(int cell, boolean open) {
         unlockedCells.put(cell, open);
         return this;
     }
 
+    @NotNull
     public Gui setUnlocked(int row, int col, boolean open) {
         final int cell = col + row * 9;
         unlockedCells.put(cell, open);
         return this;
     }
 
+    @NotNull
     public Gui setTitle(String title) {
         this.title = title;
         return this;
@@ -201,6 +213,7 @@ public class Gui {
         return rows;
     }
 
+    @NotNull
     public Gui setRows(int rows) {
         switch (inventoryType) {
             case HOPPER:
@@ -212,20 +225,24 @@ public class Gui {
         return this;
     }
 
-    public Gui setDefaultAction(Clickable action) {
+    @NotNull
+    public Gui setDefaultAction(@Nullable Clickable action) {
         defaultClicker = action;
         return this;
     }
 
+    @NotNull
     public Gui setDefaultItem(ItemStack item) {
         blankItem = item;
         return this;
     }
 
+    @Nullable
     public ItemStack getDefaultItem() {
         return blankItem;
     }
 
+    @Nullable
     public ItemStack getItem(int cell) {
         if (inventory != null && unlockedCells.getOrDefault(cell, false)) {
             return inventory.getItem(cell);
@@ -233,6 +250,7 @@ public class Gui {
         return cellItems.get(cell);
     }
 
+    @Nullable
     public ItemStack getItem(int row, int col) {
         final int cell = col + row * 9;
         if (inventory != null && unlockedCells.getOrDefault(cell, false)) {
@@ -241,7 +259,8 @@ public class Gui {
         return cellItems.get(cell);
     }
 
-    public Gui setItem(int cell, ItemStack item) {
+    @NotNull
+    public Gui setItem(int cell, @Nullable ItemStack item) {
         cellItems.put(cell, item);
         if (inventory != null && cell >= 0 && cell < inventory.getSize()) {
             inventory.setItem(cell, item);
@@ -249,7 +268,8 @@ public class Gui {
         return this;
     }
 
-    public Gui setItem(int row, int col, ItemStack item) {
+    @NotNull
+    public Gui setItem(int row, int col, @Nullable ItemStack item) {
         final int cell = col + row * 9;
         cellItems.put(cell, item);
         if (inventory != null && cell >= 0 && cell < inventory.getSize()) {
@@ -258,6 +278,7 @@ public class Gui {
         return this;
     }
 
+    @NotNull
     public Gui highlightItem(int cell) {
         ItemStack item = cellItems.get(cell);
         if (item != null && item.getType() != Material.AIR) {
@@ -266,6 +287,7 @@ public class Gui {
         return this;
     }
 
+    @NotNull
     public Gui highlightItem(int row, int col) {
         final int cell = col + row * 9;
         ItemStack item = cellItems.get(cell);
@@ -275,6 +297,7 @@ public class Gui {
         return this;
     }
 
+    @NotNull
     public Gui removeHighlight(int cell) {
         ItemStack item = cellItems.get(cell);
         if (item != null && item.getType() != Material.AIR) {
@@ -283,6 +306,7 @@ public class Gui {
         return this;
     }
 
+    @NotNull
     public Gui removeHighlight(int row, int col) {
         final int cell = col + row * 9;
         ItemStack item = cellItems.get(cell);
@@ -292,11 +316,13 @@ public class Gui {
         return this;
     }
 
-    public Gui updateItem(int row, int col, String name, String... lore) {
+    @NotNull
+    public Gui updateItem(int row, int col, @Nullable String name, @NotNull String... lore) {
         return updateItem(col + row * 9, name, lore);
     }
 
-    public Gui updateItem(int cell, String name, String... lore) {
+    @NotNull
+    public Gui updateItem(int cell, @Nullable String name, @NotNull String... lore) {
         ItemStack item = cellItems.get(cell);
         if (item != null && item.getType() != Material.AIR) {
             setItem(cell, GuiUtils.updateItem(item, name, lore));
@@ -304,11 +330,13 @@ public class Gui {
         return this;
     }
 
-    public Gui updateItem(int row, int col, String name, List<String> lore) {
+    @NotNull
+    public Gui updateItem(int row, int col, @Nullable String name, @Nullable List<String> lore) {
         return updateItem(col + row * 9, name, lore);
     }
 
-    public Gui updateItem(int cell, String name, List<String> lore) {
+    @NotNull
+    public Gui updateItem(int cell, @NotNull String name, @Nullable List<String> lore) {
         ItemStack item = cellItems.get(cell);
         if (item != null && item.getType() != Material.AIR) {
             setItem(cell, GuiUtils.updateItem(item, title, lore));
@@ -316,11 +344,13 @@ public class Gui {
         return this;
     }
 
-    public Gui updateItem(int row, int col, ItemStack itemTo, String title, String... lore) {
+    @NotNull
+    public Gui updateItem(int row, int col, @NotNull ItemStack itemTo, @Nullable String title, @NotNull String... lore) {
         return updateItem(col + row * 9, itemTo, title, lore);
     }
 
-    public Gui updateItem(int cell, ItemStack itemTo, String title, String... lore) {
+    @NotNull
+    public Gui updateItem(int cell, @NotNull ItemStack itemTo, @Nullable String title, @NotNull String... lore) {
         ItemStack item = cellItems.get(cell);
         if (item != null && item.getType() != Material.AIR) {
             setItem(cell, GuiUtils.updateItem(item, itemTo, title, lore));
@@ -328,11 +358,13 @@ public class Gui {
         return this;
     }
 
-    public Gui updateItem(int row, int col, LegacyMaterials itemTo, String title, String... lore) {
+    @NotNull
+    public Gui updateItem(int row, int col, @NotNull LegacyMaterials itemTo, @Nullable String title, @NotNull String... lore) {
         return updateItem(col + row * 9, itemTo, title, lore);
     }
 
-    public Gui updateItem(int cell, LegacyMaterials itemTo, String title, String... lore) {
+    @NotNull
+    public Gui updateItem(int cell, @NotNull LegacyMaterials itemTo, @Nullable String title, @Nullable String... lore) {
         ItemStack item = cellItems.get(cell);
         if (item != null && item.getType() != Material.AIR) {
             setItem(cell, GuiUtils.updateItem(item, itemTo, title, lore));
@@ -340,11 +372,13 @@ public class Gui {
         return this;
     }
 
-    public Gui updateItem(int row, int col, ItemStack itemTo, String title, List<String> lore) {
+    @NotNull
+    public Gui updateItem(int row, int col, @NotNull ItemStack itemTo, @Nullable String title, @Nullable List<String> lore) {
         return updateItem(col + row * 9, itemTo, title, lore);
     }
 
-    public Gui updateItem(int cell, ItemStack itemTo, String title, List<String> lore) {
+    @NotNull
+    public Gui updateItem(int cell, @NotNull ItemStack itemTo, @Nullable String title, @Nullable List<String> lore) {
         ItemStack item = cellItems.get(cell);
         if (item != null && item.getType() != Material.AIR) {
             setItem(cell, GuiUtils.updateItem(item, itemTo, title, lore));
@@ -352,11 +386,13 @@ public class Gui {
         return this;
     }
 
-    public Gui updateItem(int row, int col, LegacyMaterials itemTo, String title, List<String> lore) {
+    @NotNull
+    public Gui updateItem(int row, int col, @NotNull LegacyMaterials itemTo, @Nullable String title, @Nullable List<String> lore) {
         return updateItem(col + row * 9, itemTo, title, lore);
     }
 
-    public Gui updateItem(int cell, LegacyMaterials itemTo, String title, List<String> lore) {
+    @NotNull
+    public Gui updateItem(int cell, @NotNull LegacyMaterials itemTo, @Nullable String title, @Nullable List<String> lore) {
         ItemStack item = cellItems.get(cell);
         if (item != null && item.getType() != Material.AIR) {
             setItem(cell, GuiUtils.updateItem(item, itemTo, title, lore));
@@ -364,34 +400,40 @@ public class Gui {
         return this;
     }
 
-    public Gui setAction(int cell, Clickable action) {
+    @NotNull
+    public Gui setAction(int cell, @Nullable Clickable action) {
         setConditional(cell, null, action);
         return this;
     }
 
-    public Gui setAction(int row, int col, Clickable action) {
+    @NotNull
+    public Gui setAction(int row, int col, @Nullable Clickable action) {
         setConditional(col + row * 9, null, action);
         return this;
     }
 
-    public Gui setAction(int cell, ClickType type, Clickable action) {
+    @NotNull
+    public Gui setAction(int cell, @Nullable ClickType type, @Nullable Clickable action) {
         setConditional(cell, type, action);
         return this;
     }
 
-    public Gui setAction(int row, int col, ClickType type, Clickable action) {
+    @NotNull
+    public Gui setAction(int row, int col, @Nullable ClickType type, @Nullable Clickable action) {
         setConditional(col + row * 9, type, action);
         return this;
     }
 
-    public Gui setActionForRange(int cellFirst, int cellLast, Clickable action) {
+    @NotNull
+    public Gui setActionForRange(int cellFirst, int cellLast, @Nullable Clickable action) {
         for (int cell = cellFirst; cell <= cellLast; ++cell) {
             setConditional(cell, null, action);
         }
         return this;
     }
 
-    public Gui setActionForRange(int cellRowFirst, int cellColFirst, int cellRowLast, int cellColLast, Clickable action) {
+    @NotNull
+    public Gui setActionForRange(int cellRowFirst, int cellColFirst, int cellRowLast, int cellColLast, @Nullable Clickable action) {
         final int last = cellColLast + cellRowLast * 9;
         for (int cell = cellColFirst + cellRowFirst * 9; cell <= last; ++cell) {
             setConditional(cell, null, action);
@@ -399,14 +441,16 @@ public class Gui {
         return this;
     }
 
-    public Gui setActionForRange(int cellFirst, int cellLast, ClickType type, Clickable action) {
+    @NotNull
+    public Gui setActionForRange(int cellFirst, int cellLast, @Nullable ClickType type, @Nullable Clickable action) {
         for (int cell = cellFirst; cell <= cellLast; ++cell) {
             setConditional(cell, type, action);
         }
         return this;
     }
 
-    public Gui setActionForRange(int cellRowFirst, int cellColFirst, int cellRowLast, int cellColLast, ClickType type, Clickable action) {
+    @NotNull
+    public Gui setActionForRange(int cellRowFirst, int cellColFirst, int cellRowLast, int cellColLast, @Nullable ClickType type, @Nullable Clickable action) {
         final int last = cellColLast + cellRowLast * 9;
         for (int cell = cellColFirst + cellRowFirst * 9; cell <= last; ++cell) {
             setConditional(cell, type, action);
@@ -414,90 +458,128 @@ public class Gui {
         return this;
     }
 
+    @NotNull
     public Gui clearActions(int cell) {
         conditionalButtons.remove(cell);
         return this;
     }
 
+    @NotNull
     public Gui clearActions(int row, int col) {
         final int cell = col + row * 9;
         conditionalButtons.remove(cell);
         return this;
     }
 
-    public Gui setButton(int cell, ItemStack item, Clickable action) {
+    @NotNull
+    public Gui setButton(int cell, ItemStack item, @Nullable Clickable action) {
         setItem(cell, item);
         setConditional(cell, null, action);
         return this;
     }
 
-    public Gui setButton(int row, int col, ItemStack item, Clickable action) {
+    @NotNull
+    public Gui setButton(int row, int col, @Nullable ItemStack item, @Nullable Clickable action) {
         final int cell = col + row * 9;
         setItem(cell, item);
         setConditional(cell, null, action);
         return this;
     }
 
-    public Gui setButton(int cell, ItemStack item, ClickType type, Clickable action) {
+    @NotNull
+    public Gui setButton(int cell, @Nullable ItemStack item, @Nullable ClickType type, @Nullable Clickable action) {
         setItem(cell, item);
         setConditional(cell, type, action);
         return this;
     }
 
-    public Gui setButton(int row, int col, ItemStack item, ClickType type, Clickable action) {
+    @NotNull
+    public Gui setButton(int row, int col, @Nullable ItemStack item, @Nullable ClickType type, @Nullable Clickable action) {
         final int cell = col + row * 9;
         setItem(cell, item);
         setConditional(cell, type, action);
         return this;
     }
 
-    protected void setConditional(int cell, ClickType type, Clickable action) {
+    protected void setConditional(int cell, @Nullable ClickType type, @Nullable Clickable action) {
         Map<ClickType, Clickable> conditionals = conditionalButtons.get(cell);
-        if (action != null) {
-            if (conditionals == null) {
-                conditionalButtons.put(cell, conditionals = new HashMap());
-            }
-            conditionals.put(type, action);
+        if (conditionals == null) {
+            conditionalButtons.put(cell, conditionals = new HashMap());
         }
+        conditionals.put(type, action);
     }
 
-    public Gui setOnOpen(Openable action) {
+    @NotNull
+    public Gui setOnOpen(@Nullable Openable action) {
         opener = action;
         return this;
     }
 
-    public Gui setOnClose(Closable action) {
+    @NotNull
+    public Gui setOnClose(@Nullable Closable action) {
         closer = action;
         return this;
     }
 
-    public Gui setOnDrop(Droppable action) {
+    @NotNull
+    public Gui setOnDrop(@Nullable Droppable action) {
         dropper = action;
         return this;
     }
 
-    public Gui setOnPage(Pagable action) {
+    @NotNull
+    public Gui setOnPage(@Nullable Pagable action) {
         pager = action;
         return this;
     }
 
-    public Gui setNextPage(int row, int col, ItemStack item) {
+    public Gui setNextPage(ItemStack item) {
+        nextPage = item;
+        return this;
+    }
+
+    public Gui setPrevPage(ItemStack item) {
+        prevPage = item;
+        return this;
+    }
+
+    @NotNull
+    public Gui setNextPage(int cell, @NotNull ItemStack item) {
+        nextPageIndex = cell;
+        if (page < pages) {
+            setButton(nextPageIndex, nextPage = item, ClickType.LEFT, (event) -> this.nextPage(event.manager));
+        }
+        return this;
+    }
+
+    @NotNull
+    public Gui setNextPage(int row, int col, @NotNull ItemStack item) {
         nextPageIndex = col + row * 9;
         if (page < pages) {
-            setButton(nextPageIndex, item, ClickType.LEFT, (event) -> this.nextPage(event.manager));
+            setButton(nextPageIndex, nextPage = item, ClickType.LEFT, (event) -> this.nextPage(event.manager));
         }
         return this;
     }
 
-    public Gui setPrevPage(int row, int col, ItemStack item) {
+    @NotNull
+    public Gui setPrevPage(int cell, @NotNull ItemStack item) {
+        prevPageIndex = cell;
+        if (page > 1) {
+            setButton(prevPageIndex, prevPage = item, ClickType.LEFT, (event) -> this.prevPage(event.manager));
+        }
+        return this;
+    }
+
+    @NotNull
+    public Gui setPrevPage(int row, int col, @NotNull ItemStack item) {
         prevPageIndex = col + row * 9;
         if (page > 1) {
-            setButton(prevPageIndex, item, ClickType.LEFT, (event) -> this.prevPage(event.manager));
+            setButton(prevPageIndex, prevPage = item, ClickType.LEFT, (event) -> this.prevPage(event.manager));
         }
         return this;
     }
 
-    public void nextPage(GuiManager manager) {
+    public void nextPage(@NotNull GuiManager manager) {
         if (page < pages) {
             int lastPage = page;
             ++page;
@@ -515,7 +597,7 @@ public class Gui {
         }
     }
 
-    public void prevPage(GuiManager manager) {
+    public void prevPage(@NotNull GuiManager manager) {
         if (page > 1) {
             int lastPage = page;
             --page;
@@ -547,11 +629,13 @@ public class Gui {
         }
     }
 
-    protected Inventory getOrCreateInventory(GuiManager manager) {
+    @NotNull
+    protected Inventory getOrCreateInventory(@NotNull GuiManager manager) {
         return inventory != null ? inventory : generateInventory(manager);
     }
 
-    protected Inventory generateInventory(GuiManager manager) {
+    @NotNull
+    protected Inventory generateInventory(@NotNull GuiManager manager) {
         final int cells = rows * 9;
         InventoryType t = inventoryType == null ? InventoryType.CHEST : inventoryType.type;
         switch (t) {
@@ -573,6 +657,7 @@ public class Gui {
         return inventory;
     }
 
+    @Nullable
     public Gui getParent() {
         return parent;
     }
@@ -595,11 +680,11 @@ public class Gui {
         return title;
     }
 
-    protected boolean onClickOutside(GuiManager manager, Player player, InventoryClickEvent event) {
+    protected boolean onClickOutside(@NotNull GuiManager manager, @NotNull Player player, @NotNull InventoryClickEvent event) {
         return dropper != null ? dropper.onDrop(new GuiDropItemEvent(manager, this, player, event)) : true;
     }
 
-    protected boolean onClick(GuiManager manager, Player player, Inventory inventory, InventoryClickEvent event) {
+    protected boolean onClick(@NotNull GuiManager manager, @NotNull Player player, @NotNull Inventory inventory, @NotNull InventoryClickEvent event) {
         final int cell = event.getSlot();
         Map<ClickType, Clickable> conditionals = conditionalButtons.get(cell);
         Clickable button;
@@ -608,7 +693,7 @@ public class Gui {
             button.onClick(new GuiClickEvent(manager, this, player, event, cell, true));
         } else {
             // no event for this button
-            if(defaultClicker != null) {
+            if (defaultClicker != null) {
                 // this is a default action, not a triggered action
                 defaultClicker.onClick(new GuiClickEvent(manager, this, player, event, cell, true));
             }
@@ -617,19 +702,19 @@ public class Gui {
         return true;
     }
 
-    protected boolean onClickPlayerInventory(GuiManager manager, Player player, Inventory openInv, InventoryClickEvent event) {
+    protected boolean onClickPlayerInventory(@NotNull GuiManager manager, @NotNull Player player, @NotNull Inventory openInv, @NotNull InventoryClickEvent event) {
         // no events for this yet
         return false;
     }
 
-    public void onOpen(GuiManager manager, Player player) {
+    public void onOpen(@NotNull GuiManager manager, @NotNull Player player) {
         open = true;
         if (opener != null) {
             opener.onOpen(new GuiOpenEvent(manager, this, player));
         }
     }
 
-    public void onClose(GuiManager manager, Player player) {
+    public void onClose(@NotNull GuiManager manager, @NotNull Player player) {
         if (!allowClose) {
             manager.showGUI(player, this);
             return;

src/main/java/com/songoda/core/gui/SimplePagedGui.java

@@ -62,16 +62,6 @@ public class SimplePagedGui extends Gui {
         return this;
     }
 
-    public SimplePagedGui setNextPage(ItemStack item) {
-        nextPage = item;
-        return this;
-    }
-
-    public SimplePagedGui setPrevPage(ItemStack item) {
-        prevPage = item;
-        return this;
-    }
-
     @Override
     public SimplePagedGui setItem(int row, int col, ItemStack item) {
         return setItem(col + row * 9, item);