Create class that allows to change required entities for challenge.

src/main/java/world/bentobox/challenges/panel/admin/ManageEntitiesGUI.java

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+package world.bentobox.challenges.panel.admin;
+
+
+import org.apache.commons.lang.WordUtils;
+import org.bukkit.Material;
+import org.bukkit.World;
+import org.bukkit.entity.EntityType;
+import org.bukkit.inventory.ItemStack;
+import java.util.*;
+
+import world.bentobox.bentobox.api.panels.PanelItem;
+import world.bentobox.bentobox.api.panels.builders.PanelBuilder;
+import world.bentobox.bentobox.api.panels.builders.PanelItemBuilder;
+import world.bentobox.bentobox.api.user.User;
+import world.bentobox.challenges.ChallengesAddon;
+import world.bentobox.challenges.panel.CommonGUI;
+import world.bentobox.challenges.panel.util.NumberGUI;
+import world.bentobox.challenges.panel.util.SelectEntityGUI;
+import world.bentobox.challenges.utils.HeadLib;
+
+
+/**
+ * This class allows to edit entities that are in required entities map.
+ */
+public class ManageEntitiesGUI extends CommonGUI
+{
+	public ManageEntitiesGUI(ChallengesAddon addon,
+		World world,
+		User user,
+		Map<EntityType, Integer> requiredEntities,
+		String topLabel,
+		String permissionPrefix,
+		CommonGUI parentGUI)
+	{
+		super(addon, world, user, topLabel, permissionPrefix, parentGUI);
+		this.requiredEntities = requiredEntities;
+
+		this.entityList = new ArrayList<>(this.requiredEntities.keySet());
+		this.entityList.sort(Comparator.comparing(Enum::name));
+
+		this.selectedEntities = new HashSet<>(EntityType.values().length);
+	}
+
+
+// ---------------------------------------------------------------------
+// Section: Methods
+// ---------------------------------------------------------------------
+
+
+	/**
+	 * This method builds all necessary elements in GUI panel.
+	 */
+	@Override
+	public void build()
+	{
+		PanelBuilder panelBuilder = new PanelBuilder().user(this.user).
+			name(this.user.getTranslation("challenges.gui.admin.edit-entities"));
+
+		// create border
+		this.fillBorder(panelBuilder);
+
+		panelBuilder.item(3, this.createButton(Button.ADD));
+		panelBuilder.item(5, this.createButton(Button.REMOVE));
+
+		final int MAX_ELEMENTS = 21;
+
+		if (this.pageIndex < 0)
+		{
+			this.pageIndex = this.entityList.size() / MAX_ELEMENTS;
+		}
+		else if (this.pageIndex > (this.entityList.size() / MAX_ELEMENTS))
+		{
+			this.pageIndex = 0;
+		}
+
+		int entitiesIndex = MAX_ELEMENTS * this.pageIndex;
+
+		// I want first row to be only for navigation and return button.
+		int index = 10;
+
+		while (entitiesIndex < ((this.pageIndex + 1) * MAX_ELEMENTS) &&
+			entitiesIndex < this.entityList.size())
+		{
+			if (!panelBuilder.slotOccupied(index))
+			{
+				panelBuilder.item(index, this.createEntityButton(this.entityList.get(entitiesIndex++)));
+			}
+
+			index++;
+		}
+
+		// Navigation buttons only if necessary
+		if (this.entityList.size() > MAX_ELEMENTS)
+		{
+			panelBuilder.item(18, this.getButton(CommonButtons.PREVIOUS));
+			panelBuilder.item(26, this.getButton(CommonButtons.NEXT));
+		}
+
+		// Add return button.
+		panelBuilder.item(44, this.returnButton);
+
+		panelBuilder.build();
+	}
+
+
+	/**
+	 * This method creates PanelItem button of requested type.
+	 * @param button Button which must be created.
+	 * @return new PanelItem with requested functionality.
+	 */
+	private PanelItem createButton(Button button)
+	{
+		PanelItemBuilder builder = new PanelItemBuilder();
+
+		switch (button)
+		{
+			case ADD:
+				builder.name(this.user.getTranslation("challenges.gui.button.add"));
+				builder.icon(Material.BUCKET);
+				builder.clickHandler((panel, user1, clickType, slot) -> {
+					new SelectEntityGUI(this.user, (status, entity) -> {
+						if (status)
+						{
+							if (!this.requiredEntities.containsKey(entity))
+							{
+								this.requiredEntities.put(entity, 1);
+								this.entityList.add(entity);
+							}
+						}
+
+						this.build();
+					});
+					return true;
+				});
+				break;
+			case REMOVE:
+				builder.name(this.user.getTranslation("challenges.gui.button.remove-selected"));
+				builder.icon(Material.LAVA_BUCKET);
+				builder.clickHandler((panel, user1, clickType, slot) -> {
+					this.requiredEntities.keySet().removeAll(this.selectedEntities);
+					this.entityList.removeAll(this.selectedEntities);
+					return true;
+				});
+				break;
+		}
+
+		return builder.build();
+	}
+
+
+	/**
+	 * This method creates button for given entity.
+	 * @param entity Entity which button must be created.
+	 * @return new Button for entity.
+	 */
+	private PanelItem createEntityButton(EntityType entity)
+	{
+		return new PanelItemBuilder().
+			name(WordUtils.capitalize(entity.name().toLowerCase().replace("_", " "))).
+			icon(this.asEggs ? this.getEntityEgg(entity) : this.getEntityHead(entity)).
+			clickHandler((panel, user1, clickType, slot) -> {
+				// On right click change which entities are selected for deletion.
+				if (clickType.isRightClick())
+				{
+					if (!this.selectedEntities.add(entity))
+					{
+						// Remove entity if it is already selected
+						this.selectedEntities.remove(entity);
+					}
+
+					this.build();
+				}
+				else
+				{
+					new NumberGUI(this.user, this.requiredEntities.get(entity), 1, (status, value) -> {
+						if (status)
+						{
+							// Update value only when something changes.
+							this.requiredEntities.put(entity, value);
+						}
+
+						this.build();
+					});
+				}
+				return true;
+			}).
+			glow(this.selectedEntities.contains(entity)).
+			build();
+	}
+
+
+	/**
+	 * This method transforms entity into egg or block that corresponds given entity. If entity egg is not
+	 * found, then it is replaced by block that represents entity or barrier block.
+	 * @param entity Entity which egg must be returned.
+	 * @return ItemStack that may be egg for given entity.
+	 */
+	private ItemStack getEntityEgg(EntityType entity)
+	{
+		ItemStack itemStack;
+
+		switch (entity)
+		{
+			case PIG_ZOMBIE:
+				itemStack = new ItemStack(Material.ZOMBIE_PIGMAN_SPAWN_EGG);
+				break;
+			case ENDER_DRAGON:
+				itemStack = new ItemStack(Material.DRAGON_EGG);
+				break;
+			case WITHER:
+				itemStack = new ItemStack(Material.SOUL_SAND);
+				break;
+			case PLAYER:
+				itemStack = new ItemStack(Material.PLAYER_HEAD);
+				break;
+			case MUSHROOM_COW:
+				itemStack = new ItemStack(Material.MOOSHROOM_SPAWN_EGG);
+				break;
+			case SNOWMAN:
+				itemStack = new ItemStack(Material.CARVED_PUMPKIN);
+				break;
+			case IRON_GOLEM:
+				itemStack = new ItemStack(Material.IRON_BLOCK);
+				break;
+			case ARMOR_STAND:
+				itemStack = new ItemStack(Material.ARMOR_STAND);
+				break;
+			default:
+				Material material = Material.getMaterial(entity.name() + "_SPAWN_EGG");
+
+				if (material == null)
+				{
+					itemStack = new ItemStack(Material.BARRIER);
+				}
+				else
+				{
+					itemStack = new ItemStack(material);
+				}
+
+				break;
+		}
+
+		itemStack.setAmount(this.requiredEntities.get(entity));
+
+		return itemStack;
+	}
+
+
+	/**
+	 * This method transforms entity into player head with skin that corresponds given entity. If entity head
+	 * is not found, then it is replaced by barrier block.
+	 * @param entity Entity which head must be returned.
+	 * @return ItemStack that may be head for given entity.
+	 */
+	private ItemStack getEntityHead(EntityType entity)
+	{
+		ItemStack itemStack;
+
+		switch (entity)
+		{
+			case PLAYER:
+				itemStack = new ItemStack(Material.PLAYER_HEAD);
+				break;
+			case WITHER_SKELETON:
+				itemStack = new ItemStack(Material.WITHER_SKELETON_SKULL);
+				break;
+			case ARMOR_STAND:
+				itemStack = new ItemStack(Material.ARMOR_STAND);
+				break;
+			case SKELETON:
+				itemStack = new ItemStack(Material.SKELETON_SKULL);
+				break;
+			case GIANT:
+			case ZOMBIE:
+				itemStack = new ItemStack(Material.ZOMBIE_HEAD);
+				break;
+			case CREEPER:
+				itemStack = new ItemStack(Material.CREEPER_HEAD);
+				break;
+			case ENDER_DRAGON:
+				itemStack = new ItemStack(Material.DRAGON_HEAD);
+				break;
+			default:
+				HeadLib head = HeadLib.getHead(entity.name());
+
+				if (head == null)
+				{
+					itemStack = new ItemStack(Material.BARRIER);
+				}
+				else
+				{
+					itemStack = head.toItemStack(this.requiredEntities.get(entity));
+				}
+				break;
+		}
+
+		return itemStack;
+	}
+
+
+// ---------------------------------------------------------------------
+// Section: Enums
+// ---------------------------------------------------------------------
+
+
+	/**
+	 * Functional buttons in current GUI.
+	 */
+	private enum Button
+	{
+		ADD,
+		REMOVE
+	}
+
+
+// ---------------------------------------------------------------------
+// Section: Variables
+// ---------------------------------------------------------------------
+
+	/**
+	 * List with entities to avoid list irregularities.
+	 */
+	private List<EntityType> entityList;
+
+	/**
+	 * Set with entities that are selected.
+	 */
+	private Set<EntityType> selectedEntities;
+
+	/**
+	 * Map that contains all entities and their cound.
+	 */
+	private Map<EntityType, Integer> requiredEntities;
+
+	/**
+	 * Boolean indicate if entities should be displayed as eggs or mob heads.
+	 */
+	private boolean asEggs;
+}