Create ListLevelsGUI class that allows to view all Levels.

It has 2 modes: edit mode and delete mode.

src/main/java/world/bentobox/challenges/ChallengesManager.java

@@ -515,4 +515,10 @@ public class ChallengesManager {
     {
         return new ArrayList<>(this.challengeMap.get(challengeLevel));
     }
+
+
+    public List<ChallengeLevels> getChallengeLevelList()
+    {
+        return new ArrayList<>(this.challengeMap.keySet());
+    }
 }

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

@@ -0,0 +1,176 @@
+package world.bentobox.challenges.panel.admin;
+
+
+import org.bukkit.World;
+
+import java.util.List;
+
+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.database.object.ChallengeLevels;
+import world.bentobox.challenges.panel.CommonGUI;
+
+
+/**
+ * This class creates GUI that lists all Levels. Clicking on Level icon will be processed
+ * by input mode.
+ */
+public class ListLevelsGUI extends CommonGUI
+{
+// ---------------------------------------------------------------------
+// Section: Constructor
+// ---------------------------------------------------------------------
+
+
+	/**
+	 * {@inheritDoc}
+	 * @param mode - mode that indicate what should do icon clicking.
+	 */
+	public ListLevelsGUI(ChallengesAddon addon,
+		World world,
+		User user,
+		Mode mode,
+		String topLabel,
+		String permissionPrefix)
+	{
+		this(addon, world, user, mode, topLabel, permissionPrefix, null);
+	}
+
+
+	/**
+	 * {@inheritDoc}
+	 * @param mode - mode that indicate what should do icon clicking.
+	 */
+	public ListLevelsGUI(ChallengesAddon addon,
+		World world,
+		User user,
+		Mode mode,
+		String topLabel,
+		String permissionPrefix,
+		CommonGUI parentGUI)
+	{
+		super(addon, world, user, topLabel, permissionPrefix, parentGUI);
+		this.currentMode = mode;
+	}
+
+
+// ---------------------------------------------------------------------
+// Section: Methods
+// ---------------------------------------------------------------------
+
+
+	/**
+	 * {@inheritDoc}
+	 */
+	@Override
+	public void build()
+	{
+		PanelBuilder panelBuilder = new PanelBuilder().user(this.user).name(
+			this.user.getTranslation("challenges.gui.admin.choose-level-title"));
+
+		List<ChallengeLevels> levelList = this.addon.getChallengesManager().getChallengeLevelList();
+
+		int MAX_ELEMENTS = 45;
+		if (this.pageIndex < 0)
+		{
+			this.pageIndex = 0;
+		}
+		else if (this.pageIndex > (levelList.size() / MAX_ELEMENTS))
+		{
+			this.pageIndex = levelList.size() / MAX_ELEMENTS;
+		}
+
+		int levelIndex = MAX_ELEMENTS * this.pageIndex;
+
+		while (levelIndex < ((this.pageIndex + 1) * MAX_ELEMENTS) &&
+			levelIndex < levelList.size())
+		{
+			panelBuilder.item(this.createLevelIcon(levelList.get(levelIndex)));
+			levelIndex++;
+		}
+
+		int nextIndex = levelIndex % MAX_ELEMENTS == 0 ?
+			MAX_ELEMENTS :
+			(((levelIndex % MAX_ELEMENTS) - 1) / 9 + 1) * 9;
+
+		if (levelIndex > MAX_ELEMENTS)
+		{
+			panelBuilder.item(nextIndex + 2, this.getButton(CommonButtons.PREVIOUS));
+		}
+
+		if (levelIndex < levelList.size())
+		{
+			panelBuilder.item(nextIndex + 6, this.getButton(CommonButtons.NEXT));
+		}
+
+		panelBuilder.item(nextIndex + 8, this.returnButton);
+
+		panelBuilder.build();
+	}
+
+
+	/**
+	 * This method creates button for given level
+	 * @param challengeLevel Level which button must be created.
+	 * @return Level button.
+	 */
+	private PanelItem createLevelIcon(ChallengeLevels challengeLevel)
+	{
+		PanelItemBuilder itemBuilder = new PanelItemBuilder().
+			name(challengeLevel.getFriendlyName()).
+			description(challengeLevel.getUnlockMessage()).
+			icon(challengeLevel.getIcon()).
+			glow(false);
+
+		if (this.currentMode.equals(Mode.EDIT))
+		{
+			itemBuilder.clickHandler((panel, user1, clickType, i) -> {
+				new EditLevelGUI(this.addon,
+					this.world,
+					this.user,
+					challengeLevel,
+					this.topLabel,
+					this.permissionPrefix,
+					this).build();
+				return true;
+			});
+		}
+		else if (this.currentMode.equals(Mode.DELETE))
+		{
+			itemBuilder.clickHandler((panel, user1, clickType, i) -> {
+				// TODO: Conformation GUI for DELETING.
+				return true;
+			});
+		}
+
+		return itemBuilder.build();
+	}
+
+
+// ---------------------------------------------------------------------
+// Section: Enums
+// ---------------------------------------------------------------------
+
+
+	/**
+	 * Mode in which gui icons should processed.
+	 */
+	public enum Mode
+	{
+		EDIT,
+		DELETE
+	}
+
+
+// ---------------------------------------------------------------------
+// Section: Variables
+// ---------------------------------------------------------------------
+
+	/**
+	 * Current mode in which icons will act.
+	 */
+	private Mode currentMode;
+}