Create ConfirmationGUI class that creates GUI with 2 buttons.

On Cancel it should return to previous GUI.
On Accept it should process given command. If command does not exist, it should throw error message in chat. At the end it should return to previous GUI.

src/main/java/world/bentobox/challenges/panel/util/ConfirmationGUI.java

@@ -0,0 +1,138 @@
+package world.bentobox.challenges.panel.util;
+
+
+
+
+import org.bukkit.Material;
+
+import java.util.*;
+
+import world.bentobox.bentobox.BentoBox;
+import world.bentobox.bentobox.api.commands.CompositeCommand;
+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.panel.CommonGUI;
+
+
+/**
+ * This GUI is used to confirm that user wants to run command, that should be created from
+ * command string list.
+ */
+public class ConfirmationGUI
+{
+	/**
+	 * This constructor inits and opens ConfirmationGUI.
+	 * @param user Gui Caller.
+	 * @param parentGUI Parent GUI.
+	 * @param commandLabels Command labels.
+	 * @param variables Variables at the end of command.
+	 */
+	public ConfirmationGUI(BentoBox plugin,
+		User user,
+		CommonGUI parentGUI,
+		List<String> commandLabels,
+		String... variables)
+	{
+		this.plugin = plugin;
+		this.user = user;
+		this.parentGUI = parentGUI;
+		this.commandLabels = commandLabels;
+		this.variables = variables;
+
+		if (this.commandLabels.isEmpty())
+		{
+			this.user.sendMessage("challenges.errors.missing-command");
+		}
+		else
+		{
+			this.build();
+		}
+	}
+
+
+	/**
+	 * This method builds confirmation panel with 2 buttons.
+	 */
+	public void build()
+	{
+		PanelBuilder panelBuilder = new PanelBuilder().name(this.user.getTranslation("challenges.gui.admin.confirm-title"));
+
+		panelBuilder.item(3, new PanelItemBuilder().
+			name(this.user.getTranslation("challenges.gui.admin.buttons.proceed")).
+			icon(Material.GREEN_STAINED_GLASS_PANE).
+			clickHandler((panel, user1, clickType, index) ->
+			{
+				Iterator<String> iterator = this.commandLabels.iterator();
+
+				CompositeCommand command = this.plugin.getCommandsManager().getCommand(iterator.next());
+
+				while (iterator.hasNext() && command != null)
+				{
+					Optional<CompositeCommand> commandOptional = command.getSubCommand(iterator.next());
+
+					if (commandOptional.isPresent())
+					{
+						command = commandOptional.get();
+					}
+					else
+					{
+						this.user.sendMessage("challenges.errors.missing-command");
+						command = null;
+					}
+				}
+
+				if (command != null)
+				{
+					command.execute(this.user, "CONFIRMATION", Arrays.asList(this.variables));
+				}
+
+				this.user.closeInventory();
+				this.parentGUI.build();
+				return true;
+			}).
+			build());
+
+		panelBuilder.item(5, new PanelItemBuilder().
+			name(this.user.getTranslation("challenges.gui.admin.buttons.cancel")).
+			icon(Material.RED_STAINED_GLASS_PANE).
+			clickHandler((panel, user1, clickType, i) ->
+			{
+				this.parentGUI.build();
+				return true;
+			}).
+			build());
+
+		panelBuilder.build();
+	}
+
+
+// ---------------------------------------------------------------------
+// Section: Variables
+// ---------------------------------------------------------------------
+
+	/**
+	 * BentoBox plugin.
+	 */
+	private BentoBox plugin;
+
+	/**
+	 * User who wants to run command.
+	 */
+	private User user;
+
+	/**
+	 * Parent GUI where should return on cancel or proceed.
+	 */
+	private CommonGUI parentGUI;
+
+	/**
+	 * List of command labels.
+	 */
+	private List<String> commandLabels;
+
+	/**
+	 * List of variables.
+	 */
+	private String[] variables;
+}