Tab GUI

A Tab GUI lets you switch between different GUIs (tabs) by clicking the Tab Items associated with them.

Control Items

First, we'll need to create our Tab Items:

KotlinJava

class MyTabItem(private val tab: Int) : TabItem(tab) {

    override fun getItemProvider(gui: TabGUI): ItemProvider {
        return if (gui.currentTab == tab) {
            ItemBuilder(Material.GLOWSTONE_DUST)
                .setDisplayName("Tab $tab (selected)")
        } else {
            ItemBuilder(Material.GUNPOWDER)
                .setDisplayName("Tab $tab (not selected)")
        }
    }

}

public class MyTabItem extends TabItem {

    private final int tab;

    public MyTabItem(int tab) {
        super(tab);
        this.tab = tab;
    }

    @Override
    public ItemProvider getItemProvider(TabGUI gui) {
        if (gui.getCurrentTab() == tab) {
            return new ItemBuilder(Material.GLOWSTONE_DUST)
                .setDisplayName("Tab " + tab + " (selected)");
        } else {
            return new ItemBuilder(Material.GUNPOWDER)
                .setDisplayName("Tab " + tab + " (not selected)");
        }
    }

}

Creating the GUI

Now, lets create the actual TabGUI. In this example, I've only created two tabs, but you can have as many tabs as you want.

KotlinJava

val border = SimpleItem(ItemBuilder(Material.BLACK_STAINED_GLASS_PANE).setDisplayName("§r"))

val gui1 = SimpleGUI(9, 3)
gui1.fill(SimpleItem(ItemBuilder(Material.DIRT)), true)

val gui2 = SimpleGUI(9, 3)
gui2.fill(SimpleItem(ItemBuilder(Material.DIAMOND)), true)

val gui: GUI = GUIBuilder(GUIType.TAB)
    .setStructure(
        "# # # 0 # 1 # # #",
        "x x x x x x x x x",
        "x x x x x x x x x",
        "x x x x x x x x x")
    .addIngredient('x', Markers.ITEM_LIST_SLOT_VERTICAL)
    .addIngredient('#', border)
    .addIngredient('0', MyTabItem(0))
    .addIngredient('1', MyTabItem(1))
    .setGUIs(listOf(gui1, gui2))
    .build()

SimpleWindow(player, "InvUI", gui).show()

Item border = new SimpleItem(new ItemBuilder(Material.BLACK_STAINED_GLASS_PANE).setDisplayName("§r"));

GUI gui1 = new SimpleGUI(9, 3);
gui1.fill(new SimpleItem(new ItemBuilder(Material.DIRT)), true);

GUI gui2 = new SimpleGUI(9, 3);
gui2.fill(new SimpleItem(new ItemBuilder(Material.DIAMOND)), true);

GUI gui = new GUIBuilder<>(GUIType.TAB)
    .setStructure(
        "# # # 0 # 1 # # #",
        "x x x x x x x x x",
        "x x x x x x x x x",
        "x x x x x x x x x")
    .addIngredient('x', Markers.ITEM_LIST_SLOT_VERTICAL)
    .addIngredient('#', border)
    .addIngredient('0', new MyTabItem(0))
    .addIngredient('1', new MyTabItem(1))
    .setGUIs(Arrays.asList(gui1, gui2))
    .build();

new SimpleWindow(player, "InvUI", gui).show();

And this is how it looks like:

Info

It is also possible to, for example, put the Tab Items in a separate paged or scroll gui if you need more space.
For that to work, you'd need to set the TabGUI that the Tab items are supposed to control manually using ControlItem#setGUI.