public class GuiTest extends PaginableInventory {

    private final Player player;

    public GuiTest(int sizeGui, Player player) {
        super(sizeGui); //9
        this.player = player;
    }

    private void iconsMaker() {
        List<Icon> icons = new ArrayList<>();

        for(int i = 0; i < 29; i++) {
            if(i <= 16) {
                icons.add(new Icon(new ItemStack(Material.ANVIL), a -> { nextPage(); }));
            } else {
                icons.add(new Icon(new ItemStack(Material.DIAMOND), a -> { nextPage(); }));
            }
        }

        setIconsPage(icons);
    }

    @Override
    protected void registerItems(Player player) {
        //Paginable basic
        setPositionPreviousPage(0);
        setPositionNextPage(9);
        setPlayer(player);
        
        //Code
        player.sendMessage("Gui Open");
        iconsMaker();
        setPage(0);
    }
}