GUI

GUI is a tiny API for Bukkit/Spigot/Paper that helps you to build and manage Bukkit inventory way much easier.

Build GUI

Old way :

Inventory inventory = Bukkit.createInventory(null, 3 * 9, "Inventory Title");

New way :

GUI gui = new GUI("GUI Title", 3);
// or when you need an owner
GUI gui2 = new GUI("Owned GUI Title", 4, owner);

Notice that you no longer need to calculate the number of slots. GUI takes this number of row only between 1 and 6 included. Like the Bukkit one, the inventory owner isn't needed.

Add and remove items

Bukkit Way :

ItemStack sword = new ItemStack(Material.IRON_SWORD);

inventory.addItem(sword);
inventory.remove(sword);
inventory.setItem(3, sword);

// Multiple set
int[] slots = new int[]{4, 5, 7, 8};

for (int i : slots) {
  inventory.setItem(i, new ItemStack(Material.GOLD_INGOT));
}

GUI way :

ItemStack sword = new ItemStack(Material.IRON_SWORD);

gui.add(sword);
gui.remove(sword);
gui.set(sword, 3);
gui.set(new ItemStack(Material.GOLD_INGOT), 4, 5, 7, 8);

Fill inventory

Bukkit way :

for (int i = 0; i < 9; i++) {
  inventory.setItem(i, new ItemStack(Material.DIAMOND));
}

GUI way :

gui.fill(1, 9, new ItemStack(Material.DIAMOND));

There is more than 10 overloaded fill methods, do not hesitate to search for the most suitable one for what you need.

Horizontal and Vertical fills

If you need to fill automatiquely an item inline, there is some existing methods to do it in GUI API.

Fill the first row of the GUI of red stained glass panes.

gui.horizontalFill(0, new ItemStack(Material.RED_STAINED_GLASS_PANE));

Fill the last column of the GUI of yellow stained glass pane. Does not replace item if one's already present

gui.verticalFill(8, new ItemStack(Material.YELLOW_STAINED_GLASS_PANE), false);

Notice that both of these methods overrides existing items by default.

Using of previous and next inventories

Depending of what you build, maybe you figured about navigation inside inventories. GUI API manages to it, here's a way :

GUI first = new GUI("First GUI", 3);
GUI second = new GUI("Second GUI", 1);
first.setNextGUI(second);

first.getNextGUI().open(player);

Previous GUIs are basically the same but in reverse order.

Some methods to complete

Get an item from GUI :

int slot = 0;
ItemStack item = gui.get(slot);

Retrivered slots from an ItemStack :

int firstSlot = gui.slot(item);
Set<Integer> allSlots = gui.slots(item);

Verify if slot is free (doesn't contains not null item) :

int slot = 8;
boolean isSlotEmpty = gui.isEmpty(slot);
// gui.isEmpty() check the entire inventory

Open the GUI to a player :

// You can still do :
player.openInventory(gui.getInventory());
// But a there's a shorter way :
gui.open(player);
// If you want to open the gui to his owner, you can do :
gui.open();
// Will throw an exception if owner is null