ItemUtil.java

  /**
	 * Checks whether all of given items can be inserted into an inventory.
	 */
	public static <T extends ItemInventory & ItemInsertable> boolean canInsertAll(List<ItemStack> stacks, T inventory) {
		if (stacks.size() == 0) {
			return true;
		}
		if (stacks.size() == 1) {
			return inventory.canInsertFully(stacks.get(0));
		}
		OverlayItemInventory<T> overlay = new OverlayItemInventory<T>(inventory);
		for (ItemStack stack : stacks) {
			ItemStack excess = overlay.insert(stack, ActionMode.CHANGE);
			if (!excess.isEmpty()) {
				return false;
			}
		}
		return true;
	}

OverlayItemInventory.java

public class OverlayItemInventory<T extends ItemInventory & ItemInsertable> implements ItemInventory, ItemInsertable {
	protected final T inventory;

	protected final ItemStack[] overlay;

	public OverlayItemInventory(T inventory) {
		this.inventory = inventory;
		this.overlay = new ItemStack[inventory.size()];
	}

	@Override
	public int size() {
		return inventory.size();
	}

	@Override
	public ItemStack get(int slot) {
		if (overlay[slot] != null) {
			return overlay[slot];
		}
		return inventory.get(slot);
	}

	@Override
	public boolean isValid(int slot, ItemStack stack) {
		return inventory.isValid(slot, stack);
	}

	@Override
	public boolean set(int slot, ItemStack stack, ActionMode mode) {
		if (!isValid(slot, stack)) {
			return false;
		}
		if (mode == ActionMode.CHANGE) {
			overlay[slot] = stack;
		}
		return true;
	}

	@Override
	public ItemStack insert(ItemStack stack, ActionMode mode) {
		return InventoryUtil.insertSequentially(this, stack, mode);
	}
}