Main.java

public static IntFunction<Inventory> createGuide(List<Category> categories, Plugin plugin, int rows) {
	final MiniWindow window = new MiniWindow(plugin);
	final List<Supplier<Inventory>> pages = new ArrayList<>();
	
	//Split our list up into pages.
	final List<List<Category>> partitions = partition(categories, rows * 9);
	
	//Loop over our category lists with an index.
	for (int index = 0; index < partitions.size(); index++) {
		
		//Get the list of categories for this page.
		final List<Category> categoryList = partitions.get(index);
		
		int pageNumber = index; //This is just because java is stupid af...
		pages.add(index, cache(() -> {
			//Create a new page
			Page page = window.page();
			//Give it a title
			page.title("Page " + pageNumber);
			
			//Add items for each category on the page.
			for (Category category : categoryList) {//Create a new item for the category.
				Item item = page.element();
				
				//Let the category style it.
				category.itemLike().accept(item);
				
				//Cache the category's page as well.
				Supplier<Inventory> categoryPage = cache(() ->
						window.page(category.pageLike())
				);
				
				//Open the category's page when it's icon is clicked.
				item.onClick(player ->
						player.openInventory(categoryPage.get())
				);
			}
			
			//Footer time.
			for (int i = 0; i < 9; i++) {
				Item item = page.element(rows + 1).icon(STAINED_GLASS_PANE);
				if (pageNumber != 0 && i == 2)
					item.data(RED.getDyeData()).onClick(player ->
							//Navigate back one page.
							player.openInventory(pages.get((int) (pageNumber - 1)).get())
					);
				else if (pageNumber != partitions.size() - 1 && i == 6)
					item.data(GREEN.getDyeData()).onClick(player ->
							//Navigate forward one page.
							player.openInventory(pages.get((int) (pageNumber + 1)).get())
					);
				else
					item.data(GRAY.getDyeData());
			}
			return page.toInventory();
		}));
	}
	
	return index -> pages.get(index).get();
}

interface Category {
	static Category create(Consumer<Page> page, Consumer<Item> item) {
		return new Category() {
			@Override
			public Consumer<Page> pageLike() {
				return page;
			}
			
			@Override
			public Consumer<Item> itemLike() {
				return item;
			}
		};
	}
	
	Consumer<Page> pageLike();
	
	Consumer<Item> itemLike();
}