Main.java

package me.bhop.guide;

import org.bukkit.event.EventHandler;
import org.bukkit.event.player.PlayerJoinEvent;
import org.bukkit.inventory.Inventory;
import org.bukkit.plugin.Plugin;
import org.bukkit.plugin.java.JavaPlugin;

import java.util.ArrayList;
import java.util.List;
import java.util.function.Consumer;
import java.util.function.Supplier;

import static com.google.common.collect.Lists.partition;
import static com.google.common.collect.Streams.mapWithIndex;
import static java.util.stream.Collectors.toList;
import static me.bhop.guide.MiniWindow.Item;
import static me.bhop.guide.MiniWindow.Page;
import static org.bukkit.DyeColor.*;
import static org.bukkit.Material.STAINED_GLASS_PANE;

public class Main extends JavaPlugin {
	private final Guide guide;
	
	public Main() {
		guide = new Guide(this, 3);
	}
	
	@EventHandler
	void onJoin(PlayerJoinEvent event) {
		event.getPlayer().openInventory(guide.getPage(0));
	}
	
	
	public static Supplier<Inventory> cache(Supplier<Inventory> creator) {
		return new Supplier<Inventory>() {
			Inventory inventory = null;
			
			@Override
			public Inventory get() {
				if (inventory == null)
					inventory = creator.get();
				return inventory;
			}
		};
	}
	
	class Guide {
		final List<Category> categories = new ArrayList<>();
		final List<Supplier<Inventory>> pages = new ArrayList<>();
		
		public Guide(Plugin plugin, int rows) {
			MiniWindow window = new MiniWindow(plugin);
			List<List<Category>> partitions = partition(categories, rows * 9);
			pages.addAll(mapWithIndex(partitions.stream(), (categories, index) ->
					cache(() -> window.page(page -> {
						//Title the page.
						page.title("Page " + index);
						
						//Add items for each category on the page.
						categories.forEach(category -> {
							//Create a new item for the category.
							Item item = page.element();
							
							//Let the category style it.
							category.iconLike().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 (index != 0 && i == 2)
								item.data(RED.getDyeData()).onClick(player ->
										//Navigate back one page.
										player.openInventory(pages.get((int) (index - 1)).get())
								);
							else if (index != partitions.size() - 1 && i == 6)
								item.data(GREEN.getDyeData()).onClick(player ->
										//Navigate forward one page.
										player.openInventory(pages.get((int) (index + 1)).get())
								);
							else
								item.data(GRAY.getDyeData());
						}
					}))
			).collect(toList()));
		}
		
		public Inventory getPage(int index) {
			return pages.get(index).get();
		}
	}
	
	interface Category {
		Consumer<Page> pageLike();
		
		Consumer<Item> iconLike();
	}
}