A class for creating dynamically sizing inventories.

PageInventory.java

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;

import org.bukkit.Bukkit;
import org.bukkit.inventory.Inventory;
import org.bukkit.inventory.ItemStack;

import com.google.common.collect.Table;

public class PageInventory {
	private final String title;
	private final int size;

	private final List<ItemStack> items;
	private final List<Inventory> inventories = new ArrayList<Inventory>();

	private final Table<PageType, Integer, ItemStack> stationaryItems;

	public PageInventory(String title, int size, List<ItemStack> items, Table<PageType, Integer, ItemStack> stationaryItems) {
		this.title = title != null ? title : "Inventory";
		this.size = size;
		this.items = items;
		this.stationaryItems = stationaryItems;

		init();
	}

	public PageInventory(int size, List<ItemStack> items, Table<PageType, Integer, ItemStack> stationaryItems) {
		this(null, size, items, stationaryItems);
	}

	public PageInventory(int size, List<ItemStack> items) {
		this(null, size, items, null);
	}

	public PageInventory(List<ItemStack> items, Table<PageType, Integer, ItemStack> stationaryItems) {
		this(54, items, stationaryItems);
	}

	public PageInventory(List<ItemStack> items) {
		this(54, items);
	}

	public boolean containsInventory(Inventory inventory) {
		return inventories.contains(inventory);
	}

	public Inventory getPage(int page) {
		return inventories.get(page);
	}

	private void init() {
		Map<PageType, Map<Integer, ItemStack>> stationaryItems = this.stationaryItems != null ? this.stationaryItems.rowMap() : null;
		int itemAmount = items.size();

		if (stationaryItems == null) {
			for (int fromIndex = 0, toIndex = size; fromIndex < itemAmount; fromIndex += size, toIndex += size) {
				Inventory inventory = Bukkit.createInventory(null, size, title);

				inventory.addItem(items.subList(fromIndex, toIndex).toArray(new ItemStack[size]));
			}

			return;
		}

		Map<Integer, ItemStack> normalStationaryItems = stationaryItems.get(PageType.NORMAL);

		if (normalStationaryItems != null) {
			if (size - normalStationaryItems.size() >= items.size()) {
				final Inventory inventory = Bukkit.createInventory(null, size, title);
				
				normalStationaryItems.entrySet().stream().forEach(entry -> inventory.setItem(entry.getKey(), entry.getValue()));
				items.stream().forEach(item -> inventory.setItem(inventory.firstEmpty(), item));

				inventories.add(inventory);

				return;
			}
		}

		Inventory inventory = null;

		for (int index = 0; index < itemAmount; index++) {
			if (inventory != null) {
				int firstEmpty = inventory.firstEmpty();

				if (firstEmpty > -1) {
					inventory.setItem(firstEmpty, items.get(index));

					continue;
				}
			}

			inventory = Bukkit.createInventory(null, size, title);
			inventories.add(inventory);

			PageType type = null;
			int itemsRemaining = items.size() - index + 1;

			if (inventories.size() == 1) {
				type = PageType.NEXT;
			}

			if (type == null) {
				Map<Integer, ItemStack> backStationaryItems = stationaryItems.get(PageType.BACK);

				if (backStationaryItems != null) {
					if (itemsRemaining <= size - backStationaryItems.size()) {
						type = PageType.BACK;
					} else {
						type = PageType.BACK_AND_NEXT;
					}
				}
			}
			
			for (Entry<Integer, ItemStack> entry : stationaryItems.get(type).entrySet()) {
				inventory.setItem(entry.getKey(), entry.getValue());
			}

			inventory.setItem(inventory.firstEmpty(), items.get(index));
		}
	}

	public enum PageType {
		NORMAL, NEXT, BACK_AND_NEXT, BACK
	}
}