LinkedList test

Item.java

package linkedlist;

public class Item<T> {//ジェネリクス使用

	public final T element;//final fieldにするのでgetter,setter使わない
	private Item<T> nextItem;
	private Item<T> previousItem;

	protected Item(T element){//protected修飾子でインスタンス生成を制限
		this.element=element;
	}

	public Item<T> getNextItem() {
		return nextItem;
	}

	public void setNextItem(Item<T> nextItem) {
		this.nextItem = nextItem;
	}

	public Item<T> getPreviousItem() {
		return previousItem;
	}

	public void setPreviousItem(Item<T> previousItem) {
		this.previousItem = previousItem;
	}


}

LinkedList.java

package linkedlist;

public class LinkedList<T> {

	private int count;
	private Item<T> firstItem;
	private Item<T> lastItem;

	public LinkedList() {
		this.count = 0;
	}

	public int size(){
		return count;
	}

	private Item<T> findItem(int index) {
		if (this.count > index && index >= 0) {
			// めんどくさいので前方からのみ探索する
			Item<T> result = this.firstItem;
			for (int i = 0; i < index; i++) {
				result = result.getNextItem();
			}
			return result;
		}
		return null;
	}

	public void insert(T element, int index) {
		if (index < 0) {
			return;
		}
		if (this.count < index) {// 要素数が足りない
			while (this.count < index) {
				addLast(null);
			}
		}
		if (this.count == index) {
			addLast(element);
		} else if (index == 0) {
			addFirst(element);
		} else {
			Item<T> item = new Item<T>(element);
			Item<T> replaseItem = findItem(index);// 探索は必ず成功するはず
			if (replaseItem.getPreviousItem() != null) {
				item.setPreviousItem(replaseItem.getPreviousItem());
				replaseItem.getPreviousItem().setNextItem(item);
			}
			item.setNextItem(replaseItem);
			replaseItem.setPreviousItem(item);
			this.count++;
		}
	}

	public void addLast(T element) {
		Item<T> item = new Item<T>(element);
		Item<T> replaceItem = this.lastItem;
		this.lastItem = item;
		if (replaceItem != null) {
			replaceItem.setNextItem(item);
			item.setPreviousItem(replaceItem);
		} else {// 最後の要素がない→最初の要素もない
			this.firstItem = item;
		}
		this.count++;
	}

	public void addFirst(T element) {
		Item<T> item = new Item<T>(element);
		Item<T> replaceItem = this.firstItem;
		this.firstItem = item;
		if (replaceItem != null) {
			replaceItem.setPreviousItem(item);
			item.setNextItem(replaceItem);
		} else {// 最初の要素がない→最後の要素もない
			this.lastItem = item;
		}
		this.count++;
	}

	public void delete(int index) {
		this.remove(index);
	}

	public void remove(int index) {
		Item<T> item = findItem(index);
		if (item == null) {
			return;
		}
		Item<T> previousItem = item.getPreviousItem();
		Item<T> nextItem = item.getNextItem();
		if (previousItem == null && nextItem == null) {
			this.firstItem = null;
			this.lastItem = null;
		} else if (previousItem == null) {
			this.firstItem = nextItem;
			nextItem.setPreviousItem(null);
		} else if (nextItem == null) {
			this.lastItem = previousItem;
			previousItem.setNextItem(null);
		} else {
			previousItem.setNextItem(nextItem);
			nextItem.setPreviousItem(previousItem);
		}
		this.count--;
	}

	public int locate(T element) {
		return indexOf(element);
	}

	public int indexOf(T element) {
		Item<T> item = this.firstItem;
		for (int i = 0; i < this.count; i++) {
			if (item.element!=null&&item.element.equals(element)) {
				return i;
			}
			item = item.getNextItem();
		}
		return -1;
	}

	public T Retrieve(int index) {
		return get(index);
	}

	public T get(int index) {
		Item<T> result = findItem(index);
		return result == null ? null : result.element;
	}

	public T top() {
		Item<T> item = this.firstItem;
		return item == null ? null : item.element;
	}

	public T last() {
		Item<T> item = this.lastItem;
		return item == null ? null : item.element;
	}

}

Main.java

package linkedlisttest;

import linkedlist.LinkedList;

public class Main {

	public static void main(String[] args) {
		LinkedList<Integer> list = new LinkedList<>();
		System.out.println("count:"+list.size());
        list.insert(1, 1);
        list.insert(2, 4);
        list.insert(1, 0);
        list.insert(1, 1);
        list.addLast(3);
        list.addFirst(-1);
        list.remove(list.indexOf(2));
        for(int i=0;i<list.size();i++){
        	System.out.print(list.get(i));
        	System.out.print(' ');
        }
        System.out.print('\n');
        System.out.println("count:"+list.size());
        list.addLast(4);
        list.addLast(5);
        list.remove(list.size() - 1);
        System.out.println(list.last() == 4 ? "true" : "false");
        list.addFirst(-2);
        list.addFirst(-3);
        list.remove(0);
        System.out.println(list.top() == -2 ? "true" : "false");
        while(list.size() > 1) {
            list.remove(0);
        }
        System.out.println(list.top() == list.last() ? "true" : "false");
        list.remove(0);
        System.out.println(list.top() == null && list.last() == null ? "true" : "false");
	}

}