Java双向链表实现

public class DoublyLinkList {
	private class Data{
		private Object obj;
		private Data left = null;
		private Data right = null;
		
		Data(Object obj){
			this.obj = obj;
		}
	}
	
	private Data first = null;
	private Data last = null;
	
	public void insertFirst(Object obj){
		Data data = new Data(obj);
		if(first == null){
			last = data;
		}else{
			data.right = first;
			first.left = data;
		}
		first = data;
	}
	
	public void insertLast(Object obj){
		Data data = new Data(obj);
		if(first == null){
			first = data;
		}else{
			data.left = last;
			last.right = data;
		}
		last = data;
	}
	
	public boolean insertAfter(Object target,Object obj){
		Data data = new Data(obj);
		Data cur = first;
		while(cur != null){
			if(cur.obj.equals(target)){
				data.right = cur.right;
				data.left = cur;
				if(cur == last)
					last = data;
				else
					cur.right.left = data;
				cur.right = data;
				return true;
			}
			cur = cur.right;
		}
		return false;
	}
	
	public Object deleteFirst() throws Exception{
		if(first == null)
			throw new Exception("empty!");
		Data temp = first;
		if(first.right == null){
			first = null;
			last = null;
		}else{
			first.right.left = null;
			first = first.right;
		}
		return temp;
	}
	
	public Object deleteLast() throws Exception{
		if(first == null)
			throw new Exception("empty!");
		Data temp = last;
		if(first.right == null){
			first = null;
			last = null;
		}else{
			last.left.right = null;
			last = last.left;
		}
		return temp;
	}
	
	public Object delete(Object obj) throws Exception{
		if(first == null)
			throw new Exception("empty!");
		Data cur = first;
		while(cur != null){
			if(cur.obj.equals(obj)){
				if(cur == last)
					last = cur.left;
				else
					cur.right.left = cur.left;
				if(cur == first)
					first = cur.right;
				else
					cur.left.right = cur.right;
				return obj;
			}
			cur = cur.right;
		}
		return null;
	}
	
	public void display(){
		System.out.print("first -> last : ");
		Data data = first;
		while(data != null){
			System.out.print(data.obj.toString() + " -> ");
			data = data.right;
		}
		System.out.print("\n");
	}
	
	public static void main(String[] args) throws Exception{
		DoublyLinkList dll = new DoublyLinkList();
		dll.insertFirst(1);
		dll.insertLast(3);
		dll.insertAfter(1, 2);
		dll.insertAfter(3, 4);
		dll.insertAfter(4, 5);
		dll.display();
		dll.deleteFirst();
		dll.display();
		dll.deleteLast();
		dll.display();
		dll.delete(3);
		dll.display();
		dll.delete(2);
		dll.display();
		dll.delete(4);
		dll.display();
	}
}

first -> last : 1 -> 2 -> 3 -> 4 -> 5 -> 
first -> last : 2 -> 3 -> 4 -> 5 -> 
first -> last : 2 -> 3 -> 4 -> 
first -> last : 2 -> 4 -> 
first -> last : 4 -> 
first -> last :