java 实现简单的链式栈

package com.my;
/**
 * 链式栈
 * @author wanjn
 *
 */
public class LinkedStack {
        private Node head;
        private int size;
        //入栈，前插入构造链表，删除头结点实现栈；向后插入构造链表，删除头结点实现队列
        public void push(Object value){
            if (head ==null) {
                head = new Node(null, value);
            }else {
                head = new Node(head, value);
            }
        }
        //出栈
        public Object pop(){
            Object oldValue =null;
            if (head!=null) {
                oldValue = head.value;
                head = head.next;
            }
            return oldValue;        }
        @Override
        public String toString() {
            Node temp = head;
            String result= "";
            while (temp!=null) {
                result +=temp.value+" ,";
                temp = temp.next;
            }
            return result;
        }
        private class Node{
            Node next;
            Object value;
            Node(Node next,Object value){
                this.next = next;
                this.value = value;
            }
        }
}