Stack 源代码

1、Stack继承了Vector，它是同步的，主要包括peek()【获取栈顶元素】、pop()【出栈】、push()【进栈】、search()【查找】、empty()【是否为空】这5个方法

源代码如下：

/*
 * Copyright (c) 1994, 2010, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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package java.util;

/**
 * The Stack class represents a last-in-first-out
 * (LIFO) stack of objects. It extends class Vector with five
 * operations that allow a vector to be treated as a stack. The usual
 * push and pop operations are provided, as well as a
 * method to peek at the top item on the stack, a method to test
 * for whether the stack is empty, and a method to search
 * the stack for an item and discover how far it is from the top.
 * 

 * When a stack is first created, it contains no items.
 *
 * 
A more complete and consistent set of LIFO stack operations is
 * provided by the {@link Deque} interface and its implementations, which
 * should be used in preference to this class.  For example:
 * 
   {@code
 *   Deque stack = new ArrayDeque();}
 *
 * @author  Jonathan Payne
 * @since   JDK1.0
 */
/*
Stack 类表示后进先出（LIFO）的对象堆栈。它通过五个操作对类 Vector 进行了扩展 ，允许将向量视为堆栈。
它提供了通常的 push 和 pop 操作，以及取堆栈顶点的 peek 方法、测试堆栈是否为空的 empty 方法、
在堆栈中查找项并确定到堆栈顶距离的 search 方法。

首次创建堆栈时，它不包含元素。

Deque 接口及其实现提供了 LIFO 堆栈操作的更完整和更一致的 set，应该优先使用此接口，而非此类。例如：

   Deque stack = new ArrayDeque();
 */
public
class Stack extends Vector {
    /**
     * Creates an empty Stack.
     */
    public Stack() {
    }

    /**
     * Pushes an item onto the top of this stack. This has exactly
     * the same effect as:
     * 
     * addElement(item)
     *
     * @param   item   the item to be pushed onto this stack.
     * @return  the item argument.
     * @see     java.util.Vector#addElement
     */
    //进栈
    public E push(E item) {
        addElement(item);

        return item;
    }

    /**
     * Removes the object at the top of this stack and returns that
     * object as the value of this function.
     *
     * @return  The object at the top of this stack (the last item
     *          of the Vector object).
     * @throws  EmptyStackException  if this stack is empty.
     */
    //出栈
    public synchronized E pop() {
        E       obj;
        int     len = size();

        //先调用peek()后在移除掉元素
        obj = peek();
        removeElementAt(len - 1);

        return obj;
    }

    /**
     * Looks at the object at the top of this stack without removing it
     * from the stack.
     *
     * @return  the object at the top of this stack (the last item
     *          of the Vector object).
     * @throws  EmptyStackException  if this stack is empty.
     */
    //返回栈顶元素
    public synchronized E peek() {
        int     len = size();

        if (len == 0)
            throw new EmptyStackException();
        return elementAt(len - 1);
    }

    /**
     * Tests if this stack is empty.
     *
     * @return  true if and only if this stack contains
     *          no items; false otherwise.
     */
    //判断是偶为空
    public boolean empty() {
        return size() == 0;
    }

    /**
     * Returns the 1-based position where an object is on this stack.
     * If the object o occurs as an item in this stack, this
     * method returns the distance from the top of the stack of the
     * occurrence nearest the top of the stack; the topmost item on the
     * stack is considered to be at distance 1. The equals
     * method is used to compare o to the
     * items in this stack.
     *
     * @param   o   the desired object.
     * @return  the 1-based position from the top of the stack where
     *          the object is located; the return value -1
     *          indicates that the object is not on the stack.
     */
    //在栈中查找o，返回下标
    public synchronized int search(Object o) {
        //栈是先进后出，所以需要从后扫描
        int i = lastIndexOf(o);

        if (i >= 0) {
            //返回下标
            return size() - i;
        }
        return -1;
    }

    /** use serialVersionUID from JDK 1.0.2 for interoperability */
    private static final long serialVersionUID = 1224463164541339165L;
}