Stack源码分析

• Stack：

    /**
 * 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
 */

Java从JDK1.0提供了一种后进先出的数据结构，位于util包的Stack类，Stack类基于Vector实现，并提供了常用push、pop、peek、empty、search几个常用方法。

    public E push(E item) {
        addElement(item);

        return item;
    }
    //使用Vector的addElement方法

    //同步的出栈方法，使用Vector的removeElementAt删除栈顶元素
    public synchronized E pop() {
        E       obj;
        int     len = size();

        obj = peek();
        removeElementAt(len - 1);

        return obj;
    }

    //查看栈顶元素
    public synchronized E peek() {
        int     len = size();

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

    //返回栈是否为空
    public boolean empty() {
        return size() == 0;
    }

    //查找栈中元素
    public synchronized int search(Object o) {
        int i = lastIndexOf(o);

        if (i >= 0) {
            return size() - i;
        }
        return -1;
    }

最后，由于Vector是线程安全的，子类继承父类后，重写父类方法不带同步，需要子类自己实现自己的同步方法。