Stack类详解

前言
今天在做leetcode题目时，用到了栈Stack类，之前没有接触过，所以学习下

Stack继承关系
Stack继承了Vertor，所以他本质上是个数组，和集合类密不可分

Stack类：

1. 只有一个无参构造函数，用来创建一个空的stack

 	/**
     * Creates an empty Stack.
     */
    public Stack() {
    }

2. push方法：将一个item放置于栈顶

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

        return item;
    }

3. pop方法：将栈顶元素取出，并返回此栈顶元素

/**
     * 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.
     */
    public synchronized E pop() {
        E       obj;
        int     len = size();

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

        return obj;
    }

4. peek方法：查看栈顶元素（并不取出）

/**
     * 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.
     */
    public synchronized E peek() {
        int     len = size();

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

5. empty方法：判断此栈是否为空

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

6. search方法：返回该元素距离栈顶的距离，如果不存在返回-1，此处栈顶可以理解成是栈顶的上一层，即该元素到栈顶上一层的距离

/**
     * 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.
     */
    public synchronized int search(Object o) {
        int i = lastIndexOf(o);

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

7. 结果演示

public static void main(String args[]) {
        Stack stack = new Stack();
        stack.push("a");
        stack.push("b");
        stack.push("c");
        System.out.println(stack.search("a"));  // 3
        System.out.println(stack.search("b"));  // 2
        System.out.println(stack.search("c"));  // 1 此处c是栈顶，但是还是有一个距离
        System.out.println(stack.peek());           // c
        System.out.println(stack.pop());            // c
        System.out.println(stack.peek());           // b
        System.out.println(stack.empty());          // false
    }