Java集合系列三Vector-Stack解析

Posted 2020-09-30 wlrhnh

2017-07-29 12:59:14

一、简介

1、Vector继承关系

2、Vector类扩容

Vector类的实现和ArrayList极其相似，都使用数组存储元素，但是扩容策略不一样，ArrayList基本是按照1.5倍的思路扩容，Vector是按照创建Vector对象时设置的capacityIncrement值递增的，如果该值没有设置或者为0，则直接Double，如下：

3、Vector的线程安全性

Vector号称是线程安全的，它的安全性来自于几乎给每个方法都添加了synchronized关键字，由于锁对象是当前对象，只要有一个线程在执行某一个加锁的方法，也就是获得到当前对象这个锁对象，其他线程基本都不能再使用这个Vector对象了...这是一种很残暴的设计，估计这可能是synchronized刚出来之时，JDK团队的一时兴奋之作吧~

举几个例子：

即便是有一些方法没有直接加synchronized关键字，至少它所调用的方法都是加了的。

4、Stack

Stack继承自Vector，提供了pop、push、peek、search等栈的特殊操作，源码如下：

  1 package java.util;
  2 
  3 /**
  4  * The <code>Stack</code> class represents a last-in-first-out
  5  * (LIFO) stack of objects. It extends class <tt>Vector</tt> with five
  6  * operations that allow a vector to be treated as a stack. The usual
  7  * <tt>push</tt> and <tt>pop</tt> operations are provided, as well as a
  8  * method to <tt>peek</tt> at the top item on the stack, a method to test
  9  * for whether the stack is <tt>empty</tt>, and a method to <tt>search</tt>
 10  * the stack for an item and discover how far it is from the top.
 11  * <p>
 12  * When a stack is first created, it contains no items.
 13  *
 14  * <p>A more complete and consistent set of LIFO stack operations is
 15  * provided by the {@link Deque} interface and its implementations, which
 16  * should be used in preference to this class.  For example:
 17  * <pre>   {@code
 18  *   Deque<Integer> stack = new ArrayDeque<Integer>();}</pre>
 19  *
 20  * @author  Jonathan Payne
 21  * @since   JDK1.0
 22  */
 23 public
 24 class Stack<E> extends Vector<E> {
 25     /**
 26      * Creates an empty Stack.
 27      */
 28     public Stack() {
 29     }
 30 
 31     /**
 32      * Pushes an item onto the top of this stack. This has exactly
 33      * the same effect as:
 34      * <blockquote><pre>
 35      * addElement(item)</pre></blockquote>
 36      *
 37      * @param   item   the item to be pushed onto this stack.
 38      * @return  the <code>item</code> argument.
 39      * @see     java.util.Vector#addElement
 40      */
 41     public E push(E item) {
 42         addElement(item);
 43 
 44         return item;
 45     }
 46 
 47     /**
 48      * Removes the object at the top of this stack and returns that
 49      * object as the value of this function.
 50      *
 51      * @return  The object at the top of this stack (the last item
 52      *          of the <tt>Vector</tt> object).
 53      * @throws  EmptyStackException  if this stack is empty.
 54      */
 55     public synchronized E pop() {
 56         E       obj;
 57         int     len = size();
 58 
 59         obj = peek();
 60         removeElementAt(len - 1);
 61 
 62         return obj;
 63     }
 64 
 65     /**
 66      * Looks at the object at the top of this stack without removing it
 67      * from the stack.
 68      *
 69      * @return  the object at the top of this stack (the last item
 70      *          of the <tt>Vector</tt> object).
 71      * @throws  EmptyStackException  if this stack is empty.
 72      */
 73     public synchronized E peek() {
 74         int     len = size();
 75 
 76         if (len == 0)
 77             throw new EmptyStackException();
 78         return elementAt(len - 1);
 79     }
 80 
 81     /**
 82      * Tests if this stack is empty.
 83      *
 84      * @return  <code>true</code> if and only if this stack contains
 85      *          no items; <code>false</code> otherwise.
 86      */
 87     public boolean empty() {
 88         return size() == 0;
 89     }
 90 
 91     /**
 92      * Returns the 1-based position where an object is on this stack.
 93      * If the object <tt>o</tt> occurs as an item in this stack, this
 94      * method returns the distance from the top of the stack of the
 95      * occurrence nearest the top of the stack; the topmost item on the
 96      * stack is considered to be at distance <tt>1</tt>. The <tt>equals</tt>
 97      * method is used to compare <tt>o</tt> to the
 98      * items in this stack.
 99      *
100      * @param   o   the desired object.
101      * @return  the 1-based position from the top of the stack where
102      *          the object is located; the return value <code>-1</code>
103      *          indicates that the object is not on the stack.
104      */
105     public synchronized int search(Object o) {
106         int i = lastIndexOf(o);
107 
108         if (i >= 0) {
109             return size() - i;
110         }
111         return -1;
112     }
113 
114     /** use serialVersionUID from JDK 1.0.2 for interoperability */
115     private static final long serialVersionUID = 1224463164541339165L;
116 }

View Code

5、结尾

Vector和ArrayList的异同：

1、扩容策略不同；

2、线程安全的；

3、提供了一些独特的如removeElement()之类的方法；

4、其他方法的实现与ArrayList几乎是一致的，毕竟都是对数组的操作；

5、忽略这个类即可；