Java ArrayDeque实现Stack的功能

在j2se6引入了arraydeque类，它继承了deque(双向队列)接口，使用此类可以自己实现java.util.stack类的功能，去掉了java.util.stack的多线程同步的功能。

例如创建一个存放integer类型的stack，只要在类中创建一个arraydeque类的变量作为属性，之后定义的出栈、入栈，观察栈顶元素的操作就直接操作arraydeque的实例变量即可。 

import java.util.arraydeque; 
import java.util.deque; 
 
public class integerstack { 
  private deque<integer> data = new arraydeque<integer>(); 
 
  public void push(integer element) { 
    data.addfirst(element); 
  } 
 
  public integer pop() { 
    return data.removefirst(); 
  } 
 
  public integer peek() { 
    return data.peekfirst(); 
  } 
 
  public string tostring() { 
    return data.tostring(); 
  } 
 
  public static void main(string[] args) { 
    integerstack stack = new integerstack(); 
    for (int i = 0; i < 5; i++) { 
      stack.push(i); 
    } 
    system.out.println(stack); 
    system.out.println("after pushing 5 elements: " + stack); 
    int m = stack.pop(); 
    system.out.println("popped element = " + m); 
    system.out.println("after popping 1 element : " + stack); 
    int n = stack.peek(); 
    system.out.println("peeked element = " + n); 
    system.out.println("after peeking 1 element : " + stack); 
  } 
}  

java.util.arraydeque的源码：    

private transient e[] elements; 
 private transient int head; 
 private transient int tail; 
 
/*此处存放e的位置是从elements数组最后的位置开始存储的*/ 
 public void addfirst(e e) { 
    if (e == null) 
      throw new nullpointerexception(); 
    elements[head = (head - 1) & (elements.length - 1)] = e;//首次数组容量默认为16，head=（0-1）&(16-1)=15 
    if (head == tail) 
      doublecapacity(); 
  } 
 
/*每次扩容都按插入的先后顺序重新放入一个新的数组中，最新插入的放在数组的第一个位置。*/ 
  private void doublecapacity() { 
    assert head == tail; 
    int p = head; 
    int n = elements.length; 
    int r = n - p; // number of elements to the right of p 
    int newcapacity = n << 1; 
    if (newcapacity < 0) 
      throw new illegalstateexception("sorry, deque too big"); 
    object[] a = new object[newcapacity]; 
    system.arraycopy(elements, p, a, 0, r); 
    system.arraycopy(elements, 0, a, r, p); 
    elements = (e[])a; 
    head = 0; 
    tail = n; 
  } 
 
  public e removefirst() { 
    e x = pollfirst(); 
    if (x == null) 
      throw new nosuchelementexception(); 
    return x; 
  } 
 
  public e pollfirst() { 
    int h = head; 
    e result = elements[h]; // element is null if deque empty 
    if (result == null) 
      return null; 
    elements[h] = null;   // 重新设置数组中的这个位置为null，方便垃圾回收。 
    head = (h + 1) & (elements.length - 1);//将head的值回退，相当于将栈的指针又向下移动一格。例如，12--〉13 
    return result; 
  } 
 
  public e peekfirst() { 
    return elements[head]; // elements[head] is null if deque empty 
  } 

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