Java实现一个简单的堆栈

堆栈（Stack）是一种常见的数据结构，符合后进先出（First In Last Out）原则，通常用于实现对象存放顺序的逆序。栈的基本操作有push（添加到堆栈）,pop（从堆栈删除）,peek（检测栈顶元素且不删除）。

第一种实现方式：普通数组实现

/**
 * Created by Frank
 */
public class ToyStack {
    /**
     * 栈的最大深度
     **/
    protected int MAX_DEPTH = 10;

    /**
     * 栈的当前深度
     */
    protected int depth = 0;

    /**
     * 实际的栈
     */
    protected int[] stack = new int[MAX_DEPTH];

    /**
     * push，向栈中添加一个元素
     *
     * @param n 待添加的整数
     */
    protected void push(int n) {
        if (depth == MAX_DEPTH - 1) {
            throw new RuntimeException("栈已满，无法再添加元素。");
        }
        stack[depth++] = n;
    }

    /**
     * pop，返回栈顶元素并从栈中删除
     *
     * @return 栈顶元素
     */
    protected int pop() {
        if (depth == 0) {
            throw new RuntimeException("栈中元素已经被取完，无法再取。");
        }

        // --depth，dept先减去1再赋值给变量dept，这样整个栈的深度就减1了（相当于从栈中删除）。
        return stack[--depth];
    }

    /**
     * peek，返回栈顶元素但不从栈中删除
     *
     * @return
     */
    protected int peek() {
        if (depth == 0) {
            throw new RuntimeException("栈中元素已经被取完，无法再取。");
        }
        return stack[depth - 1];
    }
}

 
第二种实现方式：deque双向队列

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("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);
 }
}
/* 输出
After pushing 5 elements: [4, 3, 2, 1, 0]
Popped element = 4
After popping 1 element : [3, 2, 1, 0]
Peeked element = 3
After peeking 1 element : [3, 2, 1, 0]
*/

 

ArrayDeque源码 

//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
    }