栈的实现Java

 package practice;

 import java.util.Iterator;
 //栈
 public class MyStack<T> implements Iterable<T> {
     private T[] a;
     private int p = 0;
     @SuppressWarnings("unchecked")
     public MyStack() {
         a = (T[]) new Object[1];
     }

     public void push(T elements) {
         if (a.length == p) {resize(2*a.length);}
         a[p++] = elements;
     }

     public T pop() {
         if (a.length/4 == p) {resize(a.length/2);}
         return a[--p];
     }

     @SuppressWarnings("unchecked")
     public void resize(int length) {
         T[] b = (T[]) new Object[length]; //将数组长度改变
         for (int i = 0; i < p; i++) { //将数组复制
             b[i] = a[i];
         }
         a = b; //让a指向b的内存空间
     }

     public boolean isEmpty() {
         return p == 0;
     }

     public int size() {
         return p;
     }
     @Override
     public Iterator<T> iterator() { //使数据结构可以让foreach访问
         return new ArrayIterator();
     }
     private class ArrayIterator implements Iterator<T>{

         private int p2 = 0;

         @Override
         public boolean hasNext() { //return false 则foreach终止
             return p2 != p;
         }

         @Override
         public T next() { //foreach 寻找下一个元素
             return a[p2++];
         }
     }
 }