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++];
        }
    }
}