public class ResizingArrayStack<Item> implements Iterable<Item>{
    private Item[] a=(Item[]) new Object[1];
    private int N=0;
    public boolean isEmpty(){return N==0;}
    public int size(){return N;}
    private void resize(int max){
        Item[] temp=(Item[])new Object[max];
        for(int i=0;i<N;i++){
            temp[i]=a[i];
        }
        a=temp;
    }
    public  void push(Item item){
        if(N==a.length){
            resize(a.length*2);
        }
        a[N++]=item;
    }
    public Item pop(){
        if(N==0){
            return null;
        }
        Item item=a[N--];
        a[N]=null;
        if(N>0 && N==a.length/4){
            resize(a.length*2);
        }
        return item;
    }
    public Iterator<Item> iterator(){
        return new ReverseArrayIterator();
    }
    private class ReverseArrayIterator implements Iterator<Item>{
        private int i=N;

        @Override
        public boolean hasNext() {
            return i>0;
        }

        @Override
        public Item next() {
            return a[--i];
        }
    }
}

测试：

    public static void main(String[] args) {
        ResizingArrayStack<String> obj=new ResizingArrayStack<>();
        obj.push("a");
        obj.push("b");
        obj.push("c");
        Iterator<String> iter=obj.iterator();
        while (iter.hasNext()){
            String temp=iter.next();
            System.out.println(temp);
        }