简书 賈小強
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package com.lab1.test1;

import java.util.Iterator;

public class ArrayStack<Item> implements Iterable<Item> {
    private int n;
    @SuppressWarnings("unchecked")
    private Item[] a = (Item[]) new Object[2];

    @Override
    public Iterator<Item> iterator() {
        // TODO Auto-generated method stub
        return new ListIterator();
    }

    private class ListIterator implements Iterator<Item> {
        int i = 0;

        @Override
        public boolean hasNext() {
            return i < n;
        }

        @Override
        public Item next() {
            Item item = a[i];
            i++;
            return item;
        }

    }

    @Override
    public String toString() {
        StringBuilder builder = new StringBuilder();
        for (Item item : this) {
            builder.append(item + " ");
        }
        return builder.toString();
    }

    private boolean isEmpty() {
        return n == 0;
    }

    private int size() {
        return n;
    }

    private void push(Item item) {
        if (n == a.length) {
            resize(a.length * 2);
        }
        a[n++] = item;
    }

    private Item pop() {
        Item item = a[n - 1];
        a[n - 1] = null;
        n--;
        if (n > 0 && n == a.length / 4) {
            resize(a.length / 2);
        }
        return item;
    }

    @SuppressWarnings("unchecked")
    private void resize(int capacity) {
        Item[] temp = (Item[]) new Object[capacity];
        for (int i = 0; i < n; i++) {
            temp[i] = a[i];
        }
        a = temp;
    }

    public static void main(String[] args) {
        ArrayStack<String> stack = new ArrayStack<>();
        System.out.println(stack);
        System.out.println(stack.size());
        System.out.println(stack.isEmpty());

        stack.push("bill");
        stack.push("jack");
        stack.push("lucy");
        System.out.println(stack);
        System.out.println(stack.size());
        System.out.println(stack.isEmpty());

        stack.pop();
        stack.pop();
        System.out.println(stack);
        System.out.println(stack.size());
        System.out.println(stack.isEmpty());
    }

}

输出

0
true
bill jack lucy 
3
false
bill 
1
false

Happy learning !!