栈结构实现队列结构

package arithmetic;

import java.util.Stack;

public class TwoStacksQueue {
    public Stack<Integer> stackPush;
    public Stack<Integer> stackPop;

    public TwoStacksQueue() {
        stackPush = new Stack<>();
        stackPop = new Stack<>();
    }

    public void pushToPop() {
        //pop栈为空时才可以导入
        if (stackPop.isEmpty()) {
            //只要导入就需要将push栈全部导入pop栈
            while (stackPush != null) {
                stackPop.push(stackPush.pop());
            }
        }
    }

    public void add(int value) {
        stackPush.push(value);
        pushToPop();
    }

    public int poll() {
        if (stackPop.isEmpty() && stackPush.isEmpty()) {
            throw new RuntimeException("Queue is empty!");
        }
        pushToPop();
        return stackPop.pop();
    }

    public int peek() {
        if (stackPop.empty() && stackPush.empty()) {
            throw new RuntimeException("Queue is empty!");
        }
        pushToPop();
        return stackPop.peek();
    }

    public static void main(String[] args) {
        TwoStacksQueue test = new TwoStacksQueue();
        test.add(1);
        test.add(2);
        test.add(3);
        System.out.println(test.peek());
        System.out.println(test.poll());
        System.out.println(test.peek());
        System.out.println(test.poll());
        System.out.println(test.peek());
        System.out.println(test.poll());
    }
}