java进程间通信问题

[Little BigUs]

下面是代码：

import java.util.ArrayList;
import java.util.List;

public class FirstThread {
	public static void main(String[] args) throws InterruptedException {
		List<Object> goods = new ArrayList<>();	//储存物品的仓库，最多储存1
		Thread thread1 = new Thread(()->{		//线程1，生产产品
			int num = 0;
			while(true) {
				synchronized (goods) {
					if(goods.size()==0) goods.add("商品" + ++num);
					else if(goods.size()>0)
						try {
							goods.wait();
						} catch (InterruptedException e) {
							// TODO Auto-generated catch block
							e.printStackTrace();
						}
					System.out.println("生产者生产了第"+num+"个产品");
					
				}
				try {
					Thread.sleep(100);
				} catch (InterruptedException e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				}
			}
		}, "生产者");
		Thread thread2 = new Thread(()->{
			int num = 0;
			while(true) {
				synchronized (goods) {
					if(goods.size()>0) goods.remove("商品"+ ++num);
					else if (goods.size()==0)
						goods.notify();
					System.out.println("消费者消费了第"+num+"个产品");
					
				}
				try {
					Thread.sleep(100);
				} catch (InterruptedException e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				}
			}
		}, "消费者");
		thread1.start();
		thread2.start();
	}
}

按理来说每个对应的num只有一句输出，但是结果却会出现下面这样的情况

有没有好心人能解答一下，我已经知道怎么能得到我想要的结果，只是单纯想知道这是为什么，谢谢。

[ouzhijian]

应该是打印的时机不对 if(goods.size()==0) { goods.add("商品" + ++num); System.out.println("生产者生产了第"+num+"个产品"); } if(goods.size()>0) { goods.remove("商品"+ ++num); System.out.println("消费者消费了第"+num+"个产品"); }