java中生产者消费者问题

小弟编写一个生产者消费者，碰到一个非常郁闷的问题，百思不得其解。请各位高手帮忙！
程序出现的问题是产品应该是先生产才能消费。但是确有先消费后生产的情况，


截图如下：



代码如下：
package com.lcm;

public class ProducterConsumer {
public static void main(String[] args) {
int stackSize = 15;
Stack stack = new Stack(stackSize); //存储量为15的容器
Producer p = new Producer(stack, 20);
Consumer c = new Consumer(stack, 20);
new Thread(p).start();
new Thread(c).start();
}
}

class Thing {
int id;
public Thing(int id) {
this.id = id;
}
}

class Stack {
int counter = 0;
int stackSize;
Thing[] things = null;
public Stack(int stackSize) {
this.stackSize = stackSize;
things = new Thing[stackSize];
}

public synchronized void push(Thing thing) { //生产
while(counter == stackSize) { //已经满了  等待消费
System.out.println("已经存满！等待消费。。。。。");
try {
this.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
this.notify();
things[counter] = thing;
counter ++;
}

public synchronized Thing pop() { //消费
while(counter == 0