Java多线程中notify和wait的问题

程序功能：4个线程，2个线程对某个数值进行加1；2个线程进行减1，要求该数在0和1之间切换。程序如下，大家看看问题出在哪里：

DecThread类，对sample的i进行减1的线程：

public class DecThread extends Thread{
	private Sample sample;
	
	public DecThread(Sample sample, String name){
		super(name);
		this.sample = sample;
	}

	@Override
	public void run() {
		for(int i=0; i<20; i++){
			sample.dec();
		}
	}
	
}

IncThread 类，对sample的i进行加1的线程：

public class IncThread extends Thread{
	private Sample sample;

	public IncThread(Sample sample, String name){
		super(name);
		this.sample = sample;
	}

	@Override
	public void run() {
		for(int i=0; i<20; i++){
			sample.inc();
		}
	}
	
}

Sample类：

public class Sample {
	int i = 0;

	public synchronized void inc() {
		try {
			while (i == 1) {
				wait();
			}

			i++;

			System.out.println(Thread.currentThread().getName() + "-" + i);

			notify();
		} catch (InterruptedException e) {
			e.printStackTrace();
		}

	}

	public synchronized void dec() {

		try {
			while (i == 0) {
				wait();
			}

			i--;

			System.out.println(Thread.currentThread().getName() + "-" + i);

			notify();
		} catch (InterruptedException e) {
			e.printStackTrace();
		}

	}

}

测试类TestIncDec ：

public class TestIncDec {
	public static void main(String[] args) {
		Sample sample = new Sample();
		
		IncThread inc1 = new IncThread(sample,"inc1");
		IncThread inc2 = new IncThread(sample,"inc2");
		
		DecThread dec1 = new DecThread(sample,"dec1");
		DecThread dec2 = new DecThread(sample,"dec2");
		
		inc1.start();
		inc2.start();
		dec1.start();
		dec2.start();
	}
}