範例：

兩個執行緒實現加法，兩個執行緒實現減法

class Resource {
	private int num = 0;
	private boolean flag = true;
	//flag = true 表示可以進行加法操作，不能進行減法操作
	//flag = false 表示可以進行減法操作，不能進行加法操作
	
	public synchronized void add() throws InterruptedException { //執行加法操作
		if (this.flag == false) { //此處應使用while
			super.wait();
		}
		Thread.sleep(100);
		this.num++;
		System.out.println(Thread.currentThread().getName() + " 加法操作 num = " + num);
		this.flag = false;
		super.notifyAll();
	}
	
	public synchronized void sub() throws InterruptedException { //執行減法操作
		if (this.flag == true) { ////此處應使用while
			super.wait();
		}
		Thread.sleep(200);
		this.num--;
		System.out.println(Thread.currentThread().getName() + " 減法操作 num = " + num);
		this.flag = true;
		super.notifyAll();
	}
}


class AddThread implements Runnable {
	
	private Resource resource;
	public AddThread(Resource resource) {
		this.resource = resource;
	}

	@Override
	public void run() {
		for (int i = 0; i < 50; i++) {
			try {
				this.resource.add();
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}
	
}

class SubThread implements Runnable {
	
	private Resource resource;
	public SubThread(Resource resource) {
		this.resource = resource;
	}

	@Override
	public void run() {
		for (int i = 0; i < 50; i++) {
			try {
				this.resource.sub();
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}
	
}

public class ThreadDemo1 {

	public static void main(String[] args) {
		
		Resource resource = new Resource();
		AddThread addThread = new AddThread(resource);
		SubThread subThread = new SubThread(resource);
		new Thread(addThread, "加法執行緒 - A").start();
		new Thread(addThread, "加法執行緒 - B").start();
		new Thread(subThread, "減法執行緒 - X").start();
		new Thread(subThread, "減法執行緒 - Y").start();
	}

}
 

此程式碼中有一個問題，數字num會出現一直減一直加的問題，是因為在進行執行緒同步時，Object中的wait()方法執行時會釋放鎖，無法實現同步

As in the one argument version, interrupts and spurious wakeups are possible, and this method should always be used in a loop:

 synchronized (obj) {
     while (<condition does not hold>)
         obj.wait(timeout, nanos);
     ... // Perform action appropriate to condition
 }
 

上面是官方文件的說明