为什么synchronized不能同步线程？

Question

I have set the value of the flag, but the result is not 'add' and 'sub' alternates. Why? When I look at the result, it has executed twice the 'sub' method. But when the 'sub' method ended, the value of the flag will be set 'false'. But as a result, it printed "subxxxxx" twice continuously.

class Resource {
    private boolean flag = true;
    private int num = 0;

// At here I have declared an add()
    public synchronized void add() throws InterruptedException {
        if (this.flag == false) {
            super.wait();
        }
        Thread.sleep(100);
        this.num++;
        System.out.println("addition:"+Thread.currentThread().getName() + this.num);
        this.flag = false;
        super.notifyAll();
    }

// At here I have declared an sub()
    public synchronized void sub() throws InterruptedException {
        if (this.flag == true) {
            super.wait();
        }
        Thread.sleep(200);
        this.num--;
        System.out.println("subtraction:"+Thread.currentThread().getName() + this.num);
        this.flag = true;
        super.notifyAll();
    }
}

/*
* I will test it with multiple threads. For example:
*new Thread(ad, "add").start();
*new Thread(ad, "add").start();
*new Thread(sub, "sub").start();
*new Thread(sub, "sub").start();
*When threads start. it will execute alternately. For example:
Thread add:0
Thread sub:-1
Thread add:0
Thread sub:-1
Thread add:0
Thread sub:-1
Thread add:0
Thread sub:-1
But the result is like this:
Thread add:0
Thread sub:-1
Thread sub:-2
Thread add:-1
Thread sub:-3
Thread sub:-4
Why?Why?Why?
*/
        new Thread(ad, "add").start();
        new Thread(ad, "add").start();
        new Thread(sub, "sub").start();
        new Thread(sub, "sub").start();
    }
}

Answer 1

You seem to assume that when your wait() call finishes, that the flag has changed to what you wanted to have before you called wait() . There is no such guarantee, especially since you have more than two threads involved. You should check if you need to continue waiting. Also see Wait until boolean value changes it state

But in general, these constructs are too low-level to use (unless you want to learn the nitty-gritty) and you should look at the concurreny utils package for easier higher-level constructs (like Queues, Latches, Conditions).