Question

consider the following code:

public class SynchronizedCounter extends Thread {
    private int c = 0;

    public synchronized void increment() {
        c++;
    }

    public synchronized void decrement() {
        c--;
    }

    public void run() {
        for(;;)
            increment();
    }
}

static void main(String[] args) {
    SynchronizedCounter counter = new SynchronizedCounter();
    counter.start();
    for(;;)
        counter.decrement();
}

does this means that increment() and decrement() methods will wait for each other to finish or not?

EDIT:
and this does not wait?

static void main(String[] args) {
    SynchronizedCounter counter1 = new SynchronizedCounter();
    SynchronizedCounter counter2 = new SynchronizedCounter();
    counter1.start();
    for(;;)
        counter2.decrement();
}

Answer 1

Yes, the synchronized keyword is a shorthand for:

synchronized(this) {
  //...
}

So both methods are effectively locking on the same mutex object. If you want them to be independent from each other (which is a bad idea in this example as they both access the same value), see Object locking private class members – best practice? (Java).

BTW your SynchronizedCounter should implement Runnable rather than extending a Thread since you are passing it to other thread’s constructor – now it is a bit confusing.