How synchronization works in Java when i use synchronized(this.local_datamember)?

Question

Let's assume that I create 2 threads CounterRunnable and create one object of Mycounter.

In the code below, how does the synchronization work? I imagined that when I synchronize on counter, which is an object defined in the thread class, I now have 2 separate objects with two different counter objects with two different locks to each object.

I thought that I am now locking on the counter object itself not the single referenced object.

public class example {

    class CounterRunnable implements Runnable {
        private MyCounter counter;

        public CounterRunnable (MyCounter counter) {
            this.counter = counter;
        }

        public void run () {
            for (int i = 0 ; i < 1000; i++)
              synchronized(this.counter){
                counter.increment();
              }
        }
    }

    class MyCounter  {
        private int c =0;
        public synchronized void increment () {
            c++;
        }
        public int value () {return c;} 
    }

    public static void main (String [] args) throws InterruptedException {

        example ex = new example();
        MyCounter counter = ex.new MyCounter();

        Thread t1 = new Thread (ex.new CounterRunnable(counter));
        Thread t2 = new Thread (ex.new CounterRunnable(counter));

        t1.start(); t2.start();
        t1.join(); t2.join();

        System.out.println(counter.value());

    }
}

Answer 1

You are confused with how Objects and object references work

public class Foo () {
  Object bar;

  public Foo() {
    bar = new Object();
  }
}

In the example above, bar is an object reference. It is null until it is assigned in the constructor:

bar = new Object();

bar now points to the object you created. When you pass an object into a constructor or method and then assign it to an object reference in the class, the reference takes on a value.

So back to your synchronization question. Since you are passing the same object into both CounterRunnables, they are synchronizing on the same object.