Restricting method execution from other class method

Question

Somebody asked me this question and I am not able to find out the solution, I have a class A with method a1 and a2:

Class A {a1() a2()}:

and a class B with method b1 and b2:

Class B {b1() b2()}

Considering multithreading environment i want to stop execution of b1 when a1 is called and same applies for b2 and a2. Both the objects are independent. Is this possible?

Answer 1

public class A {
    private final Object lock;

    public A(Object lock) {
        this.lock = lock;
    }

    public void a1() { 
        synchronized(lock) {
            ...
        }
    }
}

public class B {
    private final Object lock;

    public B(Object lock) {
        this.lock = lock;
    }

    public void b1() { 
        synchronized(lock) {
            ...
        }
    }
}

Now if you construct an A and a B and pass them the same lock object, The a1() and b1() methods of those two objects will be mutually exclusive:

Object lock = new Object();
A a = new A(lock);
B b = new B(lock);

If you want a2() and b2() to be mutually exclusive, then pass another lock object, and make these two methods synchronized on this other lock object.