Handling collisions/contacts between bodies etc:

This really important aspect, mostly of the every physics based game, to handle collisions/contacts between bodies, we will use ContactListener. Let's say we want to execute action after contact between body A and body B, with ContactListener, its really easy.

First step, create new contact listener: 

private ContactListener createContactListener()
{
    ContactListener contactListener = new ContactListener()
    {
        @Override
        public void beginContact(Contact contact)
        {
            final Fixture x1 = contact.getFixtureA();
            final Fixture x2 = contact.getFixtureB();
        }

        @Override
        public void endContact(Contact contact)
        {
               
        }

        @Override
        public void preSolve(Contact contact, Manifold oldManifold)
        {
               
        }

        @Override
        public void postSolve(Contact contact, ContactImpulse impulse)
        {
               
        }
    };
    return contactListener;
}

• Step two, register your contact listener to your PhysicsWorld.

mPhysicsWorld.setContactListener(createContactListener());

yourBody.setUserData("player");

if (x2.getBody().getUserData().equals("player")&&x1.getBody().getUserData().equals("monster"))
{
    Log.i("CONTACT", "BETWEEN PLAYER AND MONSTER!");
}

That's all, with this simple and easy way, you may check when contact between different kind of bodies will occur, and execute your actions. Important notice: In example below, we are checking only once, and in this case we assume that x2 fixture will be our player and x1 fixture will be our monster body. It might be different (conversely) That's why sometimes you should include second check, with the same actions but with different order (x2 and x1)