Communication is always important for developers. Sometimes we need to communicate between two fragments. Two fragments should not communicate directly as fragment is a re-useable UI component and should be built independently.

Now, we will see how we can communicate between two fragments using an interface.

Suppose, we have created two fragments in an activity named FragmentA and FragmentB. We want to perform some task in FragmentA after FragmentB has inserted some data in database.

First, we will define an interface in FragmentB.

public class FragmentB extends Fragment {
	
    private Callback callback;
    
    public void setCallback(Callback callback) {
    	this.callback = callback;
    }
    
    public interface Callback {
    	void performSomeTask();
    }   
}
Defining an interface in FragmentB

Now, when the FragmentB is attached to the MainActivity, we will do the following :

public class MainActivity extends AppCompatActivity
	implements FragmentB.Callback {

    @Override
    public void onAttachFragment(Fragment fragment) {
    	if (fragment instanceof FragmentB) {
            FragmentB fragmentB = (FragmentB) fragment;
            fragmentB.setCallback(this);
        }
    }
}
MainActivity

After insertion some data in database in FragmentB, we will use the callback interface to send message in MainActivity 

public class FragmentB extends Fragment {

    //...
    
    private void insertDataOnDB() {
    	//...
        //after insertion complete
        callback.performSomeTask();
    }
}
Delivering callback to MainActivity

Now, we will implement the Callback interface in MainActivity. Callback event comes here from FragmentB after inserting into database. Here, we will perform the task in FragmentA through the method doTaskOnCallback of FragmentA. For this, we will find the FragmentA by its Tag and perform tasks on FragmentA. 

public class MainActivity extends AppCompatActivity
	implements FragmentB.Callback {
	
    @Override
    public void performSomeTask() {
    	//when creating FragmnetA from MainActivity, we will give 
        //it a tag "fragmentA"
    	FragmentA fragmentA = (FragmentA) getSupportFragmentManager()
      			.findFragmentByTag("fragmentA");
        if(fragmentA != null) {
        	fragmentA.doTaskOnCallback();
        }
    }
}
Implementing the Callback interface in MainActivity

That's all.