Define interface

//In this interface, you can define messages, which will be send to owner.
public interface MyCustomListener {
    //In this case we have two messages, 
    //the first that is sent when the process is successful.
    void onSuccess(List<Bitmap> bitmapList);
    //And The second message, when the process will fail.
    void onFailure(String error);
}

Create listener

In the next step we need to define an instance variable in the object that will send callback via MyCustomListener. And add setter for our listener.

public class SampleClassB {
    private MyCustomListener listener;

    public void setMyCustomListener(MyCustomListener listener) {
        this.listener = listener;
    }
}

Implement listener

Now, in other class, we can create instance of SampleClassB.

public class SomeActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        SampleClassB sampleClass = new SampleClassB();
    }
}

next we can set our listener, to sampleClass, in two ways:

by implements MyCustomListener in our class:

public class SomeActivity extends Activity implements MyCustomListener {
    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        SampleClassB sampleClass = new SampleClassB();
        sampleClass.setMyCustomListener(this);
    }

    @Override
    public void onSuccess(List<Bitmap> bitmapList) {

    }

    @Override
    public void onFailure(String error) {

    }
}

or just instantiate an anonymous inner class:

public class SomeActivity extends Activity {
    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        SampleClassB sampleClass = new SampleClassB();
        sampleClass.setMyCustomListener(new MyCustomListener() {

            @Override
            public void onSuccess(List<Bitmap> bitmapList) {

            }

            @Override
            public void onFailure(String error) {

            }
        });
    }
}

Trigger listener

public class SampleClassB {
    private MyCustomListener listener;

    public void setMyCustomListener(MyCustomListener listener) {
        this.listener = listener;
    }

    public void doSomething() {
        fetchImages();
    }

    private void fetchImages() {
        AsyncImagefetch imageFetch = new AsyncImageFetch();
        imageFetch.start(new Response<Bitmap>() {
            @Override
            public void onDone(List<Bitmap> bitmapList, Exception e) {
                //do some stuff if needed

                //check if listener is set or not.
                if(listener == null)
                    return;
                //Fire proper event. bitmapList or error message will be sent to
                //class which set listener.
                if(e == null)
                    listener.onSuccess(bitmapList);
                else
                    listener.onFailure(e.getMessage());
            }
        });
    }
}

The "listener" or "observer" pattern is the most common strategy for creating asynchronous callbacks in Android development.

public class MyCustomObject {       
  
  //1 - Define the interface 
  public interface MyCustomObjectListener {
      public void onAction(String action);
  }

  //2 - Declare your listener object
  private MyCustomObjectListener listener;

  // and initialize it in the costructor
  public MyCustomObject() {        
    this.listener = null; 
 }

 //3 - Create your listener setter
 public void setCustomObjectListener(MyCustomObjectListener listener) {
    this.listener = listener;
 }

 // 4 - Trigger listener event
 public void makeSomething(){
    if (this.listener != null){
       listener.onAction("hello!");
 }        
}

Now on your Activity:

public class MyActivity extends Activity {
   public final String TAG = "MyActivity";

  @Override
  protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.main_activity);
        

      MyCustomObject mObj = new MyCustomObject();
    
      //5 - Implement listener callback
      mObj.setCustomObjectListener(new MyCustomObjectListener() {
        @Override
          public void onAction(String action) {
              Log.d(TAG, "Value: "+action);
          }
      });
   }
}