How to Define Callbacks in Android?

Question

In many cases, you have an interface and pass along an object that implements it. Dialogs for example have the OnClickListener.

Just as a random example:

// The callback interface
interface MyCallback {
    void callbackCall();
}

// The class that takes the callback
class Worker {
   MyCallback callback;

   void onEvent() {
      callback.callbackCall();
   }
}

// Option 1:

class Callback implements MyCallback {
   void callbackCall() {
      // callback code goes here
   }
}

worker.callback = new Callback();

// Option 2:

worker.callback = new MyCallback() {

   void callbackCall() {
      // callback code goes here
   }
};

I probably messed up the syntax in option 2. It's early.

When something happens in my view I fire off an event that my activity is listening for:

// DECLARED IN (CUSTOM) VIEW

    private OnScoreSavedListener onScoreSavedListener;
    public interface OnScoreSavedListener {
        public void onScoreSaved();
    }
    // ALLOWS YOU TO SET LISTENER && INVOKE THE OVERIDING METHOD 
    // FROM WITHIN ACTIVITY
    public void setOnScoreSavedListener(OnScoreSavedListener listener) {
        onScoreSavedListener = listener;
    }

// DECLARED IN ACTIVITY

    MyCustomView slider = (MyCustomView) view.findViewById(R.id.slider)
    slider.setOnScoreSavedListener(new OnScoreSavedListener() {
        @Override
        public void onScoreSaved() {
            Log.v("","EVENT FIRED");
        }
    });

If you want to know more about communication (callbacks) between fragments see here: http://developer.android.com/guide/components/fragments.html#CommunicatingWithActivity

No need to define a new interface when you can use an existing one: android.os.Handler.Callback. Pass an object of type Callback, and invoke callback's handleMessage(Message msg).

Example to implement callback method using interface.

Define the interface, NewInterface.java.

package javaapplication1;

public interface NewInterface {
    void callback();
}

Create a new class, NewClass.java. It will call the callback method in main class.

package javaapplication1;

public class NewClass {

    private NewInterface mainClass;

    public NewClass(NewInterface mClass){
        mainClass = mClass;
    }

    public void calledFromMain(){
        //Do somthing...

        //call back main
        mainClass.callback();
    }
}

The main class, JavaApplication1.java, to implement the interface NewInterface - callback() method. It will create and call NewClass object. Then, the NewClass object will callback it's callback() method in turn.

package javaapplication1;
public class JavaApplication1 implements NewInterface{

    NewClass newClass;

    public static void main(String[] args) {

        System.out.println("test...");

        JavaApplication1 myApplication = new JavaApplication1();
        myApplication.doSomething();

    }

    private void doSomething(){
        newClass = new NewClass(this);
        newClass.calledFromMain();
    }

    @Override
    public void callback() {
        System.out.println("callback");
    }

}

to clarify a bit on dragon's answer (since it took me a while to figure out what to do with Handler.Callback):

Handler can be used to execute callbacks in the current or another thread, by passing it Messages. the Message holds data to be used from the callback. a Handler.Callback can be passed to the constructor of Handler in order to avoid extending Handler directly. thus, to execute some code via callback from the current thread:

Message message = new Message();
<set data to be passed to callback - eg message.obj, message.arg1 etc - here>

Callback callback = new Callback() {
    public boolean handleMessage(Message msg) {
        <code to be executed during callback>
    }
};

Handler handler = new Handler(callback);
handler.sendMessage(message);

EDIT: just realized there's a better way to get the same result (minus control of exactly when to execute the callback):

post(new Runnable() {
    @Override
    public void run() {
        <code to be executed during callback>
    }
});