Android Bound Services: Why do we cast this IBinder instance into an IBinder instance?

Question

In the developer docs for Bound Services, the following code example is given for "Extending the Binder Class" in "Creating a Bound Service". The following code snippet (I have removed irrelevant bits) is given in which the Service returns an IBinder from its onBind()method:

public class LocalService extends Service {
    // Binder given to clients
    private final IBinder mBinder = new LocalBinder();
    ...
    /**
     * Class used for the client Binder.  Because we know this service always
     * runs in the same process as its clients, we don't need to deal with IPC.
     */
    public class LocalBinder extends Binder {
        LocalService getService() {
            // Return this instance of LocalService so clients can call public methods
            return LocalService.this;
        }
    }

    @Override
    public IBinder onBind(Intent intent) {
        return mBinder; //**********************************************************
    }
    ...
}

Then in our client, we receive the mBinder object (which is an instance of LocalBinder) in the onServiceConnected() method of ServiceConnection. My question is that why are we trying to cast an instance of LocalBinder passed in as an argument to onServiceConnected() into a LocalBinder instance in the statement LocalBinder binder = (LocalBinder) service; ?

public class BindingActivity extends Activity {
    LocalService mService;
    boolean mBound = false;
    ...

    /** Defines callbacks for service binding, passed to bindService() */
    private ServiceConnection mConnection = new ServiceConnection() {

        @Override
        public void onServiceConnected(ComponentName className,
                IBinder service) {
            // We've bound to LocalService, cast the IBinder and get LocalService instance
            LocalBinder binder = (LocalBinder) service;
            mService = binder.getService();
            mBound = true;
        }

        @Override
        public void onServiceDisconnected(ComponentName arg0) {
            mBound = false;
        }
    };
    ...
}

Answer 1

The definition of ServiceConnection.onServiceConnected() is

public void onServiceConnected(ComponentName className, IBinder service)

Note that the parameter is an IBinder - ServiceConnection does not know what kind of service or what kind of IBinder implementation the service is returning - only you know that, hence why you need to cast it to the correct type.