InstanceFactory.java

public class InstanceFactory {
    public static ILogger initLogger(){
        return new FileLogger();
    }
}

MainActivity.java

public class MainActivity extends AppCompatActivity  {

    private ILogger logger = InstanceFactory.initLogger();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        logger.log("onCreate() starts");

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        logger.log("onCreate() ends");
    }
}