Call function when click in Button

public class MainActivity extends Activity {

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

        Button start = (Button) findViewById(R.id.button_start);
        start.setOnClickListener(startListener);


    }

    private View.OnClickListener startListener = new View.OnClickListener() {
        public void onClick(View v) {
            Log.d("MainActivity", "onClick() called - start button");
        }
    };
}