Android findViewById throws NullPointerException

Question

I have a ViewFlipper defined in a separate class. It looks like this:

public class Flipper extends Activity {
    public ViewFlipper view_flipper;

    /* Constructor */
    public Flipper(int flipper_id) {
        view_flipper = (ViewFlipper) findViewById(flipper_id);
    }
    ...

Then in another Activity's onCreate() method I am instantiating the Flipper like this:

private Flipper flipper;
flipper = new Flipper(R.id.login_screen_flipper);

However, I get a NullPointerException in the constructor of Flipper. When I debug, I see that flipper_id has a valid id. What am I doing wrong?

Answer 1

When you instantiate Flipper in another activity, findViewById is called in the constructor of Flipper. It is a method from Flipper class as it extends Activity and surely its call is not valid in the constructor because you do not set content view as it is normally done in Activity.onCreate. Flipper should not extend Activity and you should pass ViewFlipper in a constructor not id.