Why do I get a NullPointerException here?

Question

I have my code defined the way below. There are two crucial activities. Activity (1) shows some images in a ViewFlipper. It uses methods to load desired image directly. The onOptionsItemSelected() method fetches data from a menu defined within linked XML layout R.layout.browse. The other method, displaySelectedFlag(), gets a tag parameter passed from a different activity, let's call it activity (2).

Activity (1):

public class BrowserActivity extends AppCompatActivity implements SimpleGestureListener, View.OnClickListener {

public ViewFlipper vFlipper;

(...)

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

setContentView(R.layout.browse);
vFlipper = (ViewFlipper) findViewById(R.id.viewFlipperBrowser);

(...)

}  // onCreate() ends here

// this method below works fine:

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    vFlipper.setDisplayedChild(item.getOrder());
    return true;
}

// and this one doesn't:

public void displaySelectedFlag(int orderTag) {
    vFlipper.setDisplayedChild(orderTag); // crashes here
}
}

Activity (2):

public class ListActivity extends Activity implements View.OnClickListener {

private BrowserActivity browserActivity = new BrowserActivity();

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

    ImageButton imageA = (ImageButton) findViewById(R.id.img_a);
    imageA.setOnClickListener(this);

    ImageButton imageB = (ImageButton) findViewById(R.id.img_b);
    imageB.setOnClickListener(this);
}

public void displayImageInfo(View view) {
    String tagValue = (String) view.getTag();
    int tagId = Integer.parseInt(tagValue);
    Intent intent = new Intent(this, BrowserActivity.class);
    startActivity(intent);
    browserActivity.displaySelectedImage(imageId);
}

@Override
public void onClick(View view) {
    displayImageInfo(view);
}
}

As I checked, the method onClick() called in activity (2) fetches an ID of an ImageButton and passes it to activity (1). Unfortunately, I get a NullPointerException when calling the ViewFlipper (the line is marked in the code above, activity (1)).

Any idea why it happens?

Check this answer it explains why you shouldn't create objects of Activities. http://stackoverflow.com/questions/14956018/can-i-create-the-object-of-a-activity-in-other-class — Apoorv, Jun 29 '16 at 23:14

score 1 · Accepted Answer · edited May 23 '17 at 12:22

You cannot reference one Activity from another activity. You must let the Android OS create the Activity object via the call to "startActivity". Allocating a local variable as an instance of an Activity doesn't actually mean anything (like your instantiation of the BrowserActivity). Apoorv's comment links to a decent article on the subject.

If you want to pass data from one Activity to another, you need to pass extras within the Intent's bundle. This post goes into detail: https://stackoverflow.com/a/819427/504252

Why do I get a NullPointerException here?

1 Answers1