NullPointerException when trying to set text to a textview in activity from fragment

Question

I create the variable like this in top of Activity class:

private TextView label;

Im assigning the textView like this in onCreate method:

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

    label = (TextView) findViewById(R.id.label);
}

And in the Activity I have this function:

public void changeLabel(String label){
        label.setText(label);
}

Then in a fragment who is attached to Activity, inside of onAttach method I do this:

@Override
public void onAttach(Activity activity) {
    super.onAttach(activity);
    ((ParentActivity)activity).changeLabel("New label");
}

But Im getting NullPointerException in the Activity, inside of changeLabel function, here is the error:

01-15 10:44:43.090  28800-28800/com.app.example E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.app.example, PID: 28800
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.app.example/com.app.example.ParentActivity}: java.lang.NullPointerException
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2202)
        ...
 Caused by: java.lang.NullPointerException
        at com.app.example.Parentacitvity.changeLabel(Home.java:269)
        at com.app.example.HomeFragment.onAttach(ChildFragment.java:33)
        ...
        at android.app.Activity.onCreate(Activity.java:907)
        at com.app.example.ParentActivity.onCreate(Home.java:68)
        ...

This only happenning when I come back to the application after doing multiple tasks outside it, so I think maybe the function is called by the fragment before the onCreate is called in the Activity. Any suggestion?

Answer 1

The fragment gets restored when you're calling super.onCreate() in the activity, as shown in your call stack. You're initializing the label only after calling super.onCreate() .

As a quick fix, you can postpone the label setting to later in the fragment lifecycle, such as onActivityCreated() .

Generally, a fragment should not care about specifics of the activity it is attached in. Casting the activity to a specific type is a code smell. Consider redesigning your approach so that the fragment only needs to mind its own UI and let the containing activity handle its own.

Answer 2

A fragment's onAttach() is called before the activity's onCreate() . Use onActivityCreated() instead.