Nullpointerexception thrown when trying to findViewById

Question

I have the following Activity:

public class MainActivity extends ActionBarActivity {

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

    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new StartFragment())
                .commit();
    }

    Button login = (Button) findViewById(R.id.loginButton);

    login.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View arg0) {

            Intent intent = new Intent(MainActivity.this,LoginActivity.class);
            startActivity(intent);
        }
    });

}

I get a NPE when I try to invoke findViewByID for R.id.loginButton , and I'm guessing this is because loginButton is within a separate Fragment, which I have as:

public static class StartFragment extends Fragment {

    public StartFragment() {
    }
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

        return inflater.inflate(R.layout.fragment_main, container, false);

    }
}

However, I am unsure of how to fix this so that I can find the loginButton ID. I haven't worked with fragments before, so I realize I may be using them/implementing them incorrectly. fragment_main contains a few buttons in a LinearLayout , and activity_main has nothing but a single FrameLayout .

Answer 1

Try to implement your onCreateView(...) in Fragment like

@Override
 public View onCreateView(LayoutInflater inflater, ViewGroup container,
 Bundle savedInstanceState) {
 View rootView = inflater.inflate(R.layout.fragment_main, container,
  false);

 View something = rootView.findViewById(R.id.something);
 something.setOnClickListener(new View.OnClickListener() { ... });

return rootView;
}

The Button is in the fragment layout ( fragment_main.xml ) and not in the activity layout ( activity_main.xml ). onCreate() is too early in the lifecycle to find it in the activity view hierarchy, and a null is returned. Invoking a method on null causes the NPE.

Answer 2

Write code to initialize button from fragment becuase your button is into fragment layout not into activity's layout.

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {

    View rootView = inflater.inflate(R.layout.fragment_main, container,
            false);
    Button login = (Button) rootView.findViewById(R.id.loginButton);
    login.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View arg0) {

            Intent intent = new Intent(MainActivity.this,
                    LoginActivity.class);
            startActivity(intent);
        }
    });

    return rootView;
}

And remove the login button related code from onCreate of Activity .

Answer 3

findViewById() works with reference to a root view. Without having a view in the first place will throw a null pointer exception

In any activity you set a view by calling setContentView(someView); . Thus when you call findViewById() , its with reference to the someView . Also findViewById() finds the id only if its in that someView . So in you case null pointer exception

For fragments, adapters, activity, .... any view's findViewById() will only find if the id exixts in the view Alternately if you are inflating a view, then you can also use inflatedView.findViewById() to get a view from that inflatedView

In short make sure you have the id in your layout you are referring to or make findViewById() call in appropriate place(Ex. adapters getView() , activity's onCreate() or onResume() or onPause() , fragments onCreateView() , ....)

Also have an idea about UI & background thread's as you cannot efficiently update UI in bg-threads