NullpointerException in setadapter() method

Question 1

Error:

In your Activity, you are calling:

setContentView(R.layout.activity_about);

This displays the layout activity_about.xml whereas you created your views inside fragment_about.xml. It causes the NullPointerException because the system cannot find the views:

findViewById(R.id.ListView); // this 'id' (R.id.ListView) cannot be find

Solution:

Create your views inside activity_about.xml or change the target layout as follows:

setContentView(R.layout.fragment_about);

Question 2

You should change this

setContentView(R.layout.activity_about);

to

setContentView(R.layout.fragment_about);

It's because ListView in your fragment_about.xml file and you'll find ListView in activity_about.xml file

ListView listview = (ListView)findViewById(R.id.ListView); //Cause NULL Pointer Exception

Question 3

you are getting null pointer because you are using placeholder i.e fragment for UI and writing java code in activity. below are the solutions.

1.setContentView(R.layout.fragment_about);
2. change your activity like below: 

public class AboutActivity 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 PlaceholderFragment()).commit();
    }
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {

    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}

/**
 * A placeholder fragment containing a simple view.
 */
public static class PlaceholderFragment extends Fragment {
    ArrayAdapter<String> adapter;
    public PlaceholderFragment() {
    }

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

         String[] lessons = new String[] { "Android Introduction","Android Setup/Installation","Android Hello World","Android Layouts/Viewgroups","Android Activity & Lifecycle","Intents in Android"};

         adapter = new ArrayAdapter<String>(getActivity(), android.R.layout.simple_list_item_1,lessons);
         ListView listview = (ListView)rootView.findViewById(R.id.ListView);
         listview.setAdapter(adapter);
        return rootView;
    }
}

}

Question 4

Try replacing this line:

setContentView(R.layout.activity_about);

With:

setContentView(R.layout.fragment_about);

I hope this helps!