NullpointerException in setadapter() method

Question

Encountering a NullPointerException while using listview. I have initialized my adapter,my listview and my lessons array aswell.

Apologies if my doubt may sound vague. I am a newbie to android and cant figure out the bug

AboutActivity.java

public class AboutActivity extends Activity {

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

     setContentView(R.layout.activity_about);

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

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

fragment_about.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.chatclient.AboutActivity$PlaceholderFragment" >

    <ListView
        android:id="@+id/ListView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_centerVertical="true" >

    </ListView>

</RelativeLayout>

logcat:

05-09 01:41:59.029: E/AndroidRuntime(1579): FATAL EXCEPTION: main
05-09 01:41:59.029: E/AndroidRuntime(1579): Process: com.example.chatclient, PID: 1579
05-09 01:41:59.029: E/AndroidRuntime(1579): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.chatclient/com.example.chatclient.AboutActivity}: java.lang.NullPointerException
05-09 01:41:59.029: E/AndroidRuntime(1579):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
05-09 01:41:59.029: E/AndroidRuntime(1579):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
05-09 01:41:59.029: E/AndroidRuntime(1579):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
05-09 01:41:59.029: E/AndroidRuntime(1579):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
05-09 01:41:59.029: E/AndroidRuntime(1579):     at android.os.Handler.dispatchMessage(Handler.java:102)
05-09 01:41:59.029: E/AndroidRuntime(1579):     at android.os.Looper.loop(Looper.java:136)
05-09 01:41:59.029: E/AndroidRuntime(1579):     at android.app.ActivityThread.main(ActivityThread.java:5017)
05-09 01:41:59.029: E/AndroidRuntime(1579):     at java.lang.reflect.Method.invokeNative(Native Method)
05-09 01:41:59.029: E/AndroidRuntime(1579):     at java.lang.reflect.Method.invoke(Method.java:515)
05-09 01:41:59.029: E/AndroidRuntime(1579):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
05-09 01:41:59.029: E/AndroidRuntime(1579):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
05-09 01:41:59.029: E/AndroidRuntime(1579):     at dalvik.system.NativeStart.main(Native Method)
05-09 01:41:59.029: E/AndroidRuntime(1579): Caused by: java.lang.NullPointerException
05-09 01:41:59.029: E/AndroidRuntime(1579):     at com.example.chatclient.AboutActivity.onCreate(AboutActivity.java:29)
05-09 01:41:59.029: E/AndroidRuntime(1579):     at android.app.Activity.performCreate(Activity.java:5231)
05-09 01:41:59.029: E/AndroidRuntime(1579):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
05-09 01:41:59.029: E/AndroidRuntime(1579):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
05-09 01:41:59.029: E/AndroidRuntime(1579):     ... 11 more

Answer 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);  

Answer 2

Try replacing this line:

setContentView(R.layout.activity_about);

With:

setContentView(R.layout.fragment_about);

I hope this helps!

Answer 3

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

Answer 4

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;
    }
}

}