Simple ListView in Android

< 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" > 
   < ListView
     android:id="@android:id/list"
     android:layout_width="fill_parent"
     android:layout_height="wrap_content">  

  < /ListView> 
< /RelativeLayout> 

 public class MainActivity extends ListActivity 

      {
       String[] planets = new String[] { "Mercury", "Venus", "Earth", "Mars",
             "Jupiter", "Saturn", "Uranus", "Neptune","Ceres","Pluto"};
       ListView lstv;
       private ArrayAdapter listAdapter ;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        lstv=getListView();
        listAdapter = new ArrayAdapter(this,android.R.layout.simple_expandable_list_item_1, planets);
        setListAdapter(listAdapter);
 }}