try this worked for me:
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
myList = new ArrayList<String>();
String root_sd = Environment.getExternalStorageDirectory().toString();
file = new File( root_sd + "/external_sd" ) ;
File list[] = file.listFiles();
for( int i=0; i< list.length; i++)
{
myList.add( list[i].getName() );
}
setListAdapter(new ArrayAdapter<String>(this,
android.R.layout.simple_list_item_1, myList ));
}
protected void onListItemClick(ListView l, View v, int position, long id)
{
super.onListItemClick(l, v, position, id);
File temp_file = new File( file, myList.get( position ) );
if( !temp_file.isFile())
{
file = new File( file, myList.get( position ));
File list[] = file.listFiles();
myList.clear();
for( int i=0; i< list.length; i++)
{
myList.add( list[i].getName() );
}
Toast.makeText(getApplicationContext(), file.toString(), Toast.LENGTH_LONG).show();
setListAdapter(new ArrayAdapter<String>(this,
android.R.layout.simple_list_item_1, myList ));
}
}