麻烦各位大神帮忙解释 一下 这个继承自ActionBarActivity的MainActivity 类里面的方法

大神们帮忙解释 一下 这个MainActivity 类里面的方法；还有public static class PlaceholderFragment extends Fragment这一块，我在这里面放list怎么会出现这样的错误提示public class MainActivity extendThe method setlistAdapter(SimpleAdapter) is undefined for the type

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 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 {

public PlaceholderFragment() {
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_main, container,
false);
ArrayList<HashMap<String,String>> list = new ArrayList<HashMap<String,String>>();
HashMap<String,String> map1 = new HashMap<String,String>();
HashMap<String,String> map2 = new HashMap<String,String>();
HashMap<String,String> map3 = new HashMap<String,String>();
map1.put("user_name","zhangsan");
map1.put("user_ip","192.168.0.1");
map2.put("user_name","lisi");
map2.put("user_ip","192.168.0.2");
map3.put("user_name","wangwu");
map3.put("user_ip","192.168.0.3");
list.add(map1);
list.add(map2);
list.add(map3);
SimpleAdapter listAdapter = new SimpleAdapter(this,list,R.layout.user,new String[]{"user_name","user_ip"},new int[]{R.id.user_name,R.id.user_ip});
setListAdapter(listAdapter);
return rootView;
}


}

}