Android 开发时遇到 MainActivity is not an enclosing class

package cc.starlabs.ifevent_v2; import android.app.Activity; import android.os.Bundle; import android.support.v7.app.AppCompatActivity; import android.view.Menu; import android.view.MenuItem; import cc.starlabs.ifevent_v2.fragment.MainActivityFragment; public class MainActivity extends AppCompatActivity { @Override protected void onCreate (Bundle savedInstanceState ) { super.onCreate (savedInstanceState ); setContentView (R.layout.activity_main ); if (savedInstanceState==null ) { getSupportFragmentManager ().beginTransaction ().add (R.id.fragment,new MainActivityFragment ()).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.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 (); //noinspection SimplifiableIfStatement if (id == R.id.action_settings ) { return true; } return super.onOptionsItemSelected (item ); } }