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 );
    }
}