问无法解析符号碎片容器

EN

import android.support.v4.app.FragmentManager;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.os.Bundle;

public class MainActivity extends FragmentActivity  {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // add fragment to the activity
        FragmentManager fm = getSupportFragmentManager();

        // give fragment to manage
        Fragment fragment = fm.findFragmentById(R.id.fragmentContainer);

        if (fragment == null) {
            fragment = new HomeFragment();
            fm.beginTransaction().add(R.id.fragmentContainer, fragment)
                    .commit();
        }
    }

}