android中的add方法,出现add方法报错

package com.example.newfragmentdemo;

import android.app.Activity;

import android.app.ActionBar;

import android.os.Bundle;

import android.support.v4.app.FragmentActivity;

import android.view.Menu;

import android.view.MenuItem;

import android.os.Build;

import android.support.v4.app.Fragment;

public class MainActivity extends Activity {

@Override

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

if (savedInstanceState == null) {

getFragmentManager().beginTransaction()

.add(R.id.container, new PlaceholderFragment()).commit();

}

}

}

在add那里就报错了，报错的内容是The method add(int, Fragment) in the type FragmentTransaction is not applicable for the arguments (int, PlaceholderFragment)