android.app.fragment v4,引入android.support.v4.app.Fragment报错

ok，问题得到解决了，主要的原因出现以下几点，

1、你的MianActivity应该要继承FragmentActivity类

2、引入android.support.v4.app.FragmentActivity;

修改过的代码改为如下：

package com.example.testfragment;

import android.app.Activity;

import android.app.ActionBar;

import android.support.v4.app.Fragment;

import android.support.v4.app.FragmentActivity;

//import  android.support.v4.app.FragmentManager;

//import  android.support.v4.app.FragmentTransaction;

import android.os.Bundle;

import android.view.LayoutInflater;

import android.view.Menu;

import android.view.MenuItem;

import android.view.View;

import android.view.ViewGroup;

import android.os.Build;

public class MainActivity extends FragmentActivity {

@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();

}

}