android将fragment加入后退栈没作用

package com.example.mrwuchao.fragmenttest;

import android.app.FragmentManager;
import android.app.FragmentTransaction;
import android.app.ListFragment;
import android.os.Bundle;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.FrameLayout;
import android.widget.ListAdapter;
import android.widget.ListView;

import java.util.ArrayList;

/**

• Created by Mr.wuchao on 2016/5/17.
  */
  public class FirstFragment extends ListFragment {
  ArrayList arrayList = new ArrayList<>();
  public FirstFragment() {
  super();
  }

  @Override
  public void onListItemClick(ListView l, View v, int position, long id) {
  super.onListItemClick(l, v, position, id);
  FragmentManager manager = getFragmentManager();
  FragmentTransaction fra = manager.beginTransaction();
  fra.add(R.id.frame,new SecondFragment(),"test");
  System.out.println(fra.isAddToBackStackAllowed());
  fra.addToBackStack(null);
  fra.commit();
  }

  @Override
  public void onActivityCreated(Bundle savedInstanceState) {
  super.onActivityCreated(savedInstanceState);
  arrayList.add("娱乐");
  arrayList.add("新闻");
  arrayList.add("足球");
  arrayList.add("篮球");
  arrayList.add("八卦");
  ArrayAdapter arrayAdapter = new ArrayAdapter(getActivity(), R.layout.fragment1,
  arrayList);
  setListAdapter(arrayAdapter);

  }
  }

这是主要的内容 其他的就是简单的布局和实现类，我在fra中将添加fragment的动作加入后退栈，但是按后退键直接退出了activity，这是为什么呀

在frgamentactivity定义一个fragment栈，重写onbackpress方法进行fragment的切换操作，栈空了就退出activity
http://blog.csdn.net/q610098308/article/details/50098971