求大神解答，利用接口实现Fragment切换，为什么Log打印说Activity被销毁了

Question

MainActivity：
@Override
public void onClick(View view) {
    switch (view.getId()) {
        case R.id.bt_user:
            cf.setinitif(new Interface_ChangeFragment() {
                @Override
                public Fragment initif() {
                    return tab_user;
                }
 
                @Override
                public int initTAG() {
                    return 3;
                }
            });
            break;
        case R.id.bt_find:
            cf.setinitif(new Interface_ChangeFragment() {
                @Override
                public Fragment initif() {
                    return tab_found;
                }
 
                @Override
                public int initTAG() {
                    return 2;
                }
            });
            break;
        case R.id.bt_watchmoive:
            cf.setinitif(new Interface_ChangeFragment() {
                @Override
                public Fragment initif() {
                    return tab_watchMoive;
                }
 
                @Override
                public int initTAG() {
                    return 1;
                }
            });
            break;
        case R.id.bt_selectsong:
            cf.setinitif(new Interface_ChangeFragment() {
                @Override
                public Fragment initif() {
                    return tab_selectSong;
                }
 
                @Override
                public int initTAG() {
                    return 0;
                }
            });
            break;
        case R.id.bt_frist:
            break;
    }
     

     
     
     
    接口的实现类：
    public class ChangeFragment extends FragmentActivity {
    int TAG;
    Tab_selectSong tab_selectSong;
    Tab_watchMoive tab_watchmoive;
    Tab_Found tab_found;
    Tab_User tab_user;
    public void setinitif(Interface_ChangeFragment interface_changeFragment) {
        FragmentManager manager = getSupportFragmentManager();
        FragmentTransaction fts = manager.beginTransaction();
        if (interface_changeFragment.initif() == null) {
            TAG = interface_changeFragment.initTAG();
            switch (TAG) {
                case 0:
                   tab_selectSong = new Tab_selectSong();
                    fts.add(R.id.activity_fragment,tab_selectSong);
                    hideall();
                    fts.show(tab_selectSong);
                    fts.commit();
                    break;
                case 1:
                    tab_watchmoive = new Tab_watchMoive();
                    fts.add(R.id.activity_fragment,tab_watchmoive);
                    hideall();
                    fts.show(tab_watchmoive);
                    fts.commit();
                    break;
                case 2:
                    tab_found = new Tab_Found();
                    fts.add(R.id.activity_fragment,tab_found);
                    hideall();
                    fts.show(tab_found);
                    fts.commit();
                    break;
                case 3:
                   tab_user = new Tab_User();
                    fts.add(R.id.activity_fragment,tab_user);
                    hideall();
                    fts.show(tab_user);
                    fts.commit();
                    break;
            }
        } else {
            TAG = interface_changeFragment.initTAG();
            switch (TAG) {
                case 0:
                    hideall();
                    fts.show(tab_selectSong);
                    fts.commit();
                    break;
                case 1:
                    hideall();
                    fts.show(tab_watchmoive);
                    fts.commit();
                    break;
                case 2:
                    hideall();
                    fts.show(tab_found);
                    fts.commit();
                    break;
                case 3:
                   hideall();
                    fts.show(tab_user);
                    fts.commit();
                    break;
            }
 
        }
    }
 
    private void hideall() {
        FragmentManager manager = getSupportFragmentManager();
        FragmentTransaction fts = manager.beginTransaction();
        if(tab_selectSong!=null)
        {
            fts.hide(tab_selectSong);
        }
        if(tab_watchmoive!=null)
        {
            fts.hide(tab_watchmoive);
        }
        if(tab_found!=null)
        {
            fts.hide(tab_found);
        }
        if(tab_user!=null)
        {
            fts.hide(tab_user);
        }
    }
}
 
用的是按钮的点击事件new出一个匿名内部类，之所以这么做，是不想让主MainAcitity变得臃肿，可是
结果出错了，Log打印出来是java.lang.IllegalStateException: Activity has been destroyed
 
求大神解答为什么啊？难道不可以这么写吗？
我也是一个新手，才开始写我的第一个APP。求大神解答

Answer 1

能说下什么时候报的错吗？--->个人推测可能是ChangeFragment extends FragmentActivity与MainActivity冲突的问题。（我多嘴一下，Fragment类和FragmentActivity类并不是继承关系）【另外】你说你是新手，那我推荐一个开发中比较不错的用法吧（也是谷歌官方推荐的）：使用ViewPager+Fragment的形式，实现fragment页面之间的切换，具体使用可以参考这里：http://www.imooc.com/learn/198 或者 http://www.imooc.com/learn/273 ------可能我说的你之前都懂，个人能力低微，已经尽力啦，能有帮助就最好-----