字符串变量不会显示在subTitle中

Question

我开发了一个包含5个片段的基本应用程序。每个片段显示不同的标题，只有第一个片段也显示subTitle。

这是代码：

switch (position) {
                case 0:
                    setTitle("fragment_0",set);
                    if(searchFragment == null) searchFragment = new SearchFragment();
                    currentFragment = searchFragment;
                    mapButton.setVisibility(View.VISIBLE);
                    btTopQuery.setVisibility(View.INVISIBLE);
                    break;
                case 1:
                    setTitle("fragment_1",null);
                    if(queryFragment == null) queryFragment = new QueryFragment();
                    ((QueryFragment)queryFragment).viewStatus = 0;
                    currentFragment = queryFragment;
                    btTopQuery.setVisibility(View.VISIBLE);
                    break;
                case 2:
                    setTitle("fragment_2",null);
                    showDownLoadDialog();
                    return;
                case 3:
                    setTitle("fragment_3",null);
                    if(newsFragment == null) newsFragment = new NewsFragment();
                    currentFragment = newsFragment;
                    CustomSharedPreferences.init(getApplicationContext());
                    CustomSharedPreferences.setPreferences(Constants.READ_NEWS_COUNT,FirstProjectApplication.allNews.size());
                    menu_adapter.notifyDataSetChanged();
                    break;
                case 4:
                    setTitle("fragment_4",null);
                    if(infoFragment == null) infoFragment = new InfoFragment();
                    currentFragment = infoFragment;
                    break;
                case 5:
                    setTitle("fragment_5",null);
                    if(socialFragment == null) socialFragment = new SocialFragment();
                    currentFragment = socialFragment;
                    break;
                case 6:
                    setTitle("fragment_6",null);
                    if(settingsFragment == null) settingsFragment = new SettingsFragment();
                    currentFragment = settingsFragment;
                    break;
            }

            getSupportFragmentManager().beginTransaction()
                    .replace(R.id.frameContainer, currentFragment)
                    .commit();
            //.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_OPEN)
            mDrawerLayout.closeDrawer(navdrawer);
        }
    });

和：

if(currentFragment == null) {
            setTitle("fragment_0",set);
            if(searchFragment == null) searchFragment = new SearchFragment();
            currentFragment = searchFragment;
            mapButton.setVisibility(View.VISIBLE);
            btTopQuery.setVisibility(View.INVISIBLE);
            getSupportFragmentManager().beginTransaction().add(R.id.frameContainer, currentFragment).commit();
        }

变量set具有我从JSON读取的值，使用以下代码：

String set;

//Reading text
            String url1 ="url_json";
            StringRequest strReq1 = new StringRequest(Request.Method.GET,
                    url1, new Response.Listener<String>() {
                @Override
                public void onResponse(String response) {
                    Log.d("", response.toString());
                    try {
                        FirstProjectApplication.allNews.clear();
                        JSONObject featureCollection=new JSONObject(response);
                        String news=featureCollection.getString("TAG");
                        set = news;
                        Log.d("test_json", news);
                        Log.d("test", "news count:"+news.length());

                        for(int i=0;i<news.length();i++) {
                            NewsResponse newsResponse = new NewsResponse();
                            FirstProjectApplication.allNews.add(newsResponse);

                        }
                    } catch (JSONException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                    if(menu_adapter != null) {
                        menu_adapter.notifyDataSetChanged();
                    }
                }
            }, new Response.ErrorListener() {
                @Override
                public void onErrorResponse(VolleyError error) {
                    VolleyLog.d("", "Error: " + error.getMessage());
                }
            });
            FirstProjectApplication.getInstance().addToRequestQueue(strReq1, "str_req");

setTitle方法：

public void setTitle(String title,String subTitle) {
    getActionBar().setTitle(title);
    getActionBar().setSubtitle(subTitle);
}

现在在case 0进入setTitle，我设置了set变量并且显示正确，而当插入currentFragment的setTitle时，变量set未显示，但如果我用硬编码的字符串替换它，它就会显示出来。