Java Android问题，将片段更改为onPostExecute

我只是试图在执行后更改我的片段。 有以下几种情况：

            case 4:

                try {
                    String code = this.result.getString("code");
                    if (code.equals("200"))
                    {
                        final String stepString = "step";
                        setValue(stepString, "3");
                        FragmentManager fragmentManager = getFragmentManager();
                        fragmentManager.beginTransaction().replace(R.id.content_frame, new HomeFragment()).commit();
                    }
                    else if (code.equals("400"))
                    {
                        Toast.makeText(getApplicationContext(), this.result.getString("text"),
                                Toast.LENGTH_LONG).show();
                    }

                } catch (JSONException e) {
                    e.printStackTrace();
                }
            break;
        }

我还有一个名为HomeFragment的类

我有空指针错误。但是为什么呢？我有一节课

有Log.cat

10-16 12:09:10.744    4700-4700/com.example.android.navigationdrawerexample E/AndroidRuntime﹕ FATAL EXCEPTION: main
    java.lang.NullPointerException
            at com.example.android.navigationdrawerexample.MainActivity$PlanetFragment$JSONAsyncTask.onPostExecute(MainActivity.java:545)
            at com.example.android.navigationdrawerexample.MainActivity$PlanetFragment$JSONAsyncTask.onPostExecute(MainActivity.java:404)
            at android.os.AsyncTask.finish(AsyncTask.java:631)
            at android.os.AsyncTask.access$600(AsyncTask.java:177)
            at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:644)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:176)
            at android.app.ActivityThread.main(ActivityThread.java:5319)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1102)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:869)at dalvik.system.NativeStart.main(Native Method)

---

您在哪一行得到异常？显示您的404行号。此处为fragmentManager.beginTransaction.replaceR.id.content\u frame，new HomeFragment.commit；RootView应该是RootView，小写为RB，但我需要在哪里进行更改？检查此id内容框架是否正确？

10-16 12:09:10.744    4700-4700/com.example.android.navigationdrawerexample E/AndroidRuntime﹕ FATAL EXCEPTION: main
    java.lang.NullPointerException
            at com.example.android.navigationdrawerexample.MainActivity$PlanetFragment$JSONAsyncTask.onPostExecute(MainActivity.java:545)
            at com.example.android.navigationdrawerexample.MainActivity$PlanetFragment$JSONAsyncTask.onPostExecute(MainActivity.java:404)
            at android.os.AsyncTask.finish(AsyncTask.java:631)
            at android.os.AsyncTask.access$600(AsyncTask.java:177)
            at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:644)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:176)
            at android.app.ActivityThread.main(ActivityThread.java:5319)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1102)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:869)at dalvik.system.NativeStart.main(Native Method)