Android 每当试图保存SharedReference时，应用程序就会崩溃

我有一个应用程序，在我按下两个按钮中的一个之前运行良好，这两个按钮都将我当前的自定义类保存为JSON文件，其中存储了四个

ArrayList

下面是我在SharedReferences中加载和保存JSON文件的代码：

@Override
protected void onResume() {
    super.onResume();
    SharedPreferences myPrefs = getPreferences(MODE_PRIVATE);
    Gson gson = new Gson();
    String json = myPrefs.getString("MyObject", "");
    StudentArray obj = gson.fromJson(json, StudentArray.class);

    studentsFirstNames = obj.studentFirstName;
    studentsLastNames = obj.studentLastName;
    studentsCourses = obj.studentCourse;
    studentsYearsOfStudy = obj.studentYearOfStudy;
}

@Override
protected void onPause() {
    super.onPause();
    SharedPreferences myPrefs = getPreferences(MODE_PRIVATE);
    Editor prefsEditor = myPrefs.edit();
    Gson gson = new Gson();
    StudentArray example = new StudentArray(studentsFirstNames, studentsLastNames, studentsCourses, studentsYearsOfStudy);
    String json = gson.toJson(example);
    prefsEditor.putString("MyObject", json);
    prefsEditor.commit();
}       

Logcat碰撞日志

04-26 21:58:38.985 14971-14971/com.adam_smedley.moduleenrollment E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.adam_smedley.moduleenrollment, PID: 14971
java.lang.RuntimeException: Failure delivering result ResultInfo{who=null, request=0, result=0, data=null} to activity {com.adam_smedley.moduleenrollment/com.adam_smedley.moduleenrollment.MainActivity}: java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
    at android.app.ActivityThread.deliverResults(ActivityThread.java:4089)
    at android.app.ActivityThread.handleSendResult(ActivityThread.java:4132)
    at android.app.ActivityThread.-wrap20(ActivityThread.java)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1533)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:154)
    at android.app.ActivityThread.main(ActivityThread.java:6119)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
 Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
    at android.widget.ArrayAdapter.getCount(ArrayAdapter.java:344)
    at android.widget.ListView.setAdapter(ListView.java:493)
    at com.adam_smedley.moduleenrollment.MainActivity.onActivityResult(MainActivity.java:89)
    at android.app.Activity.dispatchActivityResult(Activity.java:6932)
    at android.app.ActivityThread.deliverResults(ActivityThread.java:4085)
    at android.app.ActivityThread.handleSendResult(ActivityThread.java:4132) 
    at android.app.ActivityThread.-wrap20(ActivityThread.java) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1533) 
    at android.os.Handler.dispatchMessage(Handler.java:102) 
    at android.os.Looper.loop(Looper.java:154) 
    at android.app.ActivityThread.main(ActivityThread.java:6119) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776) 

好的，看来我的ArrayList是空对象引用，因为我无法向它们添加其他字符串或get.size（）

有人看到下面的声明有什么问题吗

static ArrayList<String> studentsFirstNames;
studentsFirstNames = new ArrayList<String>();
studentsFirstNames.add("John");

它应该访问主活动中的静态ArrayList studentsFirstNames，并向ArrayList添加一个字符串firstName

onActivityResult代码

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);

    ListAdapter enrollmentAdapter = new CustomAdapter(this, studentsFirstNames, studentsLastNames, studentsCourses, studentsYearsOfStudy);
    ListView studentsListView = (ListView) findViewById(R.id.StudentsListView);
    studentsListView.setAdapter(enrollmentAdapter);
}

---

这肯定是由上面的onResume（）和onPause（）代码引起的，因为我已经注释掉了它，我的应用程序可以工作，但它没有保存任何内容，所以我没有任何数据持久性

在你的

onResume

方法中，你很可能得到无效的

StudentArray

。因此，在使用gson获取对象时，需要检查null或异常。大概是这样的：

@Override
protected void onResume() {
  super.onResume();
  SharedPreferences myPrefs = getPreferences(MODE_PRIVATE);
  Gson gson = new Gson();
  String json = myPrefs.getString("MyObject", "");
  try {
    StudentArray obj = gson.fromJson(json, StudentArray.class);
    if (obj != null) {
      studentsFirstNames = obj.studentFirstName;
      studentsLastNames = obj.studentLastName;
      studentsCourses = obj.studentCourse;
      studentsYearsOfStudy = obj.studentYearOfStudy;
    } else {
      // obj is null. You need to do something here.
    }
  } catch (IllegalStateException | JsonSyntaxException exception) {
    // there is something wrong when parsing the json
    // do something
  }
}

---

如果您不介意的话，您可以只在完成rollment和saveEnrollment时发布所涉及的代码。只需编辑问题。否则，您可能永远得不到任何帮助。您的inActivityResult代码在哪里，这就是崩溃发生的地方，您可以共享它吗？@MidasLefko onActivityResult代码？您不应该从其他活动访问

studentsFirstNames

。您需要在

onActivityResult

中处理

add

AddNewStudent

activity不知道主活动在做什么。根据堆栈跟踪，以下列表之一在onActivityResult中为空：studentsFirstNames、studentsLastNames、studentsScorses、studentsYearsOfStudy。你应该进行调试，找出我认为是哪种情况，以及为什么会是这种情况。在Logcat中，我在尝试获取null对象引用上的“int java.util.List.size（）”时遇到了一个致命的异常。我自己没有任何.size（）调用，所以当其他活动完成时，onResume函数是否完成了某些操作？

@Override
protected void onResume() {
  super.onResume();
  SharedPreferences myPrefs = getPreferences(MODE_PRIVATE);
  Gson gson = new Gson();
  String json = myPrefs.getString("MyObject", "");
  try {
    StudentArray obj = gson.fromJson(json, StudentArray.class);
    if (obj != null) {
      studentsFirstNames = obj.studentFirstName;
      studentsLastNames = obj.studentLastName;
      studentsCourses = obj.studentCourse;
      studentsYearsOfStudy = obj.studentYearOfStudy;
    } else {
      // obj is null. You need to do something here.
    }
  } catch (IllegalStateException | JsonSyntaxException exception) {
    // there is something wrong when parsing the json
    // do something
  }
}