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java - Json 反序列化后的 NullPointerExcetion

问题描述

我有一个 Android 应用程序，它在用户单击按钮时保存一些数据，这些数据被序列化为 JSON，然后通过 putString 保存到 SharedPreferences 中。这些数据可以通过 Json 从另一个活动中检索de-serialization。日志告诉我这样serialization做是正确的，但是当我调用返回反序列化 Json 对象的方法时，我得到了一个NullPointerException. 我的对象（HistoryItem）有 2 个属性：一个日期和一个心情。（一个可以悲伤、快乐等的枚举）以及相应的 getter 和 setter

我已经把 Log.e 放在Serialization, de-serialization, 之前和之后，甚至在我的第二个活动中，就在创建异常的方法之前，他们告诉我一切都很好，我肯定没有找对地方......

   public void savemood2(Date date, HistoryItem historyItem) {
    Gson gson = new Gson();
    String jsonHistoryItem;
    // HistoryItem --> Json String
    if (historyItem.getComment() != null || historyItem.getComment() != PREF_KEY_EMPTY_COMMENT) {
        jsonHistoryItem = gson.toJson(historyItem, HistoryItem.class);
    }
    //If there's no comment, save PREF_KEY_EMPTY_COMMENT as comment
    else {
        historyItem.setComment(PREF_KEY_EMPTY_COMMENT);
        jsonHistoryItem = gson.toJson(historyItem, HistoryItem.class);
    }

then 方法来获取我的 HistoryItem 对象

 public HistoryItem getMood2(Date date) {
    Gson gson = new Gson();
    String json = mPreferences.getString(getMoodDate(date), null);
    historyItem = gson.fromJson(json, HistoryItem.class);
    Log.e("TAGOUTPUT", json);
    return historyItem;
}

例外 ：

   Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'com.matt.android.moodtracker_v2.models.MoodEnum com.matt.android.moodtracker_v2.models.HistoryItem.getMood()' on a null object reference
    at com.matt.android.moodtracker_v2.controllers.MoodHistoryActivity.displayMood(MoodHistoryActivity.java:116)
    at com.matt.android.moodtracker_v2.controllers.MoodHistoryActivity.onCreate(MoodHistoryActivity.java:96)

显示情绪（java：116）：

  if (todayHistoryItem.getMood() == null) {
        relativeLayout.setBackgroundColor(0);
    }  else {
        // Set background color and fraction for each mood case
        switch (todayHistoryItem.getMood()) {
            case Sad:
                relativeLayout.setLayoutParams(new LinearLayout.LayoutParams(width / 5,
                        LinearLayout.LayoutParams.MATCH_PARENT, 1));
                relativeLayout.setBackgroundColor(getResources().getColor(R.color.faded_red));
                break;
            case Disappointed: etc....

我在这里调用 displayMood (java :96)：

  // Loop to display last 7 moods
    Calendar calendar = Calendar.getInstance();
    for (int i = 0; i < 7; i++) {
        calendar.add(Calendar.DAY_OF_WEEK, -1); //Subtract one day from calendar (yesterday)
        this.displayMood(calendar.getTime(), layouts[i]);
        this.displayComment(calendar.getTime(), buttons[i]);
    }

todayHistoryItem.getMood()应该返回一个心情。 在MainActivitysaveMood2()中调用。OnLickListener

编辑：完整的错误堆栈：

 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.matt.android.moodtracker_v2/com.matt.android.moodtracker_v2.controllers.MoodHistoryActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'com.matt.android.moodtracker_v2.models.MoodEnum com.matt.android.moodtracker_v2.models.HistoryItem.getMood()' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2721)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2782)
    at android.app.ActivityThread.-wrap12(ActivityThread.java)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1521)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:163)
    at android.app.ActivityThread.main(ActivityThread.java:6228)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:904)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:794)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'com.matt.android.moodtracker_v2.models.MoodEnum com.matt.android.moodtracker_v2.models.HistoryItem.getMood()' on a null object reference
    at com.matt.android.moodtracker_v2.controllers.MoodHistoryActivity.displayMood(MoodHistoryActivity.java:116)
    at com.matt.android.moodtracker_v2.controllers.MoodHistoryActivity.onCreate(MoodHistoryActivity.java:96)
    at android.app.Activity.performCreate(Activity.java:6860)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2674)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2782) 
    at android.app.ActivityThread.-wrap12(ActivityThread.java) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1521) 
    at android.os.Handler.dispatchMessage(Handler.java:102) 
    at android.os.Looper.loop(Looper.java:163) 
    at android.app.ActivityThread.main(ActivityThread.java:6228) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:904) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:794) 

标签： javaandroidjsonnullpointerexceptiongson

解决方案

看着你的错误

原因：java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法 'com.matt.android.moodtracker_v2.models.MoodEnum com.matt.android.moodtracker_v2.models.HistoryItem.getMood()'

看起来问题在于您正在调用.getMood()一个null对象。这意味着您的todayHistoryItem为空。

您可能应该将代码编辑为：

//add todayHistoryItem == null check
if (todayHistoryItem == null || todayHistoryItem.getMood() == null) {
        relativeLayout.setBackgroundColor(0);
    }  else {
        // Set background color and fraction for each mood case
        switch (todayHistoryItem.getMood()) {
            case Sad:
                relativeLayout.setLayoutParams(new LinearLayout.LayoutParams(width / 5,
                        LinearLayout.LayoutParams.MATCH_PARENT, 1));
                relativeLayout.setBackgroundColor(getResources().getColor(R.color.faded_red));
                break;
        case Disappointed: etc....

另一点

我在您的代码中没有看到您将您保存serialized object到mPreferences对象中的任何地方。

也许你忘记了？

让我知道它是否能解决您的问题！

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