我有一个Android应用程序，当用户单击按钮时，该应用程序会保存一些数据，这些数据会序列化为JSON，然后通过putString保存到SharedPreferences中。
这些数据可通过Json de-serialization从另一个活动中检索。日志告诉我serialization正常运行，但是当我调用返回反序列化Json对象的方法时，得到了NullPointerException。
我的对象（HistoryItem）具有2个属性：日期和心情。（可能会感到悲伤，快乐等的枚举）带有相应的获取器和设置器
我将Log.e放在Serialization，de-serialization之前和之后，甚至在我创建异常的方法之前的第二个活动中，它们告诉我一切都很好，我肯定不在意正确的地方...
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);
}
然后方法来获取我的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;
}
例外情况：
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)
displayMood（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....
我在这里叫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()应该返回心情。saveMood2()在MainActivity的OnLickListener处调用。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)
最佳答案
看着你的错误
原因：java.lang.NullPointerException：尝试调用
虚拟方法'com.matt.android.moodtracker_v2.models.MoodEnum
com.matt.android.moodtracker_v2.models.HistoryItem.getMood（）'
空对象引用
看起来问题在于您在.getMood()对象上调用null。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
我看不到代码中将
serialized object保存到mPreferences对象中的任何地方。也许你忘了吗？
让我知道它是否可以解决您的问题！