如何更好地理解错误以进行修复

Question

I'm receiving the following error message in LogCat:

   java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference

I'm aware of what a NullPointerException is but not 100% on how to fix this with regards to passing the correct context. The error only happens when the app is runnning in the background (multitasking) A little guidance would be much appreciated. please Logcat and offending code below. THanks

Logcat:

Process: com.app.app, PID: 17519
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
    at android.preference.PreferenceManager.getDefaultSharedPreferencesName(PreferenceManager.java:537)
    at android.preference.PreferenceManager.getDefaultSharedPreferences(PreferenceManager.java:526)
    at com.app.app.DatabaseHandling.UpdateData.<init>(UpdateData.java:70)
    at com.app.app.PushService.PushReceiver$1.run(PushReceiver.java:94)
    at java.lang.Thread.run(Thread.java:764)

PushReceiver

UpdateData updateData = new UpdateData(MainActivity.mainActivity);

UpdateData:

 private final SharedPreferences sp = PreferenceManager.getDefaultSharedPreferences(MainActivity.mainActivity);

Answer 1

most likely (based upon the limited example provided, which does not even indicate, within which context that code runs), it should rather be:

SharedPreferences sp = PreferenceManager.getDefaultSharedPreferences(MainActivity.this);

because one cannot just make up static fields, which do not exist -

while assigning Context classes (alike Activity or Context ) to static fields is generally bad practice... and should be avoided, whenever just possible.

Answer 2

I assume the problem is, that you try to initialise the SharedPreferences sp at the location, where you define it. You should define it first like:

private SharedPreferences sp;

After that, set this global variable sp in a function like "onReceive(Context context)":

sp = PreferenceManager.getDefaultSharedPreferences(context);

Or like already mentioned in the Activity itself in the function "onCreate(...)":

sp = PreferenceManager.getDefaultSharedPreferences(MainActivity.this);

The problem might be, that you try to initialize a variable with a context (like activity), that is not available at this point, but in a later step of the lifecycle.

And avoid to hand over a context supplied by a static variable from another class.

Answer 3

First you shuld check your code. you are passing MainActivity.mainAcitvity instead of MainACtivity.this or getApplicationContext()

private final SharedPreferences sp = PreferenceManager.getDefaultSharedPreferences(MainActivity.mainActivity);

always pass context of activity in which your method works ...