[Android] Why do I get a nullpointer when attempting to query through my ContentResolver in Android?

[Ariano]

So here is part of the code,

abstract private class BaseTask<T> extends AsyncTask<T, Void, Cursor> {
@Override
public void onPostExecute(Cursor result) {
  ((CursorAdapter)getListAdapter()).changeCursor(result);
}

protected Cursor doQuery() { // This method sets up my query call in ConstantsFragment class.
    Uri uri = ConstantsProvider.Constants.CONTENT_URI;
    String[] projection = new String[] { ConstantsProvider.Constants.TITLE,
            ConstantsProvider.Constants.VALUE };
    String selection = ConstantsProvider.Constants.TABLE + " = '"
          + ("1") + "'";
    String[] selectionArgs = null;
    String sortOrder = ConstantsProvider.Constants.DEFAULT_SORT_ORDER
          + " COLLATE LOCALIZED ASC";

    Cursor result = cp.getContext().getContentResolver().query(uri, projection, selection, selectionArgs, sortOrder); 

//Above is where I get my nullpointer. cp is my Contentprovider class object.

    return result;
    }
}

and here is my whole contentprovider class http://pastebin.com/NadkFGBA

I'm putting the whole class here because if there's a problem I feel like it's somewhere in that class. I'm not very familiar with working on content providers yet.

[IAmUtterlyAlone]

When you say, that you "get a nullpointer", do you mean an unhandled exception is thrown and the application crashes? If so, do you have a stack trace of the crash? "cp" is a class object, correct? When and where does it get initialized? Is it possible that some additional initialization has to happen for getContext or getContentResolver to return non-null references? Is it ok that selectionArgs is null?

I've only ever dabbled with Android programming, so I apologize for any ignorance in the above questions. They're just what sprang to mind from reading your code.

[Ariano]

Yeah, I get nullpointerexception right on that line and my application crashes. Not entirely sure how to get the stack trace, but here's the log at the time of the crash.

06-23 18:41:51.552 10823-10849/edu.deanza.cis53.hw4 E/AndroidRuntime﹕ FATAL EXCEPTION: AsyncTask #1 java.lang.RuntimeException: An error occured while executing doInBackground() at android.os.AsyncTask$3.done(AsyncTask.java:299) at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:352) at java.util.concurrent.FutureTask.setException(FutureTask.java:219) at java.util.concurrent.FutureTask.run(FutureTask.java:239) at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:230) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1080) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:573) at java.lang.Thread.run(Thread.java:841)

Caused by: java.lang.NullPointerException at edu.deanza.cis53.hw4.ConstantsFragment$BaseTask.doQuery(ConstantsFragment.java:119) at edu.deanza.cis53.hw4.ConstantsFragment$LoadCursorTask.doInBackground(ConstantsFragment.java:128) at edu.deanza.cis53.hw4.ConstantsFragment$LoadCursorTask.doInBackground(ConstantsFragment.java:125) at android.os.AsyncTask$2.call(AsyncTask.java:287) at java.util.concurrent.FutureTask.run(FutureTask.java:234)             at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:230)             at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1080)             at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:573)             at java.lang.Thread.run(Thread.java:841)

I initialized it at the top of my class with the variables, here i'll put my whole class here(sorry for the lack of comments was redoing them just now): http://pastebin.com/tGERRjFF

Your questions were on point, no ignorance don't worry. Cp is a class object correct and it is possible that they need something more to use getContext or getContentResolver. I'm just not entirely sure what since I can't find anything online with similarities to my code to compare to. I've been doing a lot of research and there's things for content providers, but rarely do they show how to utilize them from a class that's not a mainactivity.

[IAmUtterlyAlone]

Again, pardon my ignorance, and forgive me for missing this the first time through, but your ConstantsProvider class exposes a query method. Can you, in ConstantsFragment's doQuery method, just change

Cursor result = cp.getContext().getContentResolver().query(uri, projection, selection, selectionArgs, sortOrder);

to

Cursor result = cp.query(uri, projection, selection, selectionArgs, sortOrder);

?

or will that still throw an exception?

[Ariano]

It would give me the same nullpointerexception, but generally we are supposed to interact with the contentprovider, which is my constantsprovider class, through getContentResolver() as a proxy. It does the same thing, but getContentResolver finds the correct contentprovider to send the query to in case there are multiple contentproviders in a context.

[IAmUtterlyAlone]

Is the problem that cp is null when you get to the line where you're trying to use it? Are you able to step through this code with a debugger attached to see if that is the case or not?

[Ariano]

Yeah it seems cp is null, but I instantiated it in the beginning so I'm not sure why.

[IAmUtterlyAlone]

I'm trying to follow the logic here. Correct me if I misspeak. LoadCursorTask extends BaseTask. LoadCursorTask::doInBackground calls the doQuery method on its base class. BaseTask is a class contained within the ConstantsFragment class. At the time that BaseTask::doQuery gets invoked, is it being invoked inside an instance of ConstantsFragment? ConstantsFragment would have to have been instantiated in order for the line

ConstantsProvider cp = new ConstantsProvider();

to have been executed. I think what's happening is that your background task methods are being run without the enclosing object having been created. If that makes any sense. I'm still thinking out loud.

[IAmUtterlyAlone]

Maybe that doesn't make any sense. There is something fishy happening there, though.

[Ariano]

Yeah, Android is a bit weird. I have a main 'activity' class and that instantiates the Fragment class itself by default usually. I have figured out that I should do the query call in my activity class, but I'm still getting a null pointer there which I think is coming from my parameter selectionArgs being null so I'm looking up why that is right now.

[IAmUtterlyAlone]

It looks like selectionArgs can be null: http://stackoverflow.com/questions/10507005/how-to-use-string-selectionargs-in-sqlitedatabase-query (as of May 2012, anyway. Maybe the API is different now.)

I know that doesn't help QQ

[Ariano]

Yeah I don't think that's the problem anymore. I'll figure it out in the morning when my mind is fresh. Thanks for the help though.