Accessing Global Variables in Nested Class

Question

I'm having an embarrassing problem. For the life of me, I can't figure out why my code isn't working.

private void getParseObject(String title) {
    final String parseTitle = title;
    ParseQuery<ParseObject> query = ParseQuery.getQuery(title);
    query.findInBackground(new FindCallback<ParseObject>() {
        public void done(List<ParseObject> objects, ParseException e) {
            if (e == null) {
                if (objects.isEmpty()){
                    createParse(parseTitle);
                } else {
                    ParseObject event = objects.get(0);
                    int likes = event.getInt("likes");
                    JSONArray comments = event.getJSONArray("comments");
                    setParseInfo(likes, comments);
                }
             } else {
                Log.v("Miles", String.valueOf(e.getCode()));
             }
        }
    }); 
    Log.v("Miles", "LIKES FROM GOT " + likes);
}

public void setParseInfo(int likesFromParse, JSONArray commentsFromParse) {
    this.likes = likesFromParse;
    this.comments = commentsFromParse;
    Log.v("Miles", "LIKES FROM SET " + likes);
}

Within setParseInfo(int, JSONArray), I'm setting the global variable of my fragment. I can get the info fine, it's not null; In that log "LIKES FROM SET", the "likes" int appears more than normally. However, when I try to do the same within getParseObject() in that "LIKES FROM GOT" log, the "likes" int show up as 0. Any ideas on how I can fix this?

EDIT 1:

With the advice of Ryan J, I've tried doing something like this, and it has the same effect.

private void getParseObject(String title) {
    final String parseTitle = title;
    ParseQuery<ParseObject> query = ParseQuery.getQuery(title);
    query.findInBackground(new FindCallback<ParseObject>() {
        int nestedLikes = likes;
        JSONArray nestedComments = comments;

        public void done(List<ParseObject> objects, ParseException e) {
            if (e == null) {
                if (objects.isEmpty()){
                    createParse(parseTitle);
                } else {
                    ParseObject event = objects.get(0);
                    nestedLikes = event.getInt("likes");
                    nestedComments = event.getJSONArray("comments");
                    setParseInfo(nestedLikes, nestedComments);
                }
             } else {
                Log.v("Miles", String.valueOf(e.getCode()));
             }
        }
    }); 
    Log.v("Miles", "LIKES FROM GOT " + likes);
}

Which is what I think the suggestion was. However, the log "LIKES FROM GOT" still returns 0.

Set a global variable inside your nested function to the value of likes. It's not appearing as the expected value because once your inner function terminates, the variable goes out of scope. — Ryan J
– Ryan J, Commented Jul 20, 2014 at 4:12
You mean inside the findInBackground() function? Because I've tried that method to no avail. As in, I initialize an empty global variable right under findInBackground, set that to event.getInt("likes"), then pass that to setParseInfo(). It has the same effect; the "likes" int is still 0. — Miles Peele
– Miles Peele, Commented Jul 20, 2014 at 4:17
I mean inside your anonymous FindCallback class (the done method). Have you verified the proper value is being output in that class via the debugger or printing to console? Create a global variable in your top-level class, and set it to the value of likes in your anonymous class. That hasn't worked for you? — Ryan J
– Ryan J, Commented Jul 20, 2014 at 4:19
Yeah, as far as I can tell, that method doesn't work. Maybe I'm just not implementing it correctly... And yes, I have verified that. The event.getInt("likes") gets the correct value. The problem is getting that value to be assigned to the fragment's top-level global variable. — Miles Peele
– Miles Peele, Commented Jul 20, 2014 at 4:21
Maybe you can post more code where you've tried that method to no avail, with the corresponding output and we can be of further help. — Ryan J
– Ryan J, Commented Jul 20, 2014 at 4:22

Adam Gawne-Cain · Accepted Answer · 2014-07-20 05:39:18Z

1

You need to make the main thread wait for the background thread to complete.

Here is a simple way to make the main thread wait with a CountDownLatch:

import java.util.concurrent.CountDownLatch;

public class PassOutResult {
    public static void main(String[] args) {
        new PassOutResult().doMainThread();
    }

    void doMainThread() {
        try {
            final CountDownLatch latch = new CountDownLatch(1);
            Thread background = new Thread() {
                public void run() {
                    int r = 2;
                    setResult(r);
                    System.out.println("BackgroundThread: r=" + r);
                    latch.countDown();
                }
            };
            background.start();
            System.out.println("MainThread A: theResult=" + theResult);
            latch.await();
            System.out.println("MainThread B: theResult=" + theResult);
        } catch (InterruptedException ex) {
            System.err.println(ex);
        }
    }
    void setResult(int r) {
        theResult = r;
    }
    int theResult;
}

The above technique will work in Android Java, as well as Oracle Java (JRE).

By the way, if several threads are accessing a single variable (e.g. "theResult" in my sample code), then you should synchronize access to it. In my sample code the CoundDownLatch effectively synchronizes access. But in a large program where you cannot trust your knowledge of what threads are doing, it is usually simpler to wrap the variable in getter/setter methods, and make those methods synchronised.

synchronized int getResult() {
   return theResult;
}
synchronized void setResult(int r) {
   theResult=r;
}

Here is how to use CountDownLatch in the original code:

    private void getParseObject(String title) {
        final String parseTitle = title;
        final CountDownLatch latch = new CountDownLatch(1);
        ParseQuery<ParseObject> query = ParseQuery.getQuery(title);
        query.findInBackground(new FindCallback<ParseObject>() {
            public void done(List<ParseObject> objects, ParseException e) {
                if (e == null) {
                    if (objects.isEmpty()){
                        createParse(parseTitle);
                    } else {
                        ParseObject event = objects.get(0);
                        int likes = event.getInt("likes");
                        JSONArray comments = event.getJSONArray("comments");
                        setParseInfo(likes, comments);
                    }
                 } else {
                    Log.v("Miles", String.valueOf(e.getCode()));
                 }
                latch.countDown();
            }
        }); 
        latch.await();
        Log.v("Miles", "LIKES FROM GOT " + likes);
    }

edited Jul 20, 2014 at 5:39

answered Jul 20, 2014 at 4:57

Adam Gawne-Cain

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4 Comments

Miles Peele Over a year ago

Your post is very confusing, as I am unused to working with threads. I'm using Parse API, so the findinBackground() automatically runs in background in its own thread; how am I supposed to wait for it if I have not directly created its own thread?

Adam Gawne-Cain Over a year ago

Miles - I answered your question with a more general program (Android or Oracle JRE) to help other users with the same question. I have edited my answer to show using CountDownLatch in your original code too. Please vote my answer up if it works.

Miles Peele Over a year ago

Much appreciated, @Adam Gawne-Cain.

Miles Peele Over a year ago

The only problem is that calling latch.await() blocks the UI thread indefinitely.

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