Android - null object reference when iterating List

Question

I'm trying to copy a List 'usrs' which is created in an Inner class to a different list 'team_memebers'. After copying I try to iterate 'team_memebers' in the FOR loop, but I get a 'null object reference' error. The 'users' list contains the returned objects, tested via debug prints.

public class ListNodeActivity extends AppCompatActivity
{
    private ParseObject parse_task=null;
    private List<String> team_memebers=null;

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_list_node);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

    query.findInBackground(new FindCallback<ParseObject>() {
        public void done(List<ParseObject> usrs, ParseException e) {
            if (e == null) {

                team_memebers = new ArrayList<String>(usrs.size());
                for (ParseObject prso:usrs) {
                    team_memebers.add(new String(prso.getString("Username")));
                }
            } else {//handle the error
            }
        }
    });

    for (String str:team_memebers)
    {
        empolyeeSpinnerAdapter.add(str);
    }
}

Stack trace

 FATAL EXCEPTION: main
 Process: il.ac.shenkar.david.todolistex2, PID: 14490
 java.lang.RuntimeException: Unable to start activity ComponentInfo{il.ac.shenkar.david.todolistex2/il.ac.shenkar.david.todolistex2.ListNodeActivity}: java.lang.NullPointerException: Attempt to invoke interface method 'java.util.Iterator java.util.List.iterator()' on a null object reference
     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
     at android.app.ActivityThread.-wrap11(ActivityThread.java)
     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
     at android.os.Handler.dispatchMessage(Handler.java:102)
     at android.os.Looper.loop(Looper.java:148)
     at android.app.ActivityThread.main(ActivityThread.java:5417)
     at java.lang.reflect.Method.invoke(Native Method)
     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
  Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'java.util.Iterator java.util.List.iterator()' on a null object reference
     at il.ac.shenkar.david.todolistex2.ListNodeActivity.onCreate(ListNodeActivity.java:116)
     at android.app.Activity.performCreate(Activity.java:6251)
     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
     at android.app.ActivityThread.-wrap11(ActivityThread.java) 
     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
     at android.os.Handler.dispatchMessage(Handler.java:102) 
     at android.os.Looper.loop(Looper.java:148) 
     at android.app.ActivityThread.main(ActivityThread.java:5417) 
     at java.lang.reflect.Method.invoke(Native Method) 
     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

Have you tried a normal for loop with counter instead of a foreach for loop? Something like ``for (int i = 0; i< team_memebers.size(); i++) { employeeSpinnerAdapter.add(team_memebers.get(i); }`` — Angelos Chalaris, Mar 02 '16 at 20:10
@chalarangelo, Yes, it fails on .size(), as it appears team_members is null. — David Faizulaev, Mar 02 '16 at 21:24
You're statement is that the NPE is on the for(ParseObject prso: users)? The way this code is written it should probably NPE on the for(String str: teammembers) as I don't see you waiting for the query.findInBackground() call to finish before doing that interation. Can you highlight what line the NPE actually occurs on? I can't tell which it is based on the stack as you have two for(iter) loops. — Nathaniel D. Waggoner, Mar 03 '16 at 23:03
I get the error in the '(String str : team_memebers)' loop. I tried placing it in the 'query.findInBackground()' call block, but I get a message - "variable input is accessed within inner class; needs to be declared final" — David Faizulaev, Mar 04 '16 at 08:31
@NathanielD.Waggoner, to correct my previous comment, I get the error message regarding the '...needs to be declared final' on 'empolyeeSpinnerAdapter'. — David Faizulaev, Mar 04 '16 at 09:00

score 5 · Accepted Answer · answered Mar 03 '16 at 23:12

5

Your problem is that you aren't iterating after copying list. query.findInBackground() is an async callback and therefore, isn't executed immediately. Because your iteration loop is PLACED below that callback, doesn't mean it will be executed after callback executes. Just put your loop inside callback like this:

query.findInBackground(new FindCallback<ParseObject>() {
        public void done(List<ParseObject> usrs, ParseException e) {
            if (e == null) {

            team_memebers = new ArrayList<String>(usrs.size());
            for (ParseObject prso:usrs) {
                team_memebers.add(new String(prso.getString("Username")));
            }
            for (String str:team_memebers)
            {
                empolyeeSpinnerAdapter.add(str);
            }
        } else {//handle the error
        }
    }
});

answered Mar 03 '16 at 23:12

Mirza Brunjadze

544
4
10

I tried placing it in the 'query.findInBackground()' call block, but I get a message - "variable input is accessed within inner class; needs to be declared final" – David Faizulaev Mar 04 '16 at 08:32
On which variable do you get this message? `str` or `team_memebers`? – Mirza Brunjadze Mar 04 '16 at 08:46
Anyways, I suggest you to not assign `null` to `team_memebers`, instead declare it final and initialise with empty list: `private final List team_memebers = new ArrayList<>();` and remove this line: `team_memebers = new ArrayList(usrs.size());` also declare `str` as final in your loop: `for (final String str:team_memebers)` – Mirza Brunjadze Mar 04 '16 at 08:53
On 'empolyeeSpinnerAdapter' – David Faizulaev Mar 04 '16 at 08:59
Then declare `str` as final as I said. – Mirza Brunjadze Mar 04 '16 at 09:09
Resolved it by moving the creation of 'empolyeeSpinnerAdapter' out of the 'create' method. – David Faizulaev Mar 04 '16 at 09:11

Android - null object reference when iterating List

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