尝试在Realm中删除对象时，为什么会出现ArrayIndexOutOfBoundsException？

Question

There is such case, I need delete all objects from Realm and leave only last 10.

There is my method

private void invalidate() {
    final int AVAILABLE_SIZE = 10;
    final RealmResults<CardMainActivity> listToInvalidate = realmDB.where(CardMainActivity.class).findAll();
    final int sizeOfList = listToInvalidate.size();

    if (sizeOfList > AVAILABLE_SIZE) {
        realmDB.executeTransaction(new Realm.Transaction() {
            @Override
            public void execute(Realm realm) {
                for (int i = 0; i < sizeOfList - AVAILABLE_SIZE; i++) {
                    listToInvalidate.deleteFromRealm(i);
                }
            }
        });
    }
}

and there is error that I get

FATAL EXCEPTION: main
                                                                         Process: com.fittingroom.newtimezone, PID: 2438
                                                                         java.lang.RuntimeException: Unable to start activity ComponentInfo{com.fittingroom.newtimezone/com.fittingroom.newtimezone.view. activityMainNew.ActivityMainnew}: java.lang.ArrayIndexOutOfBoundsException:  rowIndex > available rows: 12 > 12
                                                                              at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3253)
                                                                           at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3349)
                                                                           at android.app.ActivityThread.access$1100(ActivityThread.java:221)
                                                                           at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1794)
                                                                           at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                           at android.os.Looper.loop(Looper.java:158)
                                                                           at android.app.ActivityThread.main(ActivityThread.java:7224)
                                                                           at java.lang.reflect.Method.invoke(Native Method)
                                                                           at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
                                                                           at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)
                                                                        Caused by: java.lang.ArrayIndexOutOfBoundsException: rowIndex > available rows: 12 > 12
                                                                           at io.realm.internal.TableView.nativeRemoveRow(Native Method)
                                                                           at io.realm.internal.TableView.remove(TableView.java:410)
                                                                           at io.realm.RealmResults.deleteFromRealm(RealmResults.java:295)
                                                                           at com.fittingroom.newtimezone.tools.realmdb.RealmDB$1.execute(RealmDB.java:49)
                                                                           at io.realm.Realm.executeTransaction(Realm.java:1304)
                                                                           at com.fittingroom.newtimezone.tools.realmdb.RealmDB.invalidate(RealmDB.java:43)
                                                                           at com.fittingroom.newtimezone.tools.realmdb.RealmDB.<init>(RealmDB.java:23)
                                                                           at com.fittingroom.newtimezone.tools.realmdb.RealmDB.getInstance(RealmDB.java:28)
                                                                           at com.fittingroom.newtimezone.view.activityMainNew.ControllerActivityMainnew.isDBContainsData(ControllerActivityMainnew.java:55)

But if I try delete my objects such way

private void invalidate() {
    final int AVAILABLE_SIZE = 10;
    final RealmResults<CardMainActivity> listToInvalidate = realmDB.where(CardMainActivity.class).findAll();
    final int sizeOfList = listToInvalidate.size();

    if (sizeOfList > AVAILABLE_SIZE) {
        realmDB.executeTransaction(new Realm.Transaction() {
            @Override
            public void execute(Realm realm) {
                for (int i = 0; i < sizeOfList - AVAILABLE_SIZE; i++) {
                    CardMainActivity card = listToInvalidate.get(i);
                    card.deleteFromRealm();

                }
            }
        });
    }
}

all works nice. But I need to use first approach listToInvalidate.deleteFromRealm(i); , so

What am I doing wrong?

Answer 1

RealmResults.deleteFromRealm() "also removes it from the collection." So you are deleting 0, then the original 1 shifts into 0, then you delete index 1 with the incremented i. Let's try it with a list of size 8 and keeping 3:

 0 1 2 3 4 5 6 7, delete i = 0, continue if i < 8-3
 1 2 3 4 5 6 7, delete i = 1, continue if i < 8-3 
 1 3 4 5 6 7, delete i = 2, continue if i < 8-3
 1 3 5 6 7, delete i = 3, continue if i < 8-3
 1 3 5 7, delete i = 4, Exception, there is no element 4

What you could do is delete index 0 each time. Your second sample does not delete it from the RealmResults collection so you don't have this problem.

Answer 2

I would suspect that when you are deleting from the list that you are looping through, the size of that list is shrinking on each iteration. So, your loop quickly gets to a point where the index is out of bounds. (John C does a great job of explaining this in his answer)

As for a solution, I would suggest looping through and 'marking' entries for invalidation, and THEN going through and removing the marked items. This way, you don't have to rely on the index (which is changing as you invalidate items).

Let me know if this makes sense, and I'll try to clarify what I mean if it does not.