LinkedHashMap entrySet's order not being preserved in a stream (Android)

Question

I'm creating a very simple form validation utility for a sign up screen, and I'm running into some unexpected behavior concerning LinkedHashMap and a stream created from its entrySet.

I'm storing validation results in a LinkedHashMap, with the following ordering of statements:

Map<ValidationResult.SignUpField, Boolean> fieldStatuses = new LinkedHashMap<>();

fieldStatuses.put(EMAIL, isValidEmail(emailAddress));
fieldStatuses.put(USERNAME, isValidUsername(username));
fieldStatuses.put(BIRTHDAY, isValidBirthday(birthday));
fieldStatuses.put(PASSWORD, isValidPassword(password));
fieldStatuses.put(CONFIRM_PASSWORD, password.equals(confirmedPassword));

List<ValidationEntry> invalidFields = aggregateInvalidFields(fieldStatuses);

One particular iteration yields all of the above fields invalid except for "confirm password". Using a simple for loop over the entry set and omitting valid results, the invalid results appear in the following order:

Email
Username
Birthday
Password

I then attempted to make use of the subset of Stream API's available on Android (targeting version 25 with a min of 19, hence the lack of Collectors.toMap()):

private static List<ValidationEntry> aggregateInvalidFields(Map<ValidationResult.SignUpField, Boolean> fields) {
    List<ValidationEntry> invalidFields = new ArrayList<>();
    fields.entrySet()
            .stream()
            .filter(entry -> !entry.getValue())
            .forEachOrdered(entry -> {
                ValidationResult.SignUpField field = entry.getKey();
                invalidFields.add(new ValidationEntry(field, lookUpErrorCode(field)));
            });
    return invalidFields;
}

But that code yields the following order:

Birthday
Password
Username
Email

What exactly is happening here, why is the result of the stream not honoring the insertion order of the LinkedHashMap? Note that if I swap out forEachOrdered with forEach, it's still not insertion ordered.

I'm using the subset of Java Stream API's available on Android. — Chris, Mar 10 '17 at 06:47
Yeah, I'll just add the rest of the method body and the call to that method. — Chris, Mar 10 '17 at 06:59
Weird. I tried if I could reproduce the problem with a `Map`, but my list comes out with the insertion order, no matter is I use `forEach()` or `forEachOrdered()`. — Ole V.V., Mar 10 '17 at 07:02
What if you used an enum value? That's what `SignUpField`'s type is. — Chris, Mar 10 '17 at 07:05
I assume that if you ditch the stream library and just iterate through it by hand that it works right? If so then I suspect that the Android stream operation may not obey `forEachOrdered(...)` or maybe the order is not sequential order out of the `entrySet()`. — Gray, Mar 10 '17 at 07:11
With an enum I still get `[EMAIL, USERNAME, BIRTHDAY, PASSWORD]`. — Ole V.V., Mar 10 '17 at 07:12
Could you try to reduce this to a [mcve]? It's hard to know if you left out something important. — shmosel, Mar 10 '17 at 07:17
@Gray, that's what I was thinking, I'll probably stick with the loop for now — Chris, Mar 10 '17 at 07:19
@Avi That doesn't make any sense. The keys and values are inserted in the same order. And he's actually iterating over entries, not values. — shmosel, Mar 10 '17 at 07:19
@shmosel I'll work on that in a bit, this has turned out to be one helluva tangent from the actual stuff I was trying to get done :P — Chris, Mar 10 '17 at 07:20

Stefan Zobel · Accepted Answer · 2017-03-10T09:20:22.250

This behavior is a known bug in Android's 7.0 / 7.1 implementation of LinkedHashMap.

The LinkedHashMap's collection views' spliterators for entrySet, values and keySet correctly report that they are ORDERED but actually they aren't because the spliterator implementation of the parent class (HashMap) is used internally.

This behavior has already been documented in the Javadoc and workarounds are also proposed there.

The fix has been commited on 2016-08-16 and will appear in the next Android release.

For clarification: Google became aware of this bug at first in 2017-01, so the "fix" mentioned above was an accidental fix. If they had known about this problem earlier, the resolution would have been included in 7.1

So I'm not crazy! Thank you for the insight. – Chris Mar 10 '17 at 16:52 — Chris, Mar 10 '17 at 16:52

LinkedHashMap entrySet's order not being preserved in a stream (Android)

1 Answers1

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