The introduction of Sequenced collections is not a source compatible change

Fri May 5 11:14:46 UTC 2023

Hi Joe, 
in this peculiar case, there are several reasons to be worried compared to other potential breaking changes that has appeared in the past (see the message of Tagir for an example). 

Unlike other changes 
- this one touch the collection API, and those interfaces/types are widely used, 
- we know that the source compatibility changes occurs mostly if 'var' or the "new" inference algorithm (the one from Java 8), so this is likely that most of the issues will be found in Java 11+ source code, 
- this changes may also affect all typed languages based on the JVM, not only Java. Corpus of codes in Groovy, Kotlin and Scala also need to be checked. In case of Kotlin and Scala, 'var' is the default behavior but they have their own collections (or type system around collections in case of Kotlin), so knowing the real impact of this change is hard here. 

The problem of using a corpus experiment is that the corpus may not represent the current state of the Java ecosystem, or at least the one that may be impacted. 
In my case, on my own repositories (public and private), i had only one occurrence of the issue in the main source codes because many of those repositories are not using 'var' or even the stream API but on the corpus of the unit tests we give to students to check their implementations, little less than a third of those JUnit classes had source compatibility issues because those tests are using 'var' and different collections heavily. 

And the situation is a little worst than that because in between now and the time people will use Java 21, a lot of codes will be written using Java 11 and 17 and may found incompatible later. 

A source incompatibility issue is not a big deal, as said in this thread, most of the time, explicitly fixing the type argument instead of inferring it make the code compile again. 
So the house is not burning, but we should raise awareness of this issue given that it may have a bigger impact than other source incompatible changes that occur previously. 

Rémi 

> From: "joe darcy" <joe.darcy at oracle.com>
> To: "Ethan McCue" <ethan at mccue.dev>, "Raffaello Giulietti"
> <raffaello.giulietti at oracle.com>
> Cc: "Remi Forax" <forax at univ-mlv.fr>, "Stuart Marks" <stuart.marks at oracle.com>,
> "core-libs-dev" <core-libs-dev at openjdk.java.net>
> Sent: Friday, May 5, 2023 4:38:16 AM
> Subject: Re: The introduction of Sequenced collections is not a source
> compatible change

> A few comments on the general compatibility policy for the JDK. Compatibility is
> looked after by the Compatibility and Specification Review (CSR) process (
> Compatibility & Specification Review). Summarizing the approach,

>> The general compatibility policy for exported APIs implemented in the JDK is:

>> * Don't break binary compatibility (as defined in the Java Language
>> Specification) without sufficient cause.
>> * Avoid introducing source incompatibilities.
>> * Manage behavioral compatibility changes.
> [ https://wiki.openjdk.org/display/csr/Main |
> https://wiki.openjdk.org/display/csr/Main ]

> None of binary, source, and behavioral compatibly are absolutes and judgement is
> used to assess the cost/benefits of changes. For example, strict source
> compatibility would preclude, say, introducing new public types in the
> java.lang package since the implicit import of types in java.lang could
> conflict with a same-named type *-imported from another package.

> When a proposed change is estimated to be sufficiently disruptive, we conduct a
> corpus experiment to evaluate the impact on the change on many public Java
> libraries. Back in Project Coin in JDK 7, that basic approach was used to help
> quantify various language design choices and the infrastructure to run such
> experiments has been built-out in the subsequent releases.

> HTH,

> -Joe
> CSR Group Lead
> On 5/4/2023 6:32 AM, Ethan McCue wrote:

>> I guess this a good time to ask, ignoring the benefit part of a cost benefit
>> analysis, what mechanisms do we have to measure the number of codebases relying
>> on type inference this will break?

>> Iirc Adoptium built/ran the unit tests of a bunch of public repos, but it's also
>> a bit shocking if the jtreg suite had nothing for this.

>> On Thu, May 4, 2023, 9:27 AM Raffaello Giulietti < [
>> mailto:raffaello.giulietti at oracle.com | raffaello.giulietti at oracle.com ] >
>> wrote:

>>> Without changing the semantics at all, you could also write

>>> final List<Collection<String>> list =
>>> Stream.<Collection<String>>of(nestedDequeue, nestedList).toList();

>>> to "help" type inference.

>>> On 2023-05-03 15:12, [ mailto:forax at univ-mlv.fr | forax at univ-mlv.fr ] wrote:
>>> > Another example sent to me by a fellow French guy,

>>> > final Deque<String> nestedDequeue = new ArrayDeque<>();
>>> > nestedDequeue.addFirst("C");
>>> > nestedDequeue.addFirst("B");
>>> > nestedDequeue.addFirst("A");

>>> > final List<String> nestedList = new ArrayList<>();
>>> > nestedList.add("D");
>>> > nestedList.add("E");
>>> > nestedList.add("F");

>>>> final List<Collection<String>> list = Stream.of(nestedDequeue,
>>> > nestedList).toList();

>>>> This one is cool because no 'var' is involved and using
>>> > collect(Collectors.toList()) instead of toList() solves the inference problem.

>>> > Rémi

>>> > ----- Original Message -----
>>>>> From: "Stuart Marks" < [ mailto:stuart.marks at oracle.com |
>>> >> stuart.marks at oracle.com ] >
>>> >> To: "Remi Forax" < [ mailto:forax at univ-mlv.fr | forax at univ-mlv.fr ] >
>>>>> Cc: "core-libs-dev" < [ mailto:core-libs-dev at openjdk.java.net |
>>> >> core-libs-dev at openjdk.java.net ] >
>>> >> Sent: Tuesday, May 2, 2023 2:44:28 AM
>>>>> Subject: Re: The introduction of Sequenced collections is not a source
>>> >> compatible change

>>> >> Hi Rémi,

>>> >> Thanks for trying out the latest build!

>>> >> I'll make sure this gets mentioned in the release note for Sequenced
>>> >> Collections.
>>> >> We'll also raise this issue when we talk about this feature in the Quality
>>> >> Outreach
>>> >> program.

>>> >> s'marks

>>> >> On 4/29/23 3:46 AM, Remi Forax wrote:
>>> >>> I've several repositories that now fails to compile with the latest jdk21, which
>>> >>> introduces sequence collections.

>>> >>> The introduction of a common supertype to existing collections is *not* a source
>>> >>> compatible change because of type inference.

>>> >>> Here is a simplified example:

>>> >>> public static void m(List<Supplier<? extends Map<String, String>>> factories) {
>>> >>> }

>>> >>> public static void main(String[] args) {
>>> >>> Supplier<LinkedHashMap<String,String>> supplier1 = LinkedHashMap::new;
>>> >>> Supplier<SortedMap<String,String>> supplier2 = TreeMap::new;
>>> >>> var factories = List.of(supplier1, supplier2);
>>> >>> m(factories);
>>> >>> }

>>> >>> This example compiles fine with Java 20 but report an error with Java 21:
>>> >>> SequencedCollectionBug.java:28: error: method m in class SequencedCollectionBug
>>> >>> cannot be applied to given types;
>>> >>> m(factories);
>>> >>> ^
>>> >>> required: List<Supplier<? extends Map<String,String>>>
>>> >>> found: List<Supplier<? extends SequencedMap<String,String>>>
>>> >>> reason: argument mismatch; List<Supplier<? extends SequencedMap<String,String>>>
>>> >>> cannot be converted to List<Supplier<? extends Map<String,String>>>

>>> >>> Apart from the example above, most of the failures I see are in the unit tests
>>> >>> provided to the students, because we are using a lot of 'var' in them so they
>>> >>> work whatever the name of the types chosen by the students.

>>> >>> Discussing with a colleague, we also believe that this bug is not limited to
>>> >>> Java, existing Kotlin codes will also fail to compile due to this bug.

>>> >>> Regards,
>>> >>> Rémi
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