PPS Various compatibility topics were covered in a talk I gave at the
JCP EC in April this year:
"Contributing to OpenJDK: Participating in stewardship for the long-term"
https://jcp.org/aboutJava/communityprocess/ec-public/materials/2023-04-12/Contributing_to_OpenJDK_2023_04_12.pdf
-Joe
On 5/4/2023 9:08 PM, Joseph D. Darcy wrote:
PS And as a general policy, over the releases we've done more to
preserve source compatibility when possible for language changes.
For example, there were many source compatibility breakages when
"assert" was added as a keyword back in JDK 1.4; the current keyword
management policies (https://openjdk.org/jeps/8223002), including
contextual keywords (JLS §3.9) and hyphenated-keywords, mitigate the
impact of analogous changes today.
-Joe
On 5/4/2023 7:47 PM, - wrote:
In addition, in the CSR of sequenced collection, it already
anticipated some other form of source incompatibility:
If a class implements List and Deque at the same time, the return type
of reversed() must extend both interfaces as well.
This alone would be a greater source incompatibility than this type
interference already; however, both are binarily compatible:
Java doesn't care about generics at runtime, and for default
reversed() overrides, reversed ()Ljava/util/List; and reversed
()Ljava/util/Deque; are distinct methods; code calling reversed via
List or Deque interfaces will get the reversed version of respective
interfaces, which are functionally correct as well.
I personally would consider this type inference usage moot. It is same
as adding another method addAll(List<String>) when there is already
addAll(String[]): existing addAll(null) calls will break, but this is
not a valid argument against adding a List<String> variant of addAll.
On Thu, May 4, 2023 at 9:38 PM Joseph D. Darcy <joe.da...@oracle.com>
wrote:
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
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
<raffaello.giulie...@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, fo...@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" <stuart.ma...@oracle.com>
To: "Remi Forax" <fo...@univ-mlv.fr>
Cc: "core-libs-dev" <core-libs-...@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
