[jira] [Commented] (FLINK-13647) Allow default methods and static methods to be added to public interfaces

["Chesnay Schepler (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/FLINK-13647?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16902801#comment-16902801 ] 

Chesnay Schepler commented on FLINK-13647:
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According to [JLS 13.5.6|https://docs.oracle.com/javase/specs/jls/se8/html/jls-13.html#jls-13.5.6], adding default methods can lead to errors at runtime:
{code}
Adding a default method, or changing a method from abstract to default, does not break compatibility with pre-existing binaries, but may cause an IncompatibleClassChangeError if a pre-existing binary attempts to invoke the method. This error occurs if the qualifying type, T, is a subtype of two interfaces, I and J, where both I and J declare a default method with the same signature and result, and neither I nor J is a subinterface of the other. 
{code}

I couldn't find anything specific on static methods added to interface, but [JLS 13.4.19|https://docs.oracle.com/javase/specs/jls/se8/html/jls-13.html#jls-13.4.19] could be relevant:
{code}
If a method that is not declared private is also declared static (that is, a class method) and is changed to not be declared static (that is, to an instance method), or vice versa, then compatibility with pre-existing binaries may be broken, resulting in a linkage time error, namely an IncompatibleClassChangeError, if these methods are used by the pre-existing binaries. Such changes are not recommended in code that has been widely distributed. 
{code}

> Allow default methods and static methods to be added to public interfaces
> -------------------------------------------------------------------------
>
>                 Key: FLINK-13647
>                 URL: https://issues.apache.org/jira/browse/FLINK-13647
>             Project: Flink
>          Issue Type: Improvement
>          Components: Build System
>    Affects Versions: 1.9.0, 1.10.0
>            Reporter: Till Rohrmann
>            Priority: Major
>             Fix For: 1.10.0
>
>
> Since Java 8, it is possible to extend interfaces by adding default methods and static methods. Flink's current japicmp plugin won't allow this and complains with a {{METHOD_ADDED_TO_INTERFACE}}.
> I think we should allow this in order to extend existing public interfaces. There is also a japicmp issue which addresses this problem: https://github.com/siom79/japicmp/issues/201.



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