[cdi-dev] [JBoss JIRA] (CDI-630) Revise javax.enterprise.util.AnnotationLiteral.cachedHashCode

[Martin Kouba (JIRA)]

    [ https://issues.jboss.org/browse/CDI-630?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13292264#comment-13292264 ] 

Martin Kouba edited comment on CDI-630 at 9/13/16 3:17 AM:
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bq. If we would change the hashCode of empty AnnotationLiterals then you would end up with 2 entries in the HashMap. 
[~struberg] No, you don't have to end up with 2 entries because those annotation instances are equal. From what I know the hash code is only used to compute an index into array of buckets. But it really depends on impl. So right now all annotation literal keys will end up in one bucket (-> hash collisions -> less effective lookup). 

Anyway, we would break the general equals/contract and also the {{java.lang.annotation.Annotation.hashCode()}} contract (I wasn't aware of its existence :-(. So it's definitely NOT POSSIBLE.

Still, I don't get the meaning of storing a zero integer if there are no members. Checking the members array length should be fine. Lowering the priority...


was (Author: mkouba):
bq. If we would change the hashCode of empty AnnotationLiterals then you would end up with 2 entries in the HashMap. 
[~struberg] No, you will not end up with 2 entries because those annotation instances are equal. From what I know the hash code is only used to compute an index into array of buckets. So right now all annotation literal keys will end up in one bucket (-> hash collisions -> less effective lookup).

But I agree we MAY NOT break the {{java.lang.annotation.Annotation.hashCode()}} contract - I wasn't aware of its existence :-(

Still, I don't get the meaning of storing a zero integer if there are no members. Checking the members array length should be fine. Lowering the priority...

> Revise javax.enterprise.util.AnnotationLiteral.cachedHashCode
> -------------------------------------------------------------
>
>                 Key: CDI-630
>                 URL: https://issues.jboss.org/browse/CDI-630
>             Project: CDI Specification Issues
>          Issue Type: Feature Request
>            Reporter: Martin Kouba
>            Priority: Optional
>             Fix For: 2.0 (discussion)
>
>
> Currently, {{AnnotationLiteral.hashCode()}} always returns {{0}} if there are no members (ignoring annotation type completely). Although it does not break {{Object.hashCode()}} contract, I believe we should -either return a number based on the annotation type (to make the annotation literal instances more usable in hash tables) or- simply return zero and don't cache the value at all.
> UPDATE: We may not return a number based on the annotation type because it would break the {{java.lang.annotation.Annotation.hashCode()}} contract.



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