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https://issues.apache.org/jira/browse/SOLR-14579?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17189086#comment-17189086
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Uwe Schindler commented on SOLR-14579:
--------------------------------------
If you are interested, here is the implementation of a Bootstrap for lambdas in
Elasticsearch's Painless (written by me), especially see the comment:
[LambdaBootstrap lines
126-133|https://github.com/elastic/elasticsearch/blob/0a135a358e4736ee54d76bad0932cab1b5cc1a74/modules/lang-painless/src/main/java/org/elasticsearch/painless/LambdaBootstrap.java#L126-L133]
If no captures are there: it's a singleton and the bootstrap code returns a
method handle that: [LambdaBootstrap lines
481-491|https://github.com/elastic/elasticsearch/blob/0a135a358e4736ee54d76bad0932cab1b5cc1a74/modules/lang-painless/src/main/java/org/elasticsearch/painless/LambdaBootstrap.java#L481-L491];
you see it returns a {{ConstantCallSite}} always returning the same singleton
created there with {{newInstance}}.
You can check similar code in LambdaBootstraps code in the JDK, but those are
doing a bit more magic, as Java is type-safe, while Elasticsearch Painless has
more freedom of types (e.g. def type), so this is easier to read.
> Comment SolrJ 'Utils' generic map functions
> -------------------------------------------
>
> Key: SOLR-14579
> URL: https://issues.apache.org/jira/browse/SOLR-14579
> Project: Solr
> Issue Type: Improvement
> Affects Versions: master (9.0)
> Reporter: Megan Carey
> Assignee: Erick Erickson
> Priority: Minor
> Fix For: 8.7
>
> Attachments: SOLR-14579.patch
>
> Time Spent: 2h 50m
> Remaining Estimate: 0h
>
> Remove the map functions like `NEW_HASHMAP_FUN` from the Utils class in solrj
> module to reduce warnings and improve code quality.
> [https://github.com/apache/lucene-solr/blob/master/solr/solrj/src/java/org/apache/solr/common/util/Utils.java#L92]
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