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https://issues.apache.org/jira/browse/SOLR-14586?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17142384#comment-17142384
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Megan Carey commented on SOLR-14586:
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Admittedly, I was not aware that Java lambda functions were a poor-performance
option, so thanks for bringing that to my attention [~noble.paul].
I am curious why we're using map.computeIfAbsent with lambda function when in
most cases we are not using the key to compute the value?
>From what I understand, map.computeIfAbsent(key, key -> value) is meant to be
>used when the value is computed from the key. However, if the value we are
>"computing" is just a new HashMap, ArrayList, etc., why don't we just use
>map.putIfAbsent?
Apologies if I'm missing something here.
> replace the second function parameter in computeIfAbsent with static vars
> -------------------------------------------------------------------------
>
> Key: SOLR-14586
> URL: https://issues.apache.org/jira/browse/SOLR-14586
> Project: Solr
> Issue Type: Bug
> Security Level: Public(Default Security Level. Issues are Public)
> Reporter: Noble Paul
> Assignee: Noble Paul
> Priority: Minor
> Time Spent: 10m
> Remaining Estimate: 0h
>
> people misunderstand lambdas
> let's look at a very innocuous looking piece of code
> {code:java}
> mapObject.computeIfAbsent(key, o -> new HashMap<>());
> {code}
> vs.
> {code:java}
> mapObject.computeIfAbsent(key, Utils.NEW_HASHMAP_FUN)
> {code}
>
> The first code fragment is executed as following
> {code:java}
> s.computeIfAbsent(key, new Function() {
> @Override
> public Object apply(String key) {
> return new HashMap<>();
> }
> }
> {code}
> So, there are two problems with this
> * A new anonymous inner class is created for that lambda. This one extra
> class becomes a part of your binary
> * a new instance of that class is created everytime the
> {{computeIfAbsent()}} method is invoked, irrespective of whether the value is
> absent for that key or not. Now imagine that method getting called millions
> of times and creating millions of such objects for no reason
> OTOH
> when I use {{Utils.NEW_HASHMAP_FUN}}
> * Only a single anonymous class is created for the entire codebase
> * Only single instance of that object is created in the VM
> Ideally, we should go all over the codebase and remove such lambdas
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