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https://issues.apache.org/jira/browse/SOLR-14586?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17142718#comment-17142718
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Noble Paul edited comment on SOLR-14586 at 6/23/20, 8:25 AM:
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[~megancarey] putIfAbsent() is particularly bad. The value is computed before
the method is called. If we have to choose between putifAbsent() &
computeIfAbsent(), almost always chgoose computeifAbsent. The Function Object
is cheap to construct, the real Object is usually far more expensive
[~nitsanw]
* static vars are always optimized irrespective of where it is used
* direct lamda uses are inefficient in certain places.
Now, do you expect a developer to run a perf benchmark every time he has to use
a {{Map#computeIfAbsent()}} or we just create some best practices and people
just follow them?
was (Author: noble.paul):
[~megancarey] {{putIfAbsent() }} is particularly bad. The value is computed
before the method is called.
[~nitsanw]
* static vars are always optimized irrespective of where it is used
* direct lamda uses are inefficient in certain places.
Now, do you expect a developer to run a perf benchmark every time he has to use
a {{Map#computeIfAbsent()}} or we just create some best practices and people
just follow them?
> replace the second function parameter in Map#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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