Tech debt week: clean up some crufty java idioms

[joegallo]

Placeholder description for now. Let's make a list of annoyances and clean them up.

Some proposed crufty idioms:

• Collections.singleton
• Collections.singletonMap
• Collections.emptyMap
• Collections.emptyList
• Collections.emptySet
• Arrays.asList(foo, bar, baz) when it can just be List.of(foo, bar, baz) (but not Arrays.asList(someArray))
• new ArrayList<>(Arrays.asList(foo, bar, baz)) might warrant an audit beyond just the above (if the mutability of the resulting list isn't actually necessary)
• String.format(Locale.ROOT, ...) probably should be org.elasticsearch.core.Strings.format(...)
• someStream.collect(Collectors.toList()) can be someStream.toList()
• Strings.collectionToDelimitedString( can be a joining() in some cases
• $stream$.map($x$ -> $x$.$method$()) (hat tip to @jbaiera)
• ...

Discussion

Question: should we (the Data Management team) limit this to just 'our' sections of the code or should it be an Elasticsearch-wide effort? My immediate answer I suppose is that we may as well do it across the whole codebase if the cost of doing a particular cleanup is low enough. Also if we do it across the whole codebase then we can mark the bad thing as being forbidden.

Source of additional issues: some of the various core/ and common/ could be used in more places, e.g. org.elasticsearch.core.Strings.format as mentioned above.

[elasticsearchmachine]

Pinging @elastic/es-data-management (Team:Data Management)