Quality: code cleaning

[EtienneLt]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Does this PR already have an issue describing the problem?
No

What kind of change does this PR introduce?
Cleaning some code:

• Use Java14 switch syntax (JEP-361), especially return switch syntax.
• Replace some Stream.collect(Collector.toList()) with Stream.toList() (when immutable lists can be returned without impact for callers).
• Use JsonParser.currentName() instead of deprecated method JsonParser.getCurrentName().
• Replace Files.write(file, string.getBytes(StandardCharsets.UTF_8)) with Files.writeString(file, string).
• Replace some Arrays.asList(...) with List.of(...).
• Replace assertTrue(obj instanceof Foo) with assertInstanceOf(Foo.class, obj).
• A few other simplifications.

Does this PR introduce a breaking change or deprecate an API?

• Yes
• No

[geofjamg]

Be careful, each time you replace a collec(Collectors.toList())) by a toList() and that it is returned from an api you break potentialy the api because list is now immutable. And using code outside of powsybl core might failed at runtime.

[EtienneLt]

Be careful, each time you replace a collec(Collectors.toList())) by a toList() and that it is returned from an api you break potentialy the api because list is now immutable. And using code outside of powsybl core might failed at runtime.

ok I can revert if needed, I left many Collectors.toList()

[geofjamg]

Be careful, each time you replace a collec(Collectors.toList())) by a toList() and that it is returned from an api you break potentialy the api because list is now immutable. And using code outside of powsybl core might failed at runtime.

ok I can revert if needed, I left many Collectors.toList()

I just say that each time a list is exposed to an API we have to think and decide if we want to expose a mutable or immutable list and report if we change from actual code as a breaking change and write it in the migration guide

[sonarcloud]

Quality Gate failed

Failed conditions
77.7% Coverage on New Code (required ≥ 80%)
C Reliability Rating on New Code (required ≥ A)

See analysis details on SonarCloud

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[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
82.7% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

All rolnico's requests were addressed.