Add support unit tests for ConcurrentHashSet, #1117

[paulirwin]

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Add support unit tests for ConcurrentHashSet

Fixes #1117

Description

This ports tests from Harmony for Collections.synchronizedSet and ConcurrentHashMap and adapts them to our ConcurrentHashSet.

This also adds nullable reference type checking to ConcurrentHashSet, which exposed some nullability issues. Given that our current use of this type is for values that should never be null (or would otherwise throw an NRE upon use as null is not checked-for), this adds a generic type constraint of notnull to ensure that it cannot be used with nullable reference types. Work would need to be done (under proper unit test coverage) to make this null-friendly if that is needed in the future.

[NightOwl888]

Thanks for the PR.

I have been considering phasing out the SystemTypesHelpers class (or at least most of the extension methods in it), so ideally any new tests we add would not have any dependencies on those extension methods (.add(), .contains(), .size(), etc.).

If we do keep them, they will definitely need a review. Some of the methods perform poorly or have better alternatives (.toString(), removeAll(), retainAll(), and ToByteArray() to name a few). The .append() overloads are generally there to allow culture invariant numbers to be appended, since .NET doesn't have that built-in. However, I am adding optional parameters to allow passing format and IFormatProvider on OpenStringBuilder in J2N. So, we should probably hold off on the review/removal of SystemTypesHelpers until after J2N has a build with OpenStringBuilder where those can be replaced directly by swapping in OpenStringBuilder for StringBuilder.

[NightOwl888]

Do note that some of these Support_ tests from Harmony have been ported in J2N. https://github.com/NightOwl888/J2N/blob/main/tests/NUnit/J2N.Tests/Collections/

[paulirwin]

That code appears to be GPL licensed, based on the license in the test project (and that there is no other license listed in this file). Would that be an issue? How would I do the license header in that case?

But we could always leave as a future TODO to port those tests. This test class gives very good test coverage already without them.

[NightOwl888]

The Support_ classes were ported from Apache Harmony, so no issue with those:

https://github.com/apache/harmony/blob/trunk/classlib/support/src/test/java/tests/support/Support_SetTest.java
https://github.com/apache/harmony/blob/trunk/classlib/support/src/test/java/tests/support/Support_CollectionTest.java

The GPL license was put there because some of the test code is directly from the JDK and it is the most restrictive license. But we don't ship the test projects. J2N is made up from Apache 2.0 and MIT licenses and should all have headers, but I haven't gone through all of the test files to add license headers.