Fix TreeMap creation from Java Map

[l0rinc]

Fixes #1418, @ruslansennov, @danieldietrich and @goerge, please review.

[l0rinc]

K isn't comparable, so we should hope the natural order works

[ruslansennov]

K is comparable because K extends Comparable<? super K>. We can't use natural comparator here. I believe this is correct fix:

final Comparator<Tuple2<K, V>> comparator = new EntryComparator<>((Comparator<? super K> & Serializable) K::compareTo);
RedBlackTree<Tuple2<K, V>> result = RedBlackTree.empty(comparator);

[ruslansennov]

Hmmm...
I looked inside Comparator.naturalOrder(), it seems that your solution also works :)

[danieldietrich]

@paplorinc, @ruslansennov We use the naturalComparator() only in the case we have no Comparable (generic) in scope. If we have a Comparable, like K here, we are able to make it Serializable like Ruslan showed. We use that solution in RedBlackTree for example.

So I would prefer to use

final Comparator<Tuple2<K, V>> comparator = new EntryComparator<>((Comparator<? super K> & Serializable) K::compareTo);
RedBlackTree<Tuple2<K, V>> result = RedBlackTree.empty(comparator);

because the naturalComparator() is not Serializable.

We should write a simple Serialize/Deserialize test for the empty case here. There is a helper for that: Serializables, e.g.

final CharSequence s = deserialize(serialize("test"));
assertThat(s).isEqualTo("test");

[codecov-io]

Current coverage is 97.25%

Merging #1420 into master will not change coverage

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[danieldietrich]

Thank you, will check that this evening...

[l0rinc]

The modified files had too many warnings, reduced and extracted them to simplify the review of next commit

[danieldietrich]

Thank you!! Will pull it now.