Testlib: Add test that putIfAbsent replaces a null value

This was shown in a recent Java Collections Puzzlers. The JavaDoc for Map.putIfAbsent states the following.

If the specified key is not already associated with a value (or is mapped to {@code null}) associates it with the given value and returns {@code null}, else returns the current value.

@Test
public void putIfAbsent_nullExistingValue() {
  var map = new HashMap<Object, Object>();
  map.put("a", null);
  map.putIfAbsent("a", "b");
  assertThat(map).containsEntry("a", "b");
}

I found that multiple custom maps which permit null values do not honor this peculiarity (and similar for computeIfAbsent). For example fastutils’ adopted the testlib and Object2ObjectOpenHashMap passes its suite, but it mistakenly does not replace the value as shown above.

[ben-manes]

The segment style in Guava’s is forked from Java 5’s ConcurrentHashMap. There are definite gotchas with atomicity and null values, and I think allowing null values at all was a mistake for Map. It is best replaced by a sentinel value, e.g. Optional, which has the same cpu and memory footprint (as no one actually makes a valueless entry object for this case in their maps). My general rule is to follow Doug Lea’s decisions, as if he found a feature too hard and confusing to support then everyone else will get it wrong (either the implementor or user, as too much of a footgun for a consistent understanding).

[ben-manes]

I think you can use Maven toolchains for handling this, but I am rusty with that build tool.