Most old empty and single-element java.util.Collections classes override hashCode, contains, containsAll, providing optimized implementations, whereas the default implementations inherited from AbstractSet/List/Map typically are implemented using iterators. The constant sized collections in ImmutableCollections (Set0,1,2, Map0,1, List0,1,2) doesn't. This leads to a surprising amount of allocations during startup when using Set.of in place of Collections.singleton etc

Implementing these for at least Set0,1,2 can provably help startup performance of jigsaw after converting from Collections.emptySet/singleton/unmodifiableSets to Set.of in the bootstrap (which has advantages to footprint).