@Beta public interface Interner<E>
String.intern() for other immutable types.
E intern(E sample)
intern(a).equals(a) always holds, and
intern(a) == intern(b) if and only if
a.equals(b). Note that
intern(a) is permitted to return one instance now and a different instance later if the original interned instance was garbage-collected.
Warning: do not use with mutable objects.
NullPointerException - if
sample is null