The Java (openjdk) hashmap putValue doesn't seem to compare types, it just uses .equals()

  if (p.hash == hash &&
                  ((k = p.key) == key || (key != null && key.equals(k))))

You can't really rely on checking types in a Hashmap anyways because the type you're hashing can have more possible values than the 32bit hash. For example you can have 2 strings that have a hash collision.

NaN's work in a Java hashmap because Float.hashCode returns floatToIntBits(float) which normalizes all NaN's to a canonical value.