Except...
Having constant-time lookups is not necessary in this case.
RS scripting should not care about lookup efficiency in most cases, since all delay comes from the game itself.
Also, I seriously doubt the OP will continually poll for available worlds; once he finds an available one, he will hop there. In the meantime, either OP won't poll or he will be waiting for a world to become available.
HashMaps will keep his code simple and clean, too.
Also, the HashMap won't contain thousands of elements; at most 50-ish.
tl;dr rs isn't performance sensitive, writing new class is useless