That is, if I pass a ruby array to something expecting a list, and it calls myArray.subList(0,1), it should receive a one-element sublist, not a two-element sublist. Similarly, myArray.subList(0, myArray.size()) should return the entire array, even though size() is one past the last valid index.

The source for subList includes:

IRubyObject subList = subseq(fromIndex, toIndex - fromIndex + 1);

It looks like subseq expects an offset and a length. However, from the Java API docs:

Returns a view of the portion of this list between the specified fromIndex, inclusive, and toIndex, exclusive.

So the length of that range should be toIndex - fromIndex. We can see this in the behavior of such a list:

public static void testList(List<?> items) {
  System.out.println("Received " + items.size() + " items");
  System.out.println("sublist(0,1).size() is " + items.subList(0,1).size());
}

Call this with an ArrayList, and it gives a sublist of size 1. Call it from JRuby with a Ruby array, and it gives a sublist of size 2. (A workaround in JRuby is to explicitly construct an ArrayList and addAll() the Ruby array, but that's silly.)

To fix: Just change it to:

IRubyObject subList = subseq(fromIndex, toIndex - fromIndex);

I'll generate a pull request if it really helps, but this should be a one-liner. Even the bounds check is correct (it allows toIndex==size() (should return a sublist ending at the end of the list), and it allows fromIndex==toIndex (should return an empty sublist)).