What you’re looking for is essentially the power set (minus perhaps the empty set). Guava actually has a method for this: Sets.powerSet(). You can view the source of the Sets class to see how the method is implemented if you want to write it yourself; you might need to modify it to return a List instead of a Set since you want to preserve order, although this change should not be too drastic. Once you have the power set, it should be trivial to iterate over it and construct the map you want.
