Introducing "Safety In Numbers", a Java library supporting interval arithmetic. Still in a very raw state but it is complete enough for you to have a play.
This library includes not only the Interval class to represent a mathematical interval, but an effecient IntervalSet class for aggregating groups of non-intersecting Intervals. See documentation for further details.
Also includes fairly extensive JUnits, which have proven essential for testing the myriad of special cases that have been discovered. Feel free to expand these, as I'm sure I haven't covered everything.