"java.lang.IllegalArgumentException: Comparison method violates its general contract!" on Java 7

The implementation of Collection#sort has changed in Java 7 and is a bit more strict.

To summarize it used to more or less only check if Comparable#compareTo result was negative, positive or 0 and is now really taking into account the actual number returned.

See http://www.oracle.com/technetwork/java/javase/compatibility-417013.html#source