Josh Bloch writes:

The Comparable.compareTo doc says:

  In the foregoing description, the notation sgn(expression)
designates the mathematical signum function, which is defined to
return one of -1, 0, or 1 according to whether the value of expression
 is negative, zero or positive. The implementor must ensure
sgn(x.compareTo(y)) == -sgn(y.compareTo(x)) for all x and y. (This
implies that x.compareTo(y) must throw an exception iff y.compareTo(x)
throws an exception.)

It looks like the first sentence of this paragraph should be moved to the end.