Question about Java style

[Mark Murphy]

My new class org.apache.poi.ss.util.CellBorders has some private methods
that respond to a subset of enum values being passed in. If the methods
were private, I would throw an exception if an invalid enum value was
passed, but since the methods are private, is that necessary? Well, it
isn't necessary, but should I validate the parameter anyway even though it
is always called with an explicit value within the class.