With the advent of default methods in Java 8, the need for *Expression classes (DoubleExpression, BooleanExpression, ...) vanished, since all their methods can be implemented as default methods in the respective interfaces (ObservableDoubleValue, ObservableBooleanValue, ...).

I realize that *Expression classes will have to be dragged along forever, but the implementation of their convenience methods could be moved up to the respective interfaces, so that alternative implementations of observable interfaces can inherit their default implementation of those methods.