In addition to being able to define property values in the built object, it would be good if builders could establish binds for properties that support them. The invocation could look either like this:

Rectangle r = RectangleBuilder.begin().x(100).bindY(otherRectangle.yProperty()).bindBidirectionalHeight(otherRectangle.heightProperty()).end();

or (as suggested by Alexander Kouznetsov) like this:

Rectangle r = RectangleBuilder.begin().x(100).y().bind(otherRectangle.yProperty()).height().bindBidirectional(otherRectangle.heightProperty()).end();

where the y() method returns an auxiliary class DoubleBinder (in this case) which has methods bind(ObservableValue<? extends Number>) and bindBidirectional(Property<Number>).