I keep coming back to variants of this problem: it probably has a very simple solution, but I can't seem to figure it out...
I have a bunch of classes of the form xQuantity, e.g. DistanceQuantity, AreaQuantity, etc., which extend a class DimensionQuantity. Now you can add or subtract DistanceQuantity's or AreaQuantity's, etc., but you can't mix them, so I think I need to have (short) add, subtract, etc., methods in the subclasses, but I want to reduce any logic duplication to a minimum. However, I need to return an object of the subclass, and this seems difficult to do from the superclass method. I believe this can be done using reflection, but AFAIK you still need to do a cast at the end in the subclass method, and I am told that reflection can be expensive... The best I have come up with so far is:
In DistanceQuantity (and the other similar ones):
public DistanceQuantity() {
}
public DistanceQuantity add(DistanceQuantity d1) {
DistanceQuantity dn = new DistanceQuantity();
Object o = super.add(dn, this, d1, DistanceUnit.REF_UNIT);
return (DistanceQuantity) o;
}
In DimensionQuantity (minus some less relevant statements):
public Object add(DimensionQuantity dn, DimensionQuantity d1, DimensionQuantity d2,
AbstractUnit au) {
dn.unit = au;
dn.scalar = d1.scalar + d2.scalar;
dn.units = dn.scalar;
return dn;
}
Can anyone come up with leaner code - that is still type-safe? TIA