Returning subclass object from superclass method

Question

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

Answer 1

You can use Generics like this :

public abstract class DimensionQuantity<T extends DimensionQuantity>{
    public abstract T add(T parameter);
}

and you extends it like this :

public class DistanceQuantity extends DimensionQuantity<DistanceQuantity>{
    public DistanceQuantity add(DistanceQuantity parameter){
        //Whatever
        return null;
    }
}

And for the initial question, it's a really bad idea (and a bad practice) to have a superclass which uses one of its sub-classes.

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Resources :

• Oracle.com - Generics

On the same topic :

• Resource for learning Java generics?