How to avoid casting and instanceOf

Question

I got a method that does the following:

private void computeHierarchie(GlobalKey key) {
    HierarchieValue hierarchieValue = hierarchieFactory.createHierarchie(selectedKey);
    ....
}

GlobalKey has 4 childs: KeyA, KeyB, KeyC and KeyD.

At the moment my factory looks like the following:

public class HierarchieFactory {

    // inject someFacade which has 4 different methods

    public HierarchieValue createHierarchie(GlobalKey key) {
        if (key instanceof KeyA) {
            return someFacade.computeHierarchie((KeyA) key);
        }
        if (key instanceof KeyB) {
            return someFacade.computeHierarchie((KeyB) key);
        }
        if (key instanceof KeyC) {
            return someFacade.computeHierarchie((KeyC) key);
        }
        if (key instanceof KeyD) {
            return someFacade.computeHierarchie((KeyD) key);
        }
        return new HierarchieValue();
    }

}

I really don't like this switch-case thing, but to be honest, I can't compute any worthy refactoring. I tried something like the following:

public abstract class AbstractHierarchieFactory {

    //inject someFacade

    abstract <T extends GlobalKey> HierarchieValue createHierarchie(T key);

}

And then the 4 classes extending this one. One would look like:

public class KonzernHierarchieFactory extends AbstractHierarchieFactory {

    @Override
    HierarchieValue createHierarchie(KonzernKey konzernKey) {
        return evaFacade.computeHierarchie(konzernKey);
    }

}

This would be so nice! But sadly this doens't work. Do you have any other suggestions? There must be a standard way to solve this problem.

Possible duplicate of http://stackoverflow.com/questions/2790144/avoiding-instanceof-in-java?rq=1 — Luca Basso Ricci, Sep 10 '14 at 07:26

score 0 · Accepted Answer · answered Jun 18 '15 at 13:39

0

This seems like a prime example for a Visitor pattern.

e.g. let's say you have a KeyVisitor interface with the methods visitA(KeyA key), …, visitD(KeyD key). Then you can code the a method into your Key classes (which must obviously also available from a common base class):

void visit(KeyVisitor visitor) { visitor.visitX(this); }

Where you replace the X with your A..D, depending on class. Now your factory can implement KeyVisitor and handle your key classes separately.

answered Jun 18 '15 at 13:39

llogiq

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Your visit-method has to return HierarchieValue. Btw: this does only work if you have one return type. If the return type is an abstract class for instance, you have no idea which subclass you receive from your visit-method. – Chris311 Jan 19 '16 at 12:53

How to avoid casting and instanceOf

1 Answers1