CODEWITHSUNDEEP
javadesign-patternsoopinstanceof
Setsuna F. Seiei
Setsuna F. Seiei

Reputation: 387

Is there a better/cleaner way to conditionally create a type than using instanceof? [Java]

Suppose I have:

public class FightingZone<MobileSuitso, Background> {

    private MobileSuitCollection<MobileSuitso> msCollection;
    private BackgroundInfo<Background> bgInfo;

    public FightingZone(MobileSuitCollection<MobileSuitso> newCollection, BackgroundInfo<Background> newInfo) {
        this.msCollection = newCollection;
        this.bgInfo = newInfo;
    }

    ...

        ...// inside a method
        MobileSuitCollection temporaryCollection = new MobileSuitCollection<MobileSuitso>(); // /!\

}

The problem is MobileSuitCollection is an Interface, so I can't instantiate it. For example, I could do:

MobileSuitCollection temporaryCollection = new GundamMeisterCollection<MobileSuitso>();
MobileSuitCollection temporaryCollection = new InnovatorCollection<MobileSuitso>();
MobileSuitCollection temporaryCollection = new CannonFolderCollection<MobileSuitso>();

etc. However, to manipulate temporaryCollection, I need it to be of the same type as the one passed via parameter to my Class. So I thought about doing this:

if (msCollection instanceof GundamMeisterCollection) {
    ...
} else if (msCollection instanceof InnovatorCollection) {
    ...
} ...

I realize that is awful however. Is there a better way to do this? Would it be possible to keep a reference to the class used by the initial type and then instantiate the temporaryCollection with that?

Upvotes: 3

Views: 204

Answers (2)

Bozho
Bozho

Reputation: 597392

The code that you place in the if-clause can be placed in a Visitor :

// Generics skipped for brevity
interface MobileSuitCollectionVisitor {
   handleCollection(GundamMeisterCollection collection);
   handleCollection(InnovatorCollection collection);
   handleCollection(CannonFolderCollection collection)
}

class ConcreteVisitor implements MobileSuitCollectionVisitor { 
    // place all of the logic in the implemented methods
}

and then let MobileSuitCollection have a method:

void visit(MobileSuitCollectionVisitor visitor);

And in each implementation of MobileSuitCollection simply have

public void visit(MobileSuitCollectionVisitor visitor) {
    visitor.handleCollection(this);
}

Upvotes: 2

Adam Crume
Adam Crume

Reputation: 15844

A quick and dirty way to do it would be to clone the original collection, then manipulate it as needed. A better way might be to add a newInstance() method to the interface, or pass in a factory to FightingZone.

Upvotes: 0

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