Inheritance and casting: is this good java?

Question

Take a look at this code (from here)

abstract class EntityA {
    AssocA myA;
    abstract void meet();
}

abstract class AssocA {
    int something;
    abstract void greet();
}

class AssocAConcrete extends AssocA {
    void greet() {
        System.out.println("hello");
    }
    void salute() {
        System.out.println("I am saluting.")
    }
}

class EntityAConcrete extends EntityA {
    void meet() {
        System.out.println("I am about to meet someone");
        ((AssocAConcrete)myA).salute();
    }
}

There are two parallel inheritance trees, for a parent class and an associated class. The problem is with line 23:

((AssocAConcrete)myA).salute();

It is a pain and I have that kind of thing all over my code. Even though that line is part of the concrete implementation of Entity, I need to remind it that I want to use the concrete implementation of AssocA, AssocAConcrete.

Is there some kind of annotation to declare that relationship? Or is there a better, more colloquial Java way to express this design? Thanks!

This is in response to @Dave, because I want to put some code in...

Interesting! So the invocation would look something like this:

AssocAConcrete myAssoc = new Assoca();
EnitityA<T extends AssocA> myEntity = new EntityA<AssocAConcrete>();
myEntity.setAssoc(myAssoc);
myAssoc.salute();

Yes? That's really cool. I think I will use it!

If you write AssocAConcrete myA; instead of AssocA myA; (line 2), it would solve it. But would that break the rest of the code ? — Radu Murzea
– Radu Murzea, Commented Feb 3, 2012 at 17:00
Yeah the whole idea is that there may be different variations of AssocAConcrete and the clients of EntityA would like to not be dependent on that. Notice that code that uses just AssocA and EntityA never know or mention one or the other Concrete class. And only the two Concrete "A" classes ever know about the implementation details. — pitosalas
– pitosalas, Commented Feb 3, 2012 at 20:23

Dave · Accepted Answer · 2012-07-29 13:38:17Z

8

I would think this is a lot neater using generics...

abstract class EntityA<T extends AssocA> {

    // Basically, this means myA is at least an AssocA but possibly more...
    T myA;
    abstract void meet();
}

abstract class AssocA {
    int something;
    abstract void greet();
}

class AssocAConcrete extends AssocA {
    void greet() {
        System.out.println("hello");
    }
    void salute() {
        System.out.println("I am saluting.");
    }
}

class EntityAConcrete extends EntityA<AssocAConcrete> {
    void meet() {
        System.out.println("I am about to meet someone");
        myA.salute();
    }
}

Aside from avoiding the casting, this also makes it much easier to add different functionality in your AssocA implementations. There should always be a way to do things without using dummy implementations (ie methods that just throw "NotImplementedException") or casting. Even though it isn't always easy or worth the refactoring time to do so. In other words, no one is going to blame you for casting (well...maybe some people will but you can't please everyone).

EDIT (Notes on instantiation):

From @pitosalas' comments below...

//Won't work...can't call 'new' on abstract class AssocA
AssocAConcrete myAssoc = new Assoca();

//Instead, do this...
AssocAConcrete myAssoc = new AssocAConcrete();

And then....

// Again, won't work.  T is only declaring the type inside your class/method.
// When using it to declare a variable, you have to say EXACTLY what you're making,
// or at least something as exact as the methods you're trying to invoke
EnitityA<T extends AssocA> myEntity = new EntityA<AssocAConcrete>();

//Instead do this...
EnitityA<AssocAConcrete> myEntity = new EntityAConcrete();

// Or this...
EntityAConcrete myEntity = new EntityAConcrete();

And then this should be good...

// Assuming this is defined as `public void setAssoc(T newAssoc) {this.myA = newAssoc;}`
myEntity.setAssoc(myAssoc);
myAssoc.salute();

edited Jul 29, 2012 at 13:38

answered Feb 3, 2012 at 17:30

Dave

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9 Comments

Dawood ibn Kareem Over a year ago

I wish I could give you +2. This is exactly what generics are for; and I don't understand why some people shy away from them. Oh, and I am one of the people who WOULD blame you for using casting to solve a problem that is better solved with generics.

pitosalas Over a year ago

Ah interesting. And I would instantiate them like this?

Dave Over a year ago

How you would instantiate varies. Where is the code you're currently using to instantiate? Could you update your question to have an example?

pitosalas Over a year ago

Yuck. I want to show you code but I cannot self-answer and I cannot put code in a comment... Here's a try...Maybe I will update the question... Interesting! So the invocation would look something like this: AssocAConcrete myAssoc = new Assoca(); EnitityA<T extends AssocA> myEntity = new EntityA<AssocAConcrete>(); myEntity.setAssoc(myAssoc); myAssoc.salute(); Yes? That's really cool. I think I will use it!

hvgotcodes Over a year ago

I like this answer, but the problem is that he might have another subclass that has yet another method not specified on the base class...what then? The point is whether or not you use generics, there needs to be a contract that is obeyed....

|

hvgotcodes · Accepted Answer · 2012-02-03 20:39:09Z

3

Looks suspicious to me. There is nothing terrible about casting, but in this case, you could resolve the issue by bringing the salute method into AssocA. Subclasses of AssocA can provide their implementations; that's part of the benefit of inheritance.

What you are doing now is saying all EntityA instances have an AssocA instance, but then in your meet method you basically force the AssocA instance to be an AssocAConcrete instance. That's the part that is suspicious; why does AssocA exist if you really need an AssocAConcrete.

Another option (based on your comments) is to invoke salute in the greet method. That way, the specific subclass has specified behavior greet, defined in the superclass, and does what it wants. In this case, salute could become private or protected. Another implementation can easily do something different, like runLikeHell.

edited Feb 3, 2012 at 20:39

answered Feb 3, 2012 at 16:55

hvgotcodes

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3 Comments

ewernli Over a year ago

This kind of logical coupling between families of classes is very common. Alas, the type system does not support expressing it.

pitosalas Over a year ago

@nwgotcodes - not really the same thing. I want to hide knowledge of the implementation of the two concrete "A" classes, so they each know about each other but only AssocAConcrete knows how to greet. AssocXConcrete might not know how to do that.

hvgotcodes Over a year ago

then invoke salute in your greet method.

ewernli · Accepted Answer · 2012-02-03 17:02:06Z

0

The problem of parallel class hierarchies is very real and really sucks. The logical coupling that AssocAConcrete always go with EntityAConcrete can not be expressed with the type system.

You can not specialize the type of myA in EntityAConcrete to be AssocAConcrete, without hiding it from a superclass. I think the closest work that addressed that was "Family polymorphism", but that's not mainstream.

answered Feb 3, 2012 at 17:02

ewernli

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Comments

Chris · Accepted Answer · 2012-02-03 17:02:51Z

0

If you have a large part of code where you are using the reference "myA" you could declare another reference like that:

public AssocAConcrete myAConcrete = (AssocAConcrete)myA;

now you can use the new reference myAConcrete and access the functions of the AssocAConcrete Class.

If you need to do this a lot like hvgotcodes mentioned you should probbably consider moving the method up to the AssocA Class.

answered Feb 3, 2012 at 17:02

Chris

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