Class inheritance and generic types

Question

I'm currently working on inheritance in Java. I would like to discuss the following case with you.

In my example, I have numerous animal and enclosure classes. All animals are derived from BaseAnimal. All enclosure are derived from BaseEnclosure. Both base classes provide various concrete methods - but also some abstract methods.

Now, when implementing a CatEnclosure, I want to specify that when CatEnclosure.resettleTo(Enclosure) is called, only one cat enclosure can be passed. In my current code, a cat could also be placed with a dog.

To my understanding, I would have to define the class of the future (derived) class when creating the abstract method resettleTo in the BaseEnclosure class.

My idea was to use a second generic. So BaseEnclosure<A> becomes BaseEnclosure<E, A>. But now I would also have to specify that E must be derived from BaseEnclosure. In addition, of course, A should also be of the BaseAnimal type.

So I get: BaseEnclosure<E extends BaseEnclosure, A extends BaseAnimal>

My IDE now complains that BaseEnclosure and BaseAnimal are raw types. If I write BaseEnclosure<E extends BaseEnclosure<?,?>, A extends BaseAnimal<?,?>>, it works. However, I don't know whether all of this makes sense in terms of design.

I look forward to your suggestions.

Enclosed you get the example code.

public abstract class BaseAnimal<E> {
        protected E enclosure;
       
        public void setEnclosure(E enclosure) {
                this.enclosure = enclosure;
        }
       
        public E getEnclosure() {
                return enclosure;
        }
       
        public abstract String getNoise();
}

public abstract class BaseEnclosure<A> {
        protected List<A> animals = new ArrayList<A>();
       
        // some methods...
       
        public List<A> getAnimals() {
                return animals;
        }
       
        public abstract void resettleTo(BaseEnclosure other);
}

public class Cat extends BaseAnimal<CatEnclosure> {
 
        @Override
        public String getNoise() {
                return "miiiaaauu";
        }
}

public class CatEnclosure extends BaseEnclosure<Cat>{
 
        @Override
        public void resettleTo(BaseEnclosure other) {
                // hm...
        }
}

public class Dog extends BaseAnimal<DogEnclosure> {
 
        @Override
        public String getNoise() {
                return "wuff";
        }
}

public class DogEnclosure extends BaseEnclosure<Dog>{
 
        // some methods...
       
        @Override
        public void resettleTo(BaseEnclosure other) {
                // hm...
        }
}

public class Main {
       
        public static void main(String[] args) {
                DogEnclosure doghouse = new DogEnclosure();
                Dog dog = new Dog();
                // later: JPA
                doghouse.getAnimals().add(dog);
                dog.setEnclosure(doghouse);
               
                CatEnclosure catbox = new CatEnclosure();
                Cat cat = new Cat();
                // later: JPA
                catbox.getAnimals().add(cat);
                cat.setEnclosure(catbox);
               
                // OHOHOH!!!
                doghouse.resettleTo(catbox);
        }
}

There is no question asked here. If you can post complete, working code that you're looking to improve, you can try codereview.stackexchange.com — jaco0646
– jaco0646, Commented Oct 20, 2020 at 22:46

Prasanna · Accepted Answer · 2020-10-21 11:19:53Z

I want to specify that when CatEnclosure.resettleTo(Enclosure) is called, only one Cat Enclosure can be passed. In my current code, a cat could also be placed with a dog.

Assuming here, you dont want CatEnclosure to resettleTo DogEnclosure.

Your scenario is a typical case of circular reference in generics. Based on this post, you need to redefine your base classes as follows:

public abstract class BaseAnimal<A extends BaseAnimal<A, E>, E extends BaseEnclosure<E, A>> {...}

public abstract class BaseEnclosure<E extends BaseEnclosure<E, A>, A extends BaseAnimal<A, E>> {...}

class Dog extends BaseAnimal<Dog, DogEnclosure> {...}
class DogEnclosure extends BaseEnclosure<DogEnclosure, Dog> {...}
// Similarly Cat and CatEnclosure

Now, to prevent cat enclosure resettling to dog enclosure, you need to change the resettleTo method signature as below:
public abstract void resettleTo(BaseEnclosure<E,A> other);

You will not be allowed to compile the below code:

CatEnclosure catEnclosure = new CatEnclosure();
Cat c = new Cat();
c.setEnclosure(catEnclosure);

DogEnclosure dogEnclosure = new DogEnclosure();
Dog d = new Dog();
d.setEnclosure(dogEnclosure);

catEnclosure.resettleTo(dogEnclosure); // Error type mismatch

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