Creating new object in abstract class in Java

Question

I have two objects which use really similar methods, save for one line. For example:

public class Cat extends Animal
public class Dog extends Animal

And they both use a breed method in the abstract class Animal. One calls new Dog(), and the other new Cat(). Right now I just have it declared as abstract public void breed(); in Animal, but is there a way I can generalize it so I don't have to make it an abstract method to be overridden?

Answer 1

There are many ways to do this, assuming by breed you mean "create children of me."

Reflection

First is to use reflection. If you have a no-args constructor for your classes, this is as easy as calling Class.newInstance:

public Animal breed() {
    try {
        return (Animal) getClass().newInstance();
    } catch (Exception ex) {
        // TODO Log me
        return null;
    }
}

If you don't have a no-args constructor in all your subclasses, you'll have to have a uniform constructor across all your subclasses. For example, if you have Cat(int, String) and Dog(int, String), then you need to get the constructor via Class.getConstructor and invoke newInstance on that:

return (Animal) getClass().getConstructor(int.class, String.class).newInstance(0, "Unnamed");

int and String here may be age and name, for example. This is how you do this with reflection.

Providers

Another way is to use this simple interface:

public interface Provider<T> {
    T create();
}

Then have your abstract class take an instance of this in its constructor:

public abstract class Animal {
    private final Provider<Animal> animalProvider;

    protected Animal( ... , Provider<Animal> animalProvider) {
        // ...
        this.animalProvider = animalProvider;
    }

    public Animal breed() {
        return animalProvider.create();
    }
}

Then your subclasses will pass a Provider<Animal> to the superclass which will create new instances of the subclass:

public class Dog extends Animal {
    public Dog( ... ) {
        super( ... , new DogProvider());
        // ...
    }

    private static class DogProvider implements Provider<Animal> {
        public Animal create() {
            return new Dog( ... );
        }
    }
}

Do the same for other subclasses as well.

Note: if by breed you mean "get the type of me," then you should edit your question to say so. If this is what you meant, then this is a viable solution:

public abstract class Animal {
    protected final Breed breed;

    protected Animal( ... , Breed breed) {
        // ...
        this.breed = breed;
    }

    public Breed getBreed() {
        return breed;
    }
}

I recommend following the get/set conventions for data container methods. Java has bean classes designed to handle these naming conventions, and it's more or less a standard across many platforms. For your subclasses:

public class Dog extends Animal {
    public Dog( ... ) {
        super( ... , new Breed( ... ));
        // ...
    }
}