Question

Update: My classes are more complex than this, I just am stuck on the ArrayList line

I have the following classes:

class CatList {

    List<Cat> cats = new ArrayList<Cat>();

}

and

class DogList {

    List<Dog> dogs = new ArrayList<Dog>();

}

Where Cat and dog are both data classes.

but I want to create an abstract class:

abstract class AnimalList {

    List<???> animals;

    AnimalList(Class animal) {

        animals = new ArrayList<???>();

    }
}

so that I can inherit my classes

AnimalList CatList = new AnimalList(Cat);
AnimalList DogList = new AnimalList(Dog);
AnimalList CowList = new AnimalList(Cow);

Hopefully that makes more sense. My question is therefore What are the ??? bits?

Answer 1

Using a type parameter might solve your problem – using the same class for different types – without inheritance:

public class AnimalList<T> {
  private List<T> list = new ArrayList<T>();

  public void add(T animal) {
    list.add(animal);
  }
  // more methods
}

Now you can parametize instances for persons and animals:

AnimalList<Cat> catList = new AnimalList<Cat>();
catList.add(new Cat());
AnimalList<Dog> dogList = new AnimalList<Dog>();
dogList.add(new Dog());