CODEWITHSUNDEEP
javagenericscollections
Jake
Jake

Reputation: 4660

Interfaces, Parameterized Types and Collections

This is probably a java 101 question. But I've been away from java for ten years, so it's new to me.

  1. I have 3 classes: Dog, Cat, Mouse

  2. Each has its own ArrayList e.g., ArrayList<Dog> dogs = new ArrayList<Dog>();

  3. Dog Cat and Mouse implement AnimalInterface (which has methods such as getFurColor() and setFurColor(Color c)).

  4. I have a method called, say, changeFurColor(ArrayList <AnimalInterface>list).

  5. Moreover, changeFurColor() sorts the input ArrayList using a comparator that implements <AnimalInterface> so I need this parameterized type in my changeFurColor() method.

  6. I call the method with changeFurColor(dogs);

However, this won't compile. The type <Dog> does not match the type <AnimalInterface> even though the former implements the latter.

I know I can simply use ? as the type for the changeFurColor argument and then cast or do instance of within the method as a check, but then I can't sort the list with my comparator (and it seems silly to have 3 different comparators).

I can't type all ArrayLists with <AnimalInterface> because I don't want to risk a dog in with the cats.

I am sure there is a simple solution, but none of my books provide it and I can't find it online

pseudoCode:

public interface AnimalInterface
{
    public Color getColor();
    ......
}

public class Dog implements AnimalInterface
{

}
public class Cat implements AnimalInterface
{

}

public void example()
{
    ArrayList<Dog> dogs = new ArrayList<Dog>();
    ArrayList<Cat> cats = new ArrayList<Cat>();
    ArrayList<Mouse> mice = new ArrayList<Mouse>();
    changeFurColor(dogs)

}

public void changeFurColor(ArrayList <AnimalInterface> list)
{
    ... ..

    Collections.sort(list, MyAnimalFurComparator);

}


public class MyAnimalFurComparator implements Comparator<AnimalInterface>
{

    @Override
    public int compare(AnimalInterface o1, AnimalInterface o2)
    {
        ...
    }
}

UPDATE changeFurColor(dogs) does not compile

Upvotes: 2

Views: 115

Answers (2)

Jake
Jake

Reputation: 4660

This is the correct answer (for neophytes that follow me)

Credit goes to Solitirios for his comment on the question.

public static <T extends AnimalInterface> void changeFurColor(ArrayList<T> list) –

Upvotes: 1

InvertedNetwork
InvertedNetwork

Reputation: 200

I don't see any issues in the design.I'd need the exact code to tell what's exactly wrong, but The possible errors are:

  1. You are not implementing the AnimalInterface method in the animal classes (Dog,Cat..), the method need's to be implemented explicitly:

    class Dog implements AnimalInterface{public Color getColor(){return new Color();}}

class Cat implements AnimalInterface{public Color getColor(){return new Color();}}

  2. You are passing a Comparator class, insteads of a instance in the Collections.sort method:

    Collections.sort(list, MyAnimalFurComparator);

    You need to change it to this:

    Collections.sort(list,new MyAnimalFurComparator());

  3. Also the compare method of your MyAnimalFurComparator should return an int

Upvotes: 0

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