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Question

Hello,
does somebody know why I can't compile this:
public interface SuperInterface{}
public class SubClass implements SuperInterface{}
public class SomeClass{
ArrayList<SubClass> list;
public ArrayList<SuperInterface> someMethod(){
return list;
I get an error "incompatible types"
Thanks
Greetrz vuystie

Answer 1

Let me ask a question from outside the programmer domain: "Is a dog collar an animal collar?"
You might say, "Sure. An animal collar is a collar for some kind of animal. A dog is certainly some kind of animal, so a dog collar is an animal collar."
Suppose I clarified thus: "An animal collar is a collar for any kind of collar. To be called an 'animal collar' It would have to work on a dog, a cat, or a goldfish." Now that's strange definition of "animal collar", but it clearly implies that a dog collar cannot be assumed to be an animal collar.
The world is full of containers and accessories that are associated with broader or narrower categories of objects. There are boxes for household goods and boxes for books. There are brushes for painting walls and brushes for painting unsealed concrete walls. There are jacks that will lift any automobile and jacks that will lift a Mazda Miata.
In Java (remember Java), this distinction (as Szobel pointed out) is expressed like this: Jack<Auto> j ;           // this jack can lift any model of automobile
Jack<? extends Auto> j ; // this jack can lift some model of automobile
Collar<Animal> c ;           // this collar fits any breed of animal
Collar<? extends Animal> c ; // this collar fits some breed of animalAnd the distinction is very important in programming. Your someMethod, is it supposed to return a list that contains only Superinterface objects? Or is is supposed to return a list into which you can put any Superinterface objects you want? You declared the method as the latter and then implemented it as the former.
Does this help at all?