Java using Generics

Question

I have an interface A like this:

public interface A{
    void myFirstMethod();
    void mySecondMethod();
}

And then I have this class:

public class MyClass{
    private List<? extends A> elements;

    public MyClass(){
        A obj = new A(){
            @Override
            public void myFirstMethod(){
              //SOME CODE
            }
            @Override
            public void mySecondMethod(){
              //SOME CODE
            }
        };
        elements.add(obj);
    }
}

I haven't use generics before (only things like List<String>...) so I can't see why this code doesn't compile. To be more precise, I get an error on the line elements.add(obj); that the method add is not applicable for these parameters.

EDIT: I've changed the code and now the elements.add(obj) compiles fine, but I have another problem.

public class MyClass{
    private List<A> elements;

    public MyClass(List<A> elements){
        this.elements = elements;
        elements.add(obj);
    }
}

When I try to do this, it doesn't compile.

//A1 implements A
List<A1> list = new ArrayList<A1>();
MyClass(list);

How could I fix this?

assylias · Accepted Answer · 2014-03-03 15:31:42Z

9

Imagine that you have two subclasses:

class A1 implements A {...}
class A2 implements A {...}

Then you could write:

List<? extends A> elements = new ArrayList<A1>(); // a list of A1

But then this should not compile:

elements.add(new A2()); //oops: an A2 is not an A1

For that reason, you can't add anything but null to a List<? extends A> because you don't know what actual generic type it is. In other words, ? extends A means a specific, but unknown, subtype of A (or A itself).

In your case, a List<A> would probably do what you expect - you would be able to add some A1s and some A2s to it.

edited Mar 3, 2014 at 15:31

answered Mar 3, 2014 at 15:26

assylias

330k84 gold badges680 silver badges806 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

Pablo Lozano Over a year ago

In other words: ? means "unknown", not "any".

Jesper Over a year ago

That's right; the notation List<? extends A> means that it's a List of some specific but unknown type. It does not mean that the list can contain a mixture of different types of objects that extend A; it means that the list contains objects of some definite type that extends A, but you don't know what the actual type is.

sakura Over a year ago

Just a follow up on it, I understood but just tried with something weird like, List<? extends A> elements = new ArrayList<A>(); This line is working, but again when I try to add an object of A1, elements.add(new A1()); it shows error. I can understand the error, but my question is why Java is allowing me at first place to write something like the first line.. new ArrayList<A>();

Kojotak Over a year ago

If you want to add something into generic collection of type A, use the List<? super A>.

Jesper Over a year ago

@sakura Yes, that won't work, exactly because of what we're explaining. The compiler looks at the type of the variable elements, which is List<? extends A>. From that type alone, the compiler doesn't know what the exact type is of the elements in the list; it only knows that they are of some unknown type that extends A. Since it doesn't know what the type really is, you can't add anything to the list - because it's impossible for the compiler to tell whether what you try to put in is of a valid type or not. It does not matter that you initialize your variable with new ArrayList<A>.

Collectives™ on Stack Overflow

Java using Generics

1 Answer 1

5 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

5 Comments

Your Answer

Sign up or log in

Post as a guest

Related