Specific Generic Type Parameter in the method is accepting other Types

[murali2489]

Dear All,

I am reading Generics, below method is confusing. It clearly mentioned in the Method parameter that it should accept only List. But when i give ArrayList as Argument it compiles. It should be only List right? eventhough Arraylist is an implementaion class. below is the code PLease explain.

public class Quiz extends StringCheck {

    public Quiz() {
        // TODO Auto-generated constructor stub
    }


    public static void main(String[] args) {
        // TODO Auto-generated method stub

    ArrayList<Number> input =null;

            List<Number> output = null;

            output = process(input);

        }  


    static <E extends Number> List<E> process(List<E> nums){
        return nums;


    }


}

** Here the input argument is arraylist but not List as specified in the method but still it compiles !! but when i change the output to ArrayList opposing the generic type mentioned in the method it throws error **

[JamesCherrill]

ArrayList implements List, so every ArrayList is also a List.

Can you post the exact syntax you used for the error version?

[murali2489]

Please have a look of the below Code which throws complile error

I just Changed the "Output" Variable from List to ArrayList.

public class Quiz extends StringCheck {

    public static void main(String[] args) {
        // TODO Auto-generated method stub

            ArrayList<Number> input =null;

            ArrayList<Number> output = null;

            output = process(input);

       }  


    static <E extends Number> List<E> process(List<E> nums){
        return nums;


    }


}

** Error is below **

Type mismatch: cannot convert from List<Number> to ArrayList<Number>

My doubt is, The code throws error if i change the output variable from List to ArrayList, why its not throwing error for Input Variable which is also an ArrayList

[JamesCherrill]

Input: you need a List. You give it an ArrayList which is a subclass All ArrayLists are Lists, so it's OK
Output: You need an ArrayList. YOu give it a List, which is a superclass. Not all Lists are ArrayLists (eg some are LinkedLists), so it's not OK

[murali2489]

Thanks James, Understood. This generic type is eating my Brain.

[JamesCherrill]

That problem is really nothing to do with generics. It's the same as

String s = "hello";
Object o;
o = s; // this is OK
s = o; // this isn't, even if it would be ok with this particular data
s = (String) o; // is OK, unless there was something at runtime that put a non-String into o

Maybe you are having a problem with the way the generics don't treat subclasses like the rest of Java. A type <T> doesn't include T's subclasses, for that you need some variant of <? extends T>. Most people have a problem with that.
To quote the Oracle tutorials:

In general, if Foo is a subtype (subclass or subinterface) of Bar, and G is some generic type declaration, it is not the case that G<Foo> is a subtype of G<Bar>. This is probably the hardest thing you need to learn about generics, because it goes against our deeply held intuitions.

[murali2489]

Thanks James
I think after practising for a while I will get a Hang of it.