CODEWITHSUNDEEP
javagenerics
Andrey
Andrey

Reputation: 23

Problem with undestanding generics for class and for method

I cant undestand why line for (Integer integer : genericClass.getList()) { have compile error "Incompatible types: requried Object, found Integer return value for getList is concrete and not depends of T

If I change for (GenericClass genericClass : list) { to for (GenericClass<String> genericClass : list) { error has gone

Why?

public class Test {
    public static void main(String[] args) {
        List<GenericClass<String>> list = new ArrayList<>();
        for (GenericClass genericClass : list) {
            for (Integer integer : genericClass.getList()) {
                System.out.println(integer);
            }
        }
    }

    private class GenericClass<T extends String> {
        private List<Integer> list = new ArrayList<>();

        private List<Integer> getList() {
            return list;
        }
    }
}

Upvotes: 2

Views: 61

Answers (4)

Mayur Kamaliya
Mayur Kamaliya

Reputation: 1

This line for (GenericClass genericClass : list) { gives error because you've declared GenericClass as private class GenericClass<T extends String>.

So, whenever you've to create the object of this class you've to declare it as;
GenericClass<String> genericClass. You've to provide the type.

Upvotes: 0

IfOnly
IfOnly

Reputation: 540

In your first for loop you didn't mentioned type. Try this

for (GenericClass<String> genericClass : list) {
    for (Integer integer : genericClass.getList()) {
        System.out.println(integer);
    }
}

Upvotes: 0

Igoranze
Igoranze

Reputation: 1486

Your

    private List<Integer> getList() {
        return list;
    }

Is private, but you are trying to call it anyway, did you override the default if there is any. Else set it to protected or public.

Upvotes: 0

Nicholas K
Nicholas K

Reputation: 15423

It works because earlier GenericClass genericClass is equivalent to GenericClass<Object> and Object doesn't extend String which is final.

Upvotes: 1

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