Question

I’m trying to create a container that would accept specific classes only. Using generics as follows:

    static class Test1<C extends Test1> {
            C field = null;
            public C getField() {
                return field;
            }
            public void setField(C field) {
                this.field = field;
            }
        }
        static class Test2 extends Test1{
        }

class MainTest {
            public static void main(String[] args) throws Exception {
                List<Test1<? extends Test1>> list = new ArrayList<Test1<? extends Test1>>();
                Test1<Test2> newInstance = new Test1<Test2>();
                list.add(newInstance);
                Test1<Test2> value = list.get(1);
    }
}

Just want to understand why this List> would accept a newInstance object, but compilation time error occurs while I’m fetching “Test1 value” back?
Is there any chance to workaround the problem?

Thanks in advance.

UPDATE
Many noticed that ” Test1<? extends Test1>, where ? MAY NOT be equal to Test2.”
I totally agree but as far as I understand in this case I need to type cast the extracted value to Test1<Test2> and that means that the whole point of generics is lost in this case… Please correct me if I’m wrong.

Answer 1

Because when you list.get(1) returns Test1<? extends Test1>, where ? MAY NOT be equal to Test2.

On the onther hand list.add(...) accepts Test1<? extends Test1>, i.e. Test1 with ANY generics parameter which is inheritor of Test1.