Instantiating a generic class in a generic class

Question

I have the following first generic class and its interface:

 public interface Generic1Interface<E extends Comparable> { .. } public class Generic1 <E extends Comparable> implements Generic1Interface<E> { .. //implementation of Doubly linked list using Node objects }

And the second generic class and its interface:

 public interface Generic2Interface<E extends Comparable> { .. } public class Generic2 <E extends Comparable> implements Generic22Interface<E> { Generic1Interface<E> list; //so nothing here but a reference to an object of type Generic1Interface<E> Generic2() { list = new Generic1(); }

Suppose we're working on a method inside the second class, and we try to instantiate a new instance of the second class, and name it "result", then try to access its Generic2 instance, it will give an error:

 public Generic2Interface<E> union (E a, E b) { Generic2Interface<E> result = new Generic2(); **result.list** = ....;

result.list will give an error: "list cannot be resolved or is not a field". I think there must be a workaround to instantiate a generic class in a generic class. Thanks.

EDIT: I need to conform to the interfaces in each implemented class. So as you can see the method union has to return object of type Generic2Interface that's why I declare result like this.

Isn't a cast to Generic2 necessary before you try to access a field defined in that class? As of now, result is of type Generic2Interface.. — Vasan
– Vasan, Commented Oct 17, 2017 at 11:39
If I read your code correctly you try to access the list field on an object of type Generic2Interface. This field is not defined on the interface, but on the Generic2 class. — Rob Obdeijn
– Rob Obdeijn, Commented Oct 17, 2017 at 11:39
@RobObdeijn Correct. I forgot to mention the reason I'm doing this. I have to conform to the interface so I need to deal with instances of type Generic2 but at the end of each method in this class I have to return object type Generic2Interface<E> — Othman H
– Othman H, Commented Oct 17, 2017 at 13:06

Vasan · Accepted Answer · 2017-10-17 11:44:50Z

1

You need a cast for result to Generic2 type. This works:

System.out.println(((Generic2)result).list);

Or this:

Generic2<E> result = new Generic2(); System.err.println(result.list);

answered Oct 17, 2017 at 11:44

Vasan

4,9564 gold badges22 silver badges41 bronze badges

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

6 Comments

Othman H Over a year ago

I tried that but didn't work. What I actually tried was: Generic2Interface<E> result = new Generic2(); then result = (Generic2) result; but still I couldn't access result.list.

Vasan Over a year ago

That's not what I've suggested. Set is from a different type hierarchy and isn't compatible with Generic2Interface. You need to use the type where you've defined the field list, which is only Generic2 and not any other type.

Othman H Over a year ago

I need to conform to the interfaces, so I can't declare result like this: Generic2<E> result = ...

Vasan Over a year ago

Not a problem, then you can use the first option i.e. wherever you need to access list, do a cast there to Generic2. The point still stands - you can only access list on a variable of type Generic2.

Othman H Over a year ago

I understood your point. I tried casting, but it appears that a simple cast like this is not working: Generic2Interface<E> result = new Generic2(); then result = (Generic2) result; I still couldn't access list.

|

Collectives™ on Stack Overflow

Instantiating a generic class in a generic class

1 Answer 1

6 Comments

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

6 Comments

Related