Generic

[Michael Monchu]

Hi

I am new to Java and the forum, and I will be taking SCJP 6 exam on 23 November. I am using the most wonderful and interesting technical KS&BB book, and currently I am working on generics as is one of the areas that I really need to improve on. I have two classes i.e. Circle and Shape, where Circle is subclass of Shape. I tried to add a Shape object to the list declared in the below method , but got an error. I just need to find out why Shape cannot be added as is the supertype of Circle? My understanding is that List can take anything that is a Circle or a supertype of Circle.

public void addShape(List<? super Circle> shape){
        shape.add(new Circle());
        shape.add(new Shape());                // compile error
}

Your assistance in this regard will be highly appreciated.

[Ankit Garg]

Hi Michael, welcome to javaranch.

The method can accept List<? super Circle> which means the list can be of type Circle or a super type of Circle. Lets say I pass this method an ArrayList<Circle>, then how can the compiler allow you to add an instance of Shape (super class of circle) to the list??

[Deepak Bala]

Welcome to JR Michael.

The difference comes from what can be added to the list against what can be passed to the method that declares the list. Ankit's example is a good one to ponder upon

[Michael Monchu]

It's a pleasure Ankit,

I do understand it now, the confusion was around what the method can accept, and what can be added to the list declared. Thanks a lot for clarity.

Kind regards,
Michael