Thread: ArrayList implementation

Hi,

I need to do the following.

Suppose I have a class A and class B and C are subtypes of A.

I want to implement an ArrayList that contains subtypes of A. But I only want to implement this ArrayList using methods in class A.

I was thinking about using the bounded wildcard, but I could not get it to work.

So, I need something like:

public class MyArrayList<? extends A> extends ArrayList<? extends A> {
 
    /**
     * Return a subtype of A
     */
    @Override
    public <? extends A> get(int index) {
        // do something with methods only in A.
    }
 
    /**
     * Add a subtype of A.
     */
    @Override
    public boolean add(<? extends A> e) {
        // do something with methods only in A
    }
 
}

Also I want to override the methods get and add in ArrayList. So when I use MyArrayList, I expect to do the following.

 
MyArrayList<B> bList = new MyArrayList<B>();
MyArrayList<C> cList = new MyArrayList<C>();
 
bList.add(new B());
B b = bList.get(0);
 
cList.add(new C());
C c = cList.get(1);

I could not get this to work. Any ideas how this should be done? Or is this not a good design?

Originally Posted by KevinWorkman

Can't you just use an ArrayList of A?

ArrayList<A> list = new ArrayList<A>();

Then how will the get method now whether to return an object of the type B or C?

Originally Posted by KevinWorkman

It won't. It'll return an A reference, which you can cast to the appropriate type. If you really need to be able to return a B or C from an ArrayList, you're going to have to keep track of two ArrayLists.

But this means I need to implement MyArrayList two times for exactly the same functionality. Is there no other way?

I believe type casting indicates a bad design...

When adding an element to the ArrayList I know that it is either a B or a C. Before actually adding it I need to check if that B or C is already contained by the ArrayList. So ArrayList.add() which I want to override must invoke a ArrayList.contains or something.

Hmm... looking at the documentation of ArrayList the method add may not return false. So I can not know if it is added or not.

I think I need to think a little more about the design.

I moved responsibility from the list to the B and C class. Now the list does not need to know whether the A is in fact a B or C. So this question is no longer relevant.

Thanks for your help Kevin.