Kotlin: how to use multiple Generic in Class?

Question

i'm trying to convert this c# class to kotlin for android:

public class ChildItemCollection<P, T, C> : ICollection<T>
    where P : class
    where T : ChildItem<P>
    where C : class, ICollection<T>
{

    private P _parent;
    private C _collection;

    public ChildItemCollection(P parent, C collection)
    {
        this._parent = parent;
        this._collection = collection;
    }


...

}

public class ChildItemCollection<P, T> : ChildItemCollection<P, T, List<T>>
        where P : class
        where T : ChildItem<P>
    {
        #region Constructors
        public ChildItemCollection(P parent)
            : base(parent, new List<T>()) { }

        public ChildItemCollection(P parent, ICollection<T> collection)
            : this(parent)
        {
            foreach (T item in collection)
                this.Add(item);
        }
        #endregion Constructors
    }

I have tried many things without success.

I didn't understand how to use the "where" lines either.

where P : class looks wrong, class is a keyword. Take a look at the last code example: kotlinlang.org/docs/reference/generics.html#upper-bounds — Miha_x64
– Miha_x64, Commented Jun 6, 2017 at 15:09
Please provide the exact compilation error in your questions (this is probably why all the down votes happened) — voddan
– voddan, Commented Jun 7, 2017 at 6:02
@Miha_x64 where P : class is a perfectly valid constraint. It requires the generic parameter to be a reference type, so for example string or List<something> would work while int wouldn't. The opposite also exists, where P : struct, which requires it to be a value type. — Kanashimi
– Kanashimi, Commented Dec 9, 2022 at 9:41

hotkey · Accepted Answer · 2017-06-06 14:35:27Z

10

In Kotlin, you can specify the type parameter upper bounds at their declaration:

class ChildItemCollection<P, T : ChildItem<P>, C : Collection<T>> : ...

If you have multiple upper bounds, they should be specified separately with where, see another Q&A.

Also, Kotlin does not have a class upper bound, because there's no distinction between value types and classes.

edited Jun 6, 2017 at 14:35

answered Jun 6, 2017 at 14:27

hotkey

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2 Comments

SWAppDev Over a year ago

Thanks, you saved me ! Removing the "class" word made it work :-)

Kevin Over a year ago

My dude, it was surprising how hard it was to find this info. Thanks!

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