Casting problems with ArrayList

Question

I can't figure out how to write this correctly so it will not generate compiler errors:

class AnElement implements IElement { ... }

protected ArrayList<AnElement> someElements() {
    ...
}

protected ArrayList<IElement> elements() {
    return (ArrayList<IElement>) someElements(); // this doesn't work...
    // return someElements();                    // ... neither does this
}

What's the correct way to do this?

hmm.. just realized the word Element is not the best for this example, it's just I had that as part of my actual class I use in my real code — ycomp, Sep 28 '15 at 23:44
Define _doesn't work_. What compiler errors? What do you think they mean? — Sotirios Delimanolis, Sep 28 '15 at 23:46
"Incompatible Types" error in the IDE. IDE has a big red line under the code. Required: ArrayList, Found: ArrayList — ycomp, Sep 28 '15 at 23:47
@SotiriosDelimanolis: You're right, that one's a better dupe-target. I'll reopen this, and you can reclose as a dupe of that? — ruakh, Sep 28 '15 at 23:53
Just for the future though: http://stackoverflow.com/questions/2745265/is-listdog-a-subclass-of-listanimal-why-arent-javas-generics-implicitly-p — Sotirios Delimanolis, Sep 28 '15 at 23:54

score 3 · Accepted Answer · answered Sep 28 '15 at 23:48

3

Generics are invariant so you could do

protected List<? extends IElement> elements() {
    return someElements(); 
}

answered Sep 28 '15 at 23:48

Reimeus

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Or even better, code to the interface and do `protected List someElements()` instead. – Mick Mnemonic Sep 28 '15 at 23:50
@MickMnemonic that probably wouldnt compile as compiler cant be sure what exact implementation of `IElement` is returned from `someElements`.....:) – Reimeus Sep 28 '15 at 23:59
It does compile with the original code. – Mick Mnemonic Sep 29 '15 at 00:01
In general, you should avoid returning wildcard types and instead _return the most specific type_. The amazing Angelika Langer [explains why](http://www.angelikalanger.com/GenericsFAQ/FAQSections/ProgrammingIdioms.html#FAQ303). (Might not make that much of a difference when `IElement` is an `interface`, but still.) – Mick Mnemonic Sep 29 '15 at 00:17
actually yes the base is an interface IElement.. but then there is AnElement implements IElement and ChildOfAnElement extends AnElement... _and more_. I simplified things in the original post. Basically all objects are guaranteed to be of IElement ... – ycomp Sep 29 '15 at 12:47

Casting problems with ArrayList

1 Answers1