Question

Consider the following code:

interface IFace {}

abstract class Supertype {}

class Subtype1 extends Supertype implements IFace {}
class Subtype2 extends Supertype implements IFace {}
class Subtype3 extends Supertype {}

class Foo {
    //Contains elements of Subtype1 and Subtype2
    List<IFace>     ifaceList = new ArrayList<IFace>();

    //Contains elements of Subtype1, Subtype2, and Subtype3
    List<Supertype> superList = new ArrayList<Supertype>();   

    void CopyItem() {
        superList.add( (Supertype) ifaceList.someElement() );
    }
}

Is it safe to cast an IFace element to Supertype if I know that only Subtypes will implement IFace? Is it even possible to ensure that only Subtypes will implement IFace?

I’m trying to use IFace as a marker interface to keep only certain Subtypes in the first list and allow any Subtypes in the second list.

Answer 1

Is it safe to cast an IFace element to Supertype if I know that only Subtypes will implement IFace?

Yes.

Is it even possible to ensure that?

If you mean “is it possible to ensure that only subclasses of Supertype implement IFace” – no. An interface can be implemented by anything.

If you mean “is it possible to ensure that the cast will succeed” – yes, you can use instanceof before casting.