I have a superclass that is Comparable, but a grandchild is not

I am implementing an hierarchy with the class definitions as follows:

public interface SomeInterface<T extends SomeInterface<T>> extends Comparable<T> {...}

Then,

public abstract class SomeClass<T extends SomeClass<T>> extends SomeInterface<T>{...}

And finally,

public class SomeChild extends SomeClass<SomeChild>{...}

For a SomeChild s, s instanceof Comparable returns false.

I want to be able to call Collections.sort() on an ArrayList of SomeChilds, but running it throws a compile-time error of a ClassCastException because SomeChild cannot be cast to a Comparable and I'm scratching my head as to why. If a superclass extends Comparable, why can't its child?

The code you show will not compile. The snippet:

extends SomeInterface<T>

should be:

implements SomeInterface<T>

Once you make that change, this code:

SomeChild s = new SomeChild();
if (s instanceof Comparable)
    System.out.println("Yup!");

WILL print "Yup!" because SomeChild must be a Comparable since its ancestors are, as you expect. Is your s possibly null? If it were, then "Yup!" would not print because null is not an instance of anything, and so this code:

SomeChild s = null;
if (s instanceof Comparable)
    System.out.println("Yup!");

will not print anything.