Creating a TreeSet with a non-Comparable class: why a run-time exception, rather than compile-time error?

Question

If I create an arbitrary class that does not implement Comparable, and try to use it as a treeset, it throws an exception at run time when an object is inserted:

public class Foo {
}

public TreeSet<Foo> fooSet = new TreeSet<Foo>();
fooSet.add(new Foo()); // Throws a ClassCastException exception here: Foo is not comparable

I'm no Java expert, but something about this seemed dynamically typed (ala Python) in a way I wasn't expecting. Is there no way for TreeSet's implementation to specify that its generic type argument must implement Comparable so that this can be caught at compile-time? Non-generic functions can take interfaces as arguments; is the same not possible with generics?

Answer 1

TreeSet is implemented that way because you can alternatively provide a Comparator, in which case the elements don't need to be Comparable. The only way to support both behaviors without splitting the implementation into multiple classes was to include runtime checks - this was simply a design decision by the author(s) of that class.

Exposing factory methods for TreeSet instead of public constructors would've been a way to maintain compile time checks using stricter generic type constraints, but that would've been a break from the core collections API's convention of exposing public no-arg and copy constructors for its implementation classes. As you noted in your comment, Guava goes the factory route with its collections and IMHO is better off for it.