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:

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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.