Question

How come I’m allowed to do this:

TreeSet<Object> treeSet = new TreeSet<Object>();
treeSet.add(new Object());

But not this:

final List<Object> objects = new ArrayList<Object>();
Collections.sort(objects);

The first one gives me an ClassCastException but the second one gives me a compile error. As I understand it the actual problem is the same in both cases: java.lang.Object does not implement the Comparable interface.

UPDATE:
Hmm for some reason this only applies for Java 7 and not 6.
Am I being stupid or tired? Could someone please shed some light on this?

UPDATE #2:
I do get different results depending on java versions. Please see picture:

Answer 1

Collections.sort requires a list of Comparable‘s. TreeSet doesn’t have that restriction in the typing. With the default constructor, the docs say it sorts by ” natural ordering of its elements”. What isn’t clear is how it would sort Object, but your problem deals with the typing.

Update:
I missed the last part of the question. Without looking at the stacktrace, I would guess that since the default constructor for TreeSet tries to sort by “natural order”, internally, it is doing a cast to Comparable, which would cause a ClassCastException.

Update:
I looked closer at the javadocs for TreeSet (JDK 6, JDK 7) and it says

Constructs a new, empty tree set, sorted according to the natural
ordering of its elements. All elements inserted into the set must
implement the Comparable interface. Furthermore, all such elements
must be mutually comparable: e1.compareTo(e2) must not throw a
ClassCastException for any elements e1 and e2 in the set. If the user
attempts to add an element to the set that violates this constraint
(for example, the user attempts to add a string element to a set whose
elements are integers), the add call will throw a ClassCastException.

for both. So, if no ClassCastException happens with JDK6, perhaps it is a bug.