The difference is that in one case, it is impossible to cause this error and in the other case (with contracts between methods) we now have to rely people on un

The difference is that in one case, it is impossible to cause this error and in the other case (with contracts between methods) we now have to rely people on understanding what they are doing. Looking hat Java's hashCode&equals, I think history has shown that it is maybe not a good idea to rely on that too much.