I have a project in which certain data has invariants which are not enforceable using language constructs, but I've intended to write my code such that they are maintained. If they are for some reason broken, it means that my code is buggy. I have some sanity check code which can discover if these invariants have been broken, but I'm not sure what is the canonical Java approach to responding to such a condition - is there some standard exception that I should throw? Use an assert?
Note that this is an issue of a value being set incorrectly at some point. The error itself does not occur at the time of the sanity check, but rather it occurred in the past, and the sanity check is just now discovering it (ie, it's not bad that the sanity check itself is running, it's just bad that the check failed).
Throw an IllegalStateException. It is meant specifically for purposes like this.