I'm taking about checking precondition and throwing IllegalArgumentException if it is not met.

1. This is private method, so preconditions can be checked outside of it - this method on its own has a bit of logic.
2. In other checks we does not check this precondition. And BTW in checkstyle there is a way to ensure that no undersired tokens will come to use - method "getAcceptableTokens" controls this. By default it returns "getDefaultTokens" result, so only ENUM_CONSTANT_DEF can possibly come here. So this check is not needed.