What exception to throw when an important parameter/dependency is missing?

Question

Take this method

/**  * @return List of group IDs the person belongs to  *  */ public List<String> getGroups() {     if (this.getId().equals("")) return null; } 

I would like to throw exception instead returning null, what's the exception to throw when an important parameter/dependency has not been set?

Answer 1

I'd use IllegalArgumentException if the parameter/argument is controlled from outside, or IllegalStateException if the method is just called at a wrong moment (state). In your specific case I think it's the latter. A (dubious) alternative is NullPointerException.

This should however be explicitly documented in the @throws so that the user understands the reason.

Answer 2

How about IllegalStateException?