@SergeyZolotarev: stackoverflow.com/q/1303123/1835769

Related question, asked on SO, and my answer.

@SSECommunity: With only 2 upvotes as of today this answer is very easy to overlook. I highly recommend the Talk by Michael Feathers that has been linked in this answer.

@AntP: No credits needed. Go ahead and add it. :)

Heads Up: Youtube video linked. There is a deep synergy between good design and testing. In my experience, if you have trouble testing, the design needs fundamental improvement. This is how unit testing essentially facilitates good design.

I am not down-voting this answer but it is not really a good answer. By just throwing IllegalStateException or InvalidOperationException out of the blue, a user will never understand what he did wrong. These exceptions shouldn't become a bypass to fixing the design flaw.