Essence : Java Glossary

You have a separate class (the essence) which has a no-argument constructor and can be initialised incrementally, with setXXX methods, and then when you’re done you can make your product object in one step; you can piggyback validity checks on the final creation step. This avoids the problem of having invalid half-initialised objects floating about. The objects don’t graduate and go out into the world until they have passed the final exam and been transformed from students into graduates. Half-baked students can’t accidentally be treated as graduates.