Do not fold, spindle, or mutilate.
—Anonymous, instructions on computer punch cards
This chapter shows how to implement business rules in programming code. In a sense, this chapter is what the whole book has been building toward. Previous chapters showed how to choose objects, define business rules, and code objects using the collaboration patterns. This chapter shows how to enforce real-world business rules with object methods organized by the collaboration patterns. Writing code without business rules is like solving physics problems without considering air resistance. This chapter considers air resistance. Sure, it takes a little more effort, but without it, your parachute would not work.