Object Design

Put Decisions Next to the Data They Need

A method can look perfectly reasonable while depending on far more knowledge than it should. Imagine checkout code that asks a customer object for membershipLevel, joinedAt, and totalOrders, then combines those values to decide whether the customer receives priority support. The calculation works, but checkout now knows both the customer’s data and the rule that gives that data meaning. When the rule changes, every caller that reconstructed it becomes a possible edit site.