The Law of Demeter / Principle of Least Knowledge

• The purpose of the law of Demeter is lower cohesion
• The low cohesion shows that the design is of good quality
• The Law of Demeter for functions requires that a method m of an object a may only invoke the methods of the following kinds of objects
  • a itself
  • m's parameters
  • any objects instantiated within m
  • a's attributes
  • global variables accessible by a in the scope of m
• In particular, an object should avoid invoking methods of an object returned by another method
  • For many modern object-oriented languages that use a dot as a field identifier, the law can be stated simply as use only one dot
  • That is, the code a.m().n() breaks the law where a.m() does not
  • As an analogy, when one wants a dog to walk, one does not command the dog's legs to walk directly
    • instead one commands the dog which then commands its own legs
• Some violations of the Law of Demeter are harmless, although others are a sign that the public interface has not been segregated correctly or has not been segregated at all
• example

  1. customer.bicycle.wheel.tire();
  2. customer.bicycle.wheel.rotate();
  3. Object.keys(...).sort(...).join(',');
  • 1st and 2nd line violates the law of demeter
  • 3rd line is reasonable and does not violate the law of demeter

Avoiding violations

• how to avoid unnecessary dependency

• problem

  • a User who have Account and this account has a plan and User has discountedPlanPrice which violate the Law of Demeter

  abstract class User {
    private account: Account;
  
    discountedPlanPrice(coupon: Coupon) {
      return coupon.discount(this.account.getPlan().getPrice());
    }
  }
  
  class Account {
    private plan: Plan;
  
    getPlan() {
      return this.plan;
    }
  }

• solution

  • One of the common ways to remove such chains is by using delegation
    • A wrapper method encapsulates or hides knowledge that would otherwise be implemented in the message chain
    • Delegation is an effective technique to avoid Law of Demeter violations, but only for behavior, not for attributes
  • To avoid such problems, you need to think over the application architecture in advance

  class Account {
    private plan: Plan;
  
    discountedPlanPrice(coupon) {
      return coupon.discount(this.plan.getPrice());
    }
  }