Law of Demeter

• The law: A method should only use objects that are
  • instance fields of its class
  • parameters
  • objects that it constructs with new
• Shouldn't use an object that is returned from a method call
• Remedy in mail system: Delegate mailbox methods to mail system
  mailSystem.getCurrentMessage(int mailboxNumber);
  mailSystem.addMessage(int mailboxNumber, Message msg);
  . . .
• Rule of thumb, not a mathematical law