Expert

Who, in the general case is responsible?

Assign a responsibility to the information expert -- the class that has the information necessary to fulfill the responsibility.

Creator

Who creates?

Assign class B the responsibility to create an instance of class A if one of the following is true:

1. B contains A
2. B aggregates A
3. B has the initializing data for A
4. B records A
5. B closely uses A

Controller

Who handles a system event?

Assign the responsibility for handling a system event message to a class representing one of these choices:

1. The business or overall organization (a façade controller).
2. The overall "system" (a façade controller).
3. An animate thing in the domain that would perform the work (a role controller).
4. An artificial class (Pure Fabrication) representing the use (a use case controller).

Low Coupling
(evaluative)

How to support low dependency and increased reuse?

Assign responsibilities so that coupling remains low.

High Cohesion
(evaluative)

How to keep complexity manageable?

Assign responsibilities so that cohesion remains high.

Polymorphism

Who, when behavior varies by type?

When related alternatives or behaviors vary by type (class), assign responsibility for the behavior—using polymorphic operations—to the types for which the behavior varies.

Pure Fabrication

Who, when you are desperate, and do not want to violate High Cohesion and Low Coupling?

Assign a highly cohesive set of responsibilities to an artificial class that does not represent anything in the problem domain, in order to support high cohesion, low coupling, and reuse.

Indirection

Who, to avoid direct coupling?

Assign the responsibility to an intermediate object to mediate between other components or services, so that they are not directly coupled.

Don't Talk to Strangers
(Law of Demeter)

Who, to avoid knowing about the structure of indirect objects?

Assign the responsibility to a client's direct object to collaborate with an indirect object, so that the client does not need to know about the indirect object. Within a method, messages can only be sent to the following objects:

• The this object (or self).
• A parameter of the method
• A attribute of self.
• An element of a collection which is an attribute of self.
• An object created within a method