Behavioral patterns are concerned with algorithms and the assignment of responsibilities between objects. Behavioral patterns describe not just patterns of objects or classes but also the patterns of communication between them. These patterns characterize complex control flow that's difficult to follow at run-time. They shift your focus away from flow of control to let you concentrate just on the way objects are interconnected.

Behavioral class patterns use inheritance to distribute behavior between classes. This chapter includes two such patterns. Template Method (325) is the simpler and more common of the two. A template method is an abstract definition of an algorithm. It defines the algorithm step by step. Each step invokes either an abstract operation or a primitive operation. A subclass fleshes out the algorithm by defining the abstract operations. The other behavioral class pattern is Interpreter (243), which represents a grammar as a class hierarchy and implements an interpreter as an operation on instances of these classes.

Behavioral object patterns use object composition rather than inheritance. Some describe how a group of peer objects cooperate to perform a task that no single object can carry out by itself. An important issue here is how peer objects know about each other. Peers could maintain explicit references to each other, but that would increase their coupling. In the extreme, every object would know about every other. The Mediator (273) pattern avoids this by introducing a mediator object between peers. The mediator provides the indirection needed for loose coupling.

Chain of Responsibility (223) provides even looser coupling. It lets you send requests to an object implicitly through a chain of candidate objects. Any candidate may fulfill the request depending on run-time conditions. The number of candidates is open-ended, and you can select which candidates participate in the chain at run-time.

The Observer (293) pattern defines and maintains a dependency between objects. The classic example of Observer is in Smalltalk Model/View/Controller, where all views of the model are notified whenever the model's state changes.

Other behavioral object patterns are concerned with encapsulating behavior in an object and delegating requests to it. The Strategy (315) pattern encapsulates an algorithm in an object. Strategy makes it easy to specify and change the algorithm an object uses. The Command (233) pattern encapsulates a request in an object so that it can be passed as a parameter, stored on a history list, or manipulated in other ways. The State (305) pattern encapsulates the states of an object so that the object can change its behavior when its state object changes. Visitor (331) encapsulates behavior that would otherwise be distributed across classes, and Iterator (257) abstracts the way you access and traverse objects in an aggregate.

Chain of Responsibility
Discussion of Structural Patterns