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Essence and Design Patterns

The following table relates design patterns to relative essence. Column 'GoF' refers to the page in the Gang of Four's "Design Patterns" book describing this pattern. Column 'Kind' represents the kind of pattern based on the GoF book. Column 'Essence' describes the correlation of the pattern with relative essence: "+" (presence of pattern increases relative essence), "0" (no direct relationship between pattern and relative essence), and "-" (presence of pattern decreases relative essence).

GoF Kind Design Pattern Essence Comment
page 87 Creational Abstract Factory - introduces a factory method (in addition to the necessary constructor)
page 97 Creational Builder - decouples construction by going via builder's methods
page 107 Creational Factory Method - introduces a factory method (in addition to the necessary constructor)
page 117 Creational Prototype - introduces a clone method (in addition to the necessary constructor)
page 127 Creational Singleton - introduces a method (in addition to the necessary constructor)
page 139 Structural Adapter - introduces delegation methods
page 151 Structural Bridge - decouples "interface" methods from "implementation" methods, causing extra indirection
page 163 Structural Composite 0 this is more about using polymorphism to treat atomic and composite objects the same, not necessarily about introducing or eliminating indirection
page 175 Structural Decorator - replaces inheritance with delegation
page 185 Structural Facade - introduces indirection via facade's methods
page 195 Structural Flyweight 0 this is more about reducing the number of instances and about introducing extra parameters to pass in extrinsic state than about introducing or eliminating indirection
page 207 Structural Proxy - introduces indirection via proxy's methods
page 223 Behavioral Chain of Responsibility (-) 0 may introduce an extra level of indirection, e.g. by calling a method invoking the next handler in a chain; however, this pattern seems to be more about replacing iteration with recursion than introducing or eliminating indirection
page 233 Behavioral Command - create a class to represent a method, may directly introduce indirection (command's execute method calls original method), also adds constructor and possibly accessors
page 243 Behavioral Interpreter - create a separate class for each language rule, may directly introduce indirection (separating monolithic interpret method into many interpret methods), also adds a constructors and possibly accessors
page 257 Behavioral Iterator - decouples algorithm from collection, adding next and hasNext methods
page 273 Behavioral Mediator - introduces indirection via mediator
page 283 Behavioral Memento (-) (0) this is more about an extra feature (being able to externalize state), not necessarily about introducing or eliminating indirection
page 293 Behavioral Observer - introduces indirection through extra notify methods
page 305 Behavioral State - introduces delegation to state
page 315 Behavioral Strategy - introduces delegation to strategy
page 325 Behavioral Template Method - breaks algorithm into smaller steps that can be factored out into template methods
page 331 Behavioral Visitor - double dispatch requires two methods (visit and accept)

Analysis

Of the 23 GoF design patterns, none of them should imply a higher relative essence. Only 4 patterns are mostly uncorrelated with relative essence. The vast majority (19) represent a design with a low relative essence.