Design Patterns & RefactoringComparison of the Structural Patterns

Oliver Haase

HTWG Konstanz

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Magic Circle of Structural Patterns — Ok, Just Kidding...

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Magic Circle of Structural Patterns — Ok, Just Kidding...

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Adapter vs. Bridge

Commonalities:

Level of Indirection for the access to the actual object

Delegation from an interface that the actual object doesn’t provide

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Adapter vs. Bridge

Differences:

Purpose:

Adapter intends to match an implementation with a (different)interfaceBridge intends to separate implementation from abstract to enableboth to evolve separately

Time of application:

Adapter is employed rather later, i.e. when two existing types need tobe brought togetherBridge is employed early, to foresee separate evolution of abstractionand implementation

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Composite vs. Decorator

Commonalities

Structures of composite and decorator very similar (both userecursion to structure hierarchies of objects

Decorator structure might be mistaken for degenerated compositestructure→ composite builds tree, decorator builds chain of objects

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Composite vs. Decorator

Differences are in their purpose:

Purpose:

Decorator intends to add functionality to a type without changing it→ avoids expontential explosion of number of classesComposite intends to treat leaves and inner nodes (container objects)of an object hierarchy homogeneously

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Proxy vs. Decorator

Commonalities

Similar structures, in both cases indirect access to actual object viaupstream object

in both cases, upstream object maintains reference to actual object(subject) and delegates requests to it

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Proxy vs. Decorator

Differences are in their purpose:

Proxy is not about adding functionality, but about avoiding directaccess to the subject, for varying reasons (protection, efficiency,transparent remote access)

With proxy pattern, proxy implements key functionality, withdecorator pattern, functionality is split across levels of indirection.

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