Design Patterns and Frameworks – Structural Patterns: Summary

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Design Patterns and Frameworks – Structural Patterns: Summary

Oliver Haase

Oliver Haase Emfra — Structural Patterns: Summary 1/9

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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:

I Level of Indirection for the access to the actual object

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

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Adapter vs. Bridge

Differences:

I Purpose:

I Adapterintends to match an implementation with a (different) interface

I Bridgeintends to separate implementation from abstract to enable both to evolve separately

I Time of application:

I Adapteris employed rather later, i.e. when two existing types need to be brought together

I Bridgeis employed early, to foresee separate evolution of abstraction and implementation

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Composite vs. Decorator

Commonalities

I Structures of composite and decorator very similar (both use recursion to structure hierarchies of objects

I Decorator structure might be mistaken for degenerated composite structure

→ composite builds tree, decorator builds chain of objects

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Composite vs. Decorator

Differences are in their purpose:

I Purpose:

I Decoratorintends to add functionality to a type without changing it

→avoids expontential explosion of number of classes

I Compositeintends to treat leaves and inner nodes (container objects) of an object hierarchy homogeneously

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Proxy vs. Decorator

Commonalities

I Similar structures, in both cases indirect access to actual object via upstream object

I 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:

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

I With proxy pattern, proxy implements key functionality, with decorator pattern, functionality is split across levels of indirection.