Class 39: Design Patterns (4): Structural Patterns

Held: Friday, 3 December 2004

Summary: Today we consider patterns that support interesting object-oriented structures.

Related Pages:

• EBoard.
• Summary of Patterns.

Notes:

• Swing lessons this weekend!

Overview:

• Builders, revisited
• Basic wrapper patterns
• Group wrapper patterns
• Other wrapper patterns
• Other creational patterns

Builders, Revisited

• Note that Builders may relate to the way that we typically describe lists.
• Consider a form of read-only lists. You create the list and then can only iterate it.
• Note that we have many possible implementations of lists.
• How do we decouple the potentially complex creation steps from the particulars of the implementation?

Structural Patterns

• Structural patterns build structures.
• Many of these structures involve objects "wrapping" other objects.
• Some don't.

Primary Wrappers

• There are two primary wrapper patterns: Adapter and Bridge
• Adapters wrap existing classes to meet new interfaces
• Bridges handle overlapping hierarchies.

Group Wrappers

• There are two patterns that wrap groups of objects: Composite, and Facade
• Composite wraps groups of similar objects into a single object.
• Facade wraps groups of dissimilar objects into a single object.

Additional Wrappers

• There are two additional patterns that I think of as wrappers: Decorator, and Proxy.
• Decorator wraps an object to add functionality.
• Proxy wraps an object to permit decoupling.

Other Structual Patterns

• There is one additional structural pattern: Flyweight
• Flyweight simply says "Sometimes you want to share objects by moving some of the information into the owning object."