Chapter 07 — Structural Design Patterns

Adapter — Key Points 4

1. Object adapter uses composition; class adapter uses inheritance where supported
2. Changes the interface, not the core responsibility
3. Common at legacy and third-party boundaries
4. A two-way adapter implements both interfaces so either side can use the object

Bridge — Key Points 4

1. Prevents subclass explosion across two independent dimensions of change
2. The abstraction delegates implementation-specific work to an implementor
3. m abstractions × n implementations becomes m + n classes instead of m × n
4. Refined abstractions extend one side without touching the other

Composite — Key Points 4

1. Clients treat leaves and composites uniformly — no conditionals
2. Recursive operations make tree processing simple
3. Needs care with ownership, cycles and traversal policy
4. Participants: Component, Leaf, Composite, Client

Composite — Transparent vs Safe 2

1. Transparent — child management lives on the Component; uniform but leaves expose meaningless operations
2. Safe — child management lives only on the Composite; type-safe but clients must check types

Decorator — Key Points 4

1. The decorator shares the component's interface and holds a reference to it
2. It adds behavior before, after or instead of forwarding
3. Composes features without subclass explosion
4. Order of wrapping can change behavior; many small wrappers complicate identity and debugging

Facade — Key Points 4

1. Simplifies an existing interface without adding functionality
2. Reduces client coupling but does not forbid direct subsystem access
3. Often the natural entry point of a layer
4. Risk: it grows into an all-knowing god object

Proxy — The Four Kinds 4 + 1

1. Remote — represents an object in another address space
2. Virtual — defers creating an expensive object until it is needed
3. Protection — checks access rights before forwarding
4. Caching — stores results of expensive calls
5. Smart reference — adds reference counting, lazy loading or locking

Emphasized in This Course 3

The lecture's own summary table names these three as the structural patterns covered.

1. Adapter
2. Composite
3. Facade