Pattern Design Basic Design Pattern 4 — Questions and Answers
Question 1: What is the key difference between the Strategy and Template Method patterns?
- Strategy uses inheritance; Template Method uses composition
- Strategy changes the whole algorithm; Template Method changes only steps (Correct answer)
- Strategy works at compile time; Template Method works at runtime
- Strategy is structural; Template Method is creational
Correct answer: Strategy changes the whole algorithm; Template Method changes only steps
Strategy replaces an entire algorithm via composition, while Template Method fixes the algorithm's skeleton in a base class and lets subclasses override individual steps.
Question 2: The Chain of Responsibility pattern is most appropriate when:
- Multiple objects may handle a request and the handler isn't known a priori (Correct answer)
- A single object must coordinate multiple subsystems
- An algorithm must be split across multiple classes
- Objects must be notified of state changes
Correct answer: Multiple objects may handle a request and the handler isn't known a priori
Chain of Responsibility passes a request along a chain of handlers until one processes it, decoupling senders from receivers.
Question 3: Which pattern provides a way to access elements of an aggregate object sequentially without exposing its internal representation?
- Visitor
- Memento
- Iterator (Correct answer)
- Mediator
Correct answer: Iterator
The Iterator pattern abstracts traversal logic into an iterator object, so clients can loop over a collection without knowing its underlying structure.
Question 4: In the Mediator pattern, what role does the mediator object play?
- It caches expensive computations for multiple objects
- It centralizes communication between colleague objects (Correct answer)
- It provides a unified interface to subsystems
- It stores and restores object state
Correct answer: It centralizes communication between colleague objects
The Mediator encapsulates how a set of objects interact, promoting loose coupling by keeping objects from referring to each other directly.
Question 5: What is the intent of the Memento pattern?
- Share expensive object state across instances
- Capture and externalize an object's state so it can be restored later (Correct answer)
- Define a grammar for a language and interpret sentences
- Allow incompatible interfaces to work together
Correct answer: Capture and externalize an object's state so it can be restored later
The Memento pattern saves a snapshot of an object's internal state outside the object so it can be restored to that state later without violating encapsulation.
Question 6: Abstract Factory differs from Factory Method primarily in that Abstract Factory:
- Returns a single product; Factory Method returns a family of products
- Creates families of related products; Factory Method creates one product (Correct answer)
- Uses object composition; Factory Method uses class inheritance only
- Is always implemented as a Singleton; Factory Method is not
Correct answer: Creates families of related products; Factory Method creates one product
Abstract Factory produces families of related or dependent objects through multiple factory methods, while Factory Method defines one creation method for a single product type.
Question 7: Which pattern would best decouple a UI layer from business logic by having UI elements communicate only through a single coordinator?
- Observer
- Mediator (Correct answer)
- Facade
- Proxy
Correct answer: Mediator
The Mediator centralizes control and communication, so UI components don't reference each other directly, reducing coupling across the interface.
Pattern Design Basic Design Pattern 4
What is the key difference between the Strategy and Template Method patterns?