Design Patterns Practice Exam

Design patterns are reusable solutions to commonly occurring problems in software design. They were popularized by the Gang of Four (GoF) in their 1994 book. They are not code, but templates for solving problems that can be adapted to many situations.

Benefits:

• Provide a shared vocabulary among developers
• Promote proven, tested solutions
• Improve code maintainability and flexibility

The three categories are:

• Creational Patterns – Deal with object creation mechanisms (e.g., Singleton, Factory Method, Builder)
• Structural Patterns – Deal with object composition and relationships (e.g., Adapter, Decorator, Facade)
• Behavioral Patterns – Deal with communication between objects (e.g., Observer, Strategy, Command)

The Singleton pattern ensures a class has only one instance and provides a global point of access to it.

Key traits:

• Private constructor
• Static method to get the instance
• Lazy or eager initialization

Singleton drawbacks include:

• Introduces global state, making code harder to test
• Violates the Single Responsibility Principle (manages its own lifecycle)
• Thread safety issues if not implemented carefully
• Tight coupling to the Singleton class
• Often considered an anti-pattern when overused

The Factory Method defines an interface for creating objects but lets subclasses decide which class to instantiate.

abstract class Creator {
  abstract Product factoryMethod();
}

Benefits:

• Follows the Open/Closed Principle
• Decouples client code from concrete classes
• Subclasses control instantiation