Design Patterns 1

Question 1

a reusable solution to a
common problem that occurs in a given
context within software design.

Answer 1

design pattern

Question 2

• It captures best practices and proven strategies
  from experienced developers and architects.
• Inspired by architectural patterns in mature
  engineering fields (e.g., civil, automotive).

Answer 2

design pattern

Question 3

Why Use Design Patterns?

Answer 3

EHER

• Encourage reuse of design structures rather than
  writing code from scratch.
• Help in creating systems that are scalable,
  flexible, and maintainable.
• Establish a common vocabulary for developers
  (e.g., “Use a Singleton here”).
• Reduce design errors by reusing well-tested
  solutions.

Question 4

• Introduced by Gamma, Helm, Johnson, and
  Vlissides in their 1995 book “Design Patterns:
  Elements of Reusable Object-Oriented
  Software.”
• Cataloged 23 patterns classified into three
  categories: Creational, Structural, and
  Behavioral.

Answer 4

The “Gang of Four” (GoF) Design Patterns

Question 5

Gang of Four Design Patterns was introduced by

Answer 5

Gamma, Helm, Johnson, and Vlissides in their 1995 book “Design Patterns: Elements of Reusable Object-Oriented Software.”

Question 6

The “Gang of Four” (GoF) Design Patterns Cataloged 23 patterns classified into three

Answer 6

CSB

Creational,
Structural, and
Behavioral.

Question 7

Essential Elements of a Design Pattern

Answer 7

Pattern Name
Problem
Solution
Consequences

Question 8

Unique identifier to facilitate communication (essential elements of design pattern)

Answer 8

Pattern Name

Question 9

Describes the scenario where the pattern applies (essential elements of design pattern)

Answer 9

Problem

Question 10

Abstract design, note code, often expressed in UML (essential elements of design pattern)

Answer 10

Solution

Question 11

Results and trade-offs of applying the pattern (essential elements of design pattern)

Answer 11

Consequences

Question 12

Types of Design Patterns

Answer 12

CSB

• Creational patterns
• Structural patterns
• Behavioral patterns

Question 13

Focus on object creation mechanisms, trying
to create objects in a manner suitable to the
situation.

Answer 13

Creational Patterns

Question 14

Creational Patterns Examples

Answer 14

SFABP

Singleton
Factory Method
Abstract Factory
Builder
Prototype

Question 15

Ensure only one instance of a class exist (CP)

Answer 15

Singleton

Question 16

Let subclasses decide which class to instantiate (CP)

Answer 16

Factory method

Question 17

Interface for creating families of related objects (CP)

Answer 17

Abstract Factory

Question 18

Construct complex objects step-by-step (CP)

Answer 18

Builder

Question 19

Clone an object to create a new one (CP)

Answer 19

Prototype

Question 20

Concerned with composing classes and
objects to form larger structures.

Answer 20

Structural Patterns

Question 20

Structural Patterns Examples

Answer 20

ABCDFFP

Adapter
Bridge
Composite
Decorator
Facade
Flyweight
Proxy

Question 21

Converts one interface into another (SP)

Answer 21

Adapter

Question 22

Separates abstraction from implementation. (SP)

Answer 22

Bridge

Question 23

Treats individual objects and compositions uniformly (SP)

Answer 23

Composite

Question 24

Adds responsibilities to objects dynamically (SP)

Answer 24

Decorator

Question 25

Provides a simplified interface to a complex subsystem (SP)

Answer 25

Facade

Question 26

Reduces memory usage by sharing objects (SP)

Answer 26

Flyweight

Question 27

Acts as a placeholder for another object (SP)

Answer 27

Proxy

Question 28

Deal with communication between objects, and the delegation of responsibility.

Answer 28

Behavioral Patterns

Question 29

Behavioral Patterns Examples

Answer 29

COSTV Command Observer Strategy Template Method Visitor, Mediator, Memento, Chain of Responsibility, Interpreter, State (VMMCIS)

Question 30

Encapsulates a request as an object (BP)

Answer 30

Command

Question 31

Notifies dependents of changes to an object (BP)

Answer 31

Observer

Question 32

Defines a family of interchangeable algorithms (BP)

Answer 32

Strategy

Question 33

Defines skeleton of algorithm; subclasses define steps (BP)

Answer 33

Template Method

Question 34

Problem: * Ensure that a class has only one instance and provide a global point of access to it. * This is useful when exactly one object is needed to coordinate actions across the system—such as in the case of a configuration manager, logging service, or database connection handler.

Answer 34

Singleton Pattern

Question 35

Solution: * Make the class responsible for keeping track of its sole instance. * Restrict instantiation of the class using private or protected constructors (in Python, simulate this using naming conventions and internal logic). * Provide a static access method (e.g., get_instance()) that returns the same instance every time it is called.

Answer 35

Singleton Pattern

Question 36

Singleton Pattern Advantages

Answer 36

Consequences: Advantages: * Controlled access to the sole instance. * Reduced memory usage (only one object is created). * Useful for shared resources like logging or configuration.

Question 37

Singleton Pattern Advantages Disadvantages

Answer 37

Disadvantages: * Difficult to unit test due to global state. * Can introduce hidden dependencies across the system. * May violate the Single Responsibility Principle by controlling its own lifecycle.

Question 38

Problem: * You have an existing class with a useful implementation, but its interface doesn't match the one a client (or another part of your code) expects

Answer 38

Adapter Pattern

Question 39

Solution: * Create an adapter class that translates the interface of the existing class into the interface the client expects. * The adapter sits in between and "converts" requests from the client into a form the adaptee can understand, and then may also convert the response back.

Answer 39

Adapter Pattern

Question 40

Adapter Pattern Advantages

Answer 40

Advantages: * Allows reuse of existing functionality without modifying original code. * Promotes loose coupling between client code and legacy/third-party code. * Makes systems more flexible and extensible.

Question 41

Adapter Pattern Disadvantages:

Answer 41

Disadvantages: * Adds an extra layer of complexity. * Overuse can lead to too many layers or overly abstract systems. * If behavior translation is complex, the adapter itself can become bloated.

Question 42

Problem: You want to decouple the object that issues a request from the object that knows how to perform it. This is especially useful when you need to support operations like undo/redo, queuing requests, or logging actions.

Answer 42

Command Pattern

Question 43

Solution: Encapsulate a request as an object, separating the command execution from the command invocation. Each command object implements a standard interface (e.g., execute()), and contains all the details needed to perform an action.

Answer 43

Command Pattern

Question 44

Command Pattern Advantages

Answer 44

Advantages: * Decouples sender and receiver of a request. * Makes it easy to add new commands without changing existing code. * Supports undo/redo, logging, and transaction-based operations.

Question 45

Command Pattern Disadvantages

Answer 45

Disadvantages: * Can introduce a large number of small command classes. * Code can become harder to follow due to indirection.