Design Patterns

Design Pattern Definition

A proven advice for solving a recurring class of problems by providing a template-like solution rather than a finished design.

Benefits of Design Patterns

Increase reusability, maintainability, clarity, and communication among developers.

Are design patterns language independent? Are they finished code?

They are language-independent and serve as conceptual templates, not finished code.

Architectural Design Pattern

A high-level solution that defines the structure of a system, dividing it into components/layers and specifying their communication rules.

Scope of Architectural Patterns

Consider the whole application, providing the first level of abstraction.

Common Architectural Patterns

Multi-Layer, MVC

Multi-Layer Pattern Definition

Divides an application into independent layers that provide services to their neighboring layers.

3-Layer (Multi layer) Architecture

Presentation layer, Business Rules layer, Data Management layer.

Presentation Layer Responsibilities

Handles UI/UX and interacts with the user.

Business Rules Processing Layer Responsibilities

Handles computations, logic, and application flow in Multilayer Architectural Design Pattern

Data Management Layer Responsibilities

Handles storage, retrieval, and database/file operations.

Do the layers all know each other?

Each layer knows only its neighbors, reducing tight coupling.

Benefits of Multi-Layer Pattern

Separation of concerns, fewer dependencies, readability, reusability, testability, maintainability.

What are the names of the MVC Pattern layers?

Divides an application into Model, View, and Controller to separate logic from UI.

Model (MVC) Responsibilities

Manages data, business rules, and state changes

View (MVC) Responsibilities

Displays information to the user and receives no direct data-manipulation ability.

Controller (MVC) Responsibilities

Handles user input, interprets View events, updates the Model.

MVC Interaction Flow

User → View → Controller → Model → View updates.

Benefits of MVC

Structural clarity, maintainability, reusable UI components, prevents direct user manipulation of data.

Can there be multiple Views in MVC with one model?

One model can update many views simultaneously.

Class Design Pattern

Lower-level design solutions that focus on interactions among a few classes.

What are the Three Types of Class Design Patterns?

Creational, Structural, Behavioral.

Singleton Pattern Definition

Ensures exactly one instance of a class exists at runtime and provides global access to it.

How Singleton Works

Private constructor, static instance variable, and a public static getter method.

Why Constructor Is Private in Singleton

Prevents creation of multiple instances.

Singleton Instance Lifetime

Instantiated once, usually when the class loads.

Composite Pattern

Treats individual objects and composite groups uniformly by having both implement the same interface/abstraction.

Why Composite Helps

Simplifies code, supports extension, and unifies behavior of single items and groups.

Composite Structure

Component interface + leaf classes + composite classes containing children.

Façade Pattern

Provides a simplified interface to a complex subsystem, exposing only necessary operations.

Problem Scenario that Façade solves

A client depends on many classes and sees methods it shouldn’t or doesn’t need.

Façade Solution

A wrapper class delegates tasks to subsystem classes but exposes a clean interface.

Façade Benefit

Reduces coupling, hides complexity, and improves client-side code clarity.

Strategy Pattern

Allows selecting an algorithm’s behavior at runtime by encapsulating each variant in its own class.

What is a Context Class in (Strategy)

Uses a Strategy interface reference and executes whichever strategy is assigned.

Template Method Pattern Definition

Defines the skeleton of an algorithm in a superclass method, leaving certain steps abstract for subclasses to implement.

What does the concrete method do in Template Method?

A concrete method calls one or more abstract methods and provide their own implementations to complete its algorithm.

Benefits of Template Method

Allows algorithm structure to remain fixed while subclasses customize steps.