Programming Fundamentals

Part of the Interview Prep Roadmap. Coverage ratings are explained there.

Object-Oriented Programming

What problem OOP solves, the fundamentals of classes and objects, the four pillars, relationships between classes, and the keywords that tie it together.

Coverage: rich

Resources · Java OOP — W3Schools · OOP concepts — GeeksforGeeks

Motivation for OOP

• Problems OOP solves
• Difference from procedural programming

Practice

• Why OOP instead of procedural programming — Relisource
• Principles of OOP, and why we use it — Exabyting, with answer

Classes & Objects

• Identity, state, and behavior of an object
• Constructors
• Instance vs. object

Practice

• Which characteristics represent an object — Brain Station 23
• Constructor overloading, traced — Brain Station 23
• Which classes cannot be instantiated — Brain Station 23

Pillars of OOP

• Encapsulation
• Abstraction
• Inheritance
• Polymorphism

Practice

• Which is not a fundamental OOP concept — Brain Station 23
• The four principles, explained — Exabyting, with answer
• Overriding vs. overloading — bKash, with answer
• Method overloading rules — Brain Station 23

Class Relationships

• Inheritance (is-a)
• Composition (has-a)
• Aggregation
• Association
• Abstract classes vs. interfaces
• Upcasting and downcasting

Practice

• Inheritance, from first principles — bKash, with answer
• Which relationship is IS-A — Brain Station 23
• Abstract class vs. interface — WSD
• Identify the casting in this code — Brain Station 23
• What casting is demonstrated — Brain Station 23, with answer

Special Keywords & Gotchas

• static
• final
• this
• Multiple inheritance and the diamond problem

Practice

• What static means on a method — Brain Station 23, with answer
• Explain the static keyword — DSI
• Purpose of final — Brain Station 23, with answer
• final on variables and classes — WSD
• What this refers to — Brain Station 23, with answer
• Static counter output trace — Brain Station 23
• The diamond problem — Brain Station 23, with answer

Object-Oriented / Low-Level Design

Turning a word problem into a class design — identifying entities, modeling relationships between them, and handling systems with more than one kind of actor.

Coverage: thin

Resources · Awesome Low Level Design — free problem set with worked solutions, good to work through end to end

Modeling Entities & Relationships

• Identifying entities from a word problem (ParkingLot, Vehicle, Product)
• Entity attributes
• Relationships between entities (a Car is-a Vehicle; a VendingMachine has Products)

Practice

• Design a parking management system — Therap, with answer
• Design a vending machine — Therap, with answer

Role-Based System Modeling

• Multiple actors acting on the same data (admins, cashiers, checkers, users)
• Class design driven by actor responsibilities

Practice

• Ticket management system with five roles — Brain Station 23, with answer

Design Patterns & SOLID Principles

What a design pattern is and why they exist, the SOLID principles that guide good design, and Singleton as a concrete pattern built on top of them.

Coverage: thin

Resources · Refactoring.Guru — Design Patterns · Refactoring.Guru — SOLID

Design Pattern Fundamentals

• Reusable solutions to recurring design problems
• When to reach for a design pattern
• Creational, structural, and behavioral categories

Practice

• What design patterns are, and why they matter — Stamford University
• Choosing a pattern for an auth service — ShellBeeHaken, with answer

SOLID Principles

• Single Responsibility Principle
• Open/Closed Principle
• Liskov Substitution Principle
• Interface Segregation Principle
• Dependency Inversion Principle

Practice

• Describe SOLID with examples — Astha IT
• What are the SOLID principles — REVE Systems

Singleton Pattern

• Definition and purpose
• Implementation
• Thread-safety issues in the naive implementation

Practice

• Implement Singleton — Kite Games, full Java implementation plus a thread-safety warning
• Describe Singleton and write it in Java — REVE Systems
• Write and explain the Singleton pattern — REVE Systems
• Describe the Singleton pattern — WellDev