Java Backend Foundations

This repository tracks my learning of Java for backend development, progressing towards Spring Boot, microservices, and scalable systems.

---

📌 Objective

• Build strong core Java fundamentals
• Write clean, structured, and logical code
• Prepare for backend development using Spring Boot

---

📅 Day 01 – Java Basics & Control Flow

Topics Covered

• JDK setup and IntelliJ IDEA configuration
• Java program structure and execution flow
• Variables and data types
• Literals and type conversion

Operators:

• Arithmetic
• Relational
• Logical

Conditional Statements:

• if-else
• else-if
• ternary operator
• switch-case

Loops:

• while
• do-while
• for loop
• choosing the right loop

---

💻 Practice Work

• Implemented basic Java programs for each concept
• Solved exercises on:
  • Operators
  • Conditionals (grading logic)
  • Loops
• Built basic console logic (calculator-style program)

---

📂 Folder Structure

• day-01/
  • basics/
  • operators/
  • conditionals/
  • loops/

---

⚙️ Key Learnings

• Understanding how Java code is compiled and executed
• Writing clean and structured control flow logic
• Choosing appropriate loops based on use-case

---

📅 Day 02 – Arrays, Methods & Basic OOP

Topics Covered

Arrays:

• 1D arrays
• Multi-dimensional arrays
• Jagged arrays
• Array of objects
• Enhanced for loop

Methods:

• Method creation and usage
• Method overloading

Memory Concepts:

• Stack vs Heap

Classes & Objects:

• Class structure
• Object creation
• Basic encapsulation

---

💻 Practice Work

Array Problems:

• Second largest element
• Reverse array (in-place)
• Check if array is sorted
• Find duplicate elements
• Rotate array

Methods:

• Prime number checker
• Factorial
• Fibonacci
• Find max element

Classes:

• Student (average & grade calculation)
• BankAccount (deposit & withdraw logic)
• Book (price discount logic)

---

📂 Folder Structure

• day-02/
  • arrays/
  • methods/
  • classesAndObjects/
  • self-practice/

---

⚙️ Key Learnings

• Applying array concepts to solve real problems
• Writing reusable logic using methods
• Understanding how objects encapsulate data and behavior
• Identifying and correcting logical mistakes through practice

---

---

📅 Day 03 – Strings & Core OOP Concepts

Topics Covered

Strings:

• String creation and immutability
• Mutable vs Immutable strings
• StringBuffer and StringBuilder

OOP Concepts:

• Encapsulation
• Getters and setters
• this keyword
• Constructors (default and parameterized)

Static Concepts:

• Static variables
• Static methods
• Static block

Misc:

• Naming conventions
• Anonymous objects

---

💻 Practice Work

String Problems:

• Reverse string
• Palindrome check
• Count vowels and consonants
• Remove duplicate characters
• Compare == vs .equals()

OOP Practice:

• Encapsulation with validation (Student, BankAccount)
• Constructor usage (Book, Employee)
• Static keyword practice (Counter, utility class)
• Demonstrated this keyword and anonymous objects

---

📂 Folder Structure

• day-03/
  • strings/
  • encapsulation/
  • constructors/
  • static/
  • misc/

---

⚙️ Key Learnings

• Understanding string immutability and performance implications
• Applying encapsulation for data protection and validation
• Using constructors for proper object initialization
• Understanding shared behavior using static members
• Writing cleaner and more structured object-oriented code

---

📅 Day 04 – Inheritance, Polymorphism & Advanced OOP

Topics Covered

Inheritance:

• Need of inheritance
• Single and multilevel inheritance
• Multiple inheritance (conceptual in Java)

Core Concepts:

• this and super keyword
• Method overriding

Polymorphism:

• Compile-time vs runtime polymorphism
• Dynamic method dispatch

Java Structure:

• Packages
• Access modifiers

Advanced Concepts:

• final keyword
• Object class methods (equals, toString, hashCode)
• Upcasting and downcasting
• Wrapper classes

---

💻 Practice Work

• Completed quiz and coding exercises on OOP concepts
• Understood inheritance hierarchy and method overriding
• Explored polymorphism and dynamic method dispatch behavior
• Practiced usage of access modifiers and package structure
• Analyzed object comparison using equals and hashCode

---

📂 Folder Structure

• day-04/
  • inheritance/
  • polymorphism/
  • packages/
  • object-class/
  • misc/

---

⚙️ Key Learnings

• Understanding code reuse through inheritance
• Applying polymorphism for flexible and scalable design
• Managing access and structure using packages and modifiers
• Understanding object behavior through Object class methods
• Gaining clarity on type casting and wrapper classes

---

📅 Day 05 – Mini Project: Quiz Console Application

Topics Covered

• Applying OOP concepts in a real use-case
• Class design and object interaction
• Arrays of objects
• Basic input/output handling

---

💻 Practice Work

Mini Project:

• Built a console-based quiz application

Features:

• Question class with encapsulated fields
• QuizService to manage quiz flow
• Display multiple-choice questions
• Accept user input
• Calculate and display final score

---

📂 Folder Structure

• day-05/
  • quiz-app/
    • Question.java
    • QuizService.java
    • QuizConsoleApp.java

---

⚙️ Key Learnings

• Structuring a small application using multiple classes
• Managing data using arrays of objects
• Understanding flow control in real scenarios
• Identifying design limitations (tight coupling, input handling)
• Transitioning from concept-based coding to building small applications

---

🔁 Discipline Rule

• Code is written and pushed daily to maintain consistency and track progress