Unit 1: Introduction to Java

Opening Hook — Java Runs Your India

Learning Outcomes — Bloom's Taxonomy Mapped

Concept Explanation — Introduction to Java from Scratch

1. History of Java

In 1991, a small team at Sun Microsystems led by James Gosling started "Project Green" — an initiative to build software for consumer electronic devices like set-top boxes and TVs. They needed a language that was platform-independent (worked on any chip), robust, and secure. C++ wasn't cutting it — too many memory bugs, too platform-dependent.

Gosling created a new language and named it "Oak" after an oak tree he could see from his office window. When they discovered "Oak" was already trademarked by another company, the team brainstormed over coffee (literally — Java coffee from Indonesia) and renamed it "Java".

2. Features of Java

Java became the world's most popular enterprise language because of these key features:

Java vs C vs C++ vs Python — Comparison Table

3. Java Program Structure

Every Java program follows a specific structure. Let's break it down piece by piece:

Java
// 1. Package declaration (optional) — organises classes into folders
package com.eduartha.demo;

// 2. Import statement — brings in pre-built classes
import java.util.Scanner;

// 3. Class declaration — every Java program must have at least one class
public class HelloWorld {

    // 4. Main method — the entry point where execution begins
    public static void main(String[] args) {
        // 5. Statements — actual instructions
        System.out.println("Hello, India! 🇮🇳");
    }
}

Keyword-by-Keyword Explanation

4. Writing Your First Java Program

Step 1: Write the Code

Open any text editor (Notepad, VS Code, or Notepad++) and type:

Java — HelloWorld.java
public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
        System.out.println("Welcome to Java Programming!");
        System.out.println("India's #1 Enterprise Language ☕");
    }
}

Step 2: Save the File

Save as HelloWorld.java — the file name MUST match the public class name exactly (case-sensitive).

Step 3: Compile

Open Command Prompt / Terminal and navigate to the file's folder:

Terminal
javac HelloWorld.java

This creates HelloWorld.class — the bytecode file that the JVM can execute.

Step 4: Run

Terminal
java HelloWorld

5. Command-Line Arguments

Command-line arguments let you pass data to your program when you run it — without using Scanner or hardcoding values. They arrive in the String[] args array.

Java — NamePrinter.java
public class NamePrinter {
    public static void main(String[] args) {
        System.out.println("Hello, " + args[0] + " from " + args[1] + "!");
        System.out.println("Welcome to Java programming.");
    }
}

Terminal
javac NamePrinter.java
java NamePrinter Rahul Mumbai

How it works: When you type java NamePrinter Rahul Mumbai, Java puts "Rahul" in args[0] and "Mumbai" in args[1]. The program name is NOT stored in args (unlike C).

6. JDK vs JRE vs JVM — The Java Trinity

Understanding the difference between JDK, JRE, and JVM is one of the most asked interview and exam questions. Let's break it down with an Indian analogy:

Relationship: JDK ⊃ JRE ⊃ JVM — JDK contains JRE, JRE contains JVM. Think of it as Russian nesting dolls (matryoshka).

7. Bytecode → ClassLoader → JIT Pipeline

Understanding what happens between typing javac and seeing output is crucial. Here's the complete pipeline:

Pipeline Summary:

Pipeline
HelloWorld.java  →  javac  →  HelloWorld.class (bytecode)
                                    ↓
                              ClassLoader
                                    ↓
                            Bytecode Verifier
                                    ↓
                             JIT Compiler
                                    ↓
                         Native Machine Code
                                    ↓
                              EXECUTION ✅

8. Installing Java — Setup Guide

Windows Installation

1. Go to oracle.com/java/technologies/downloads/
2. Download JDK 21 (LTS) for Windows x64 Installer
3. Run the installer → Next → Next → Install
4. Set Environment Variables:
   • JAVA_HOME = C:\Program Files\Java\jdk-21
   • Add %JAVA_HOME%\bin to PATH
5. Verify: Open CMD → type java -version and javac -version

Linux (Ubuntu/Debian) Installation

Terminal
sudo apt update
sudo apt install openjdk-21-jdk
java -version
javac -version

IDE Setup

9. Your First 5 Java Programs

Program 1: HelloWorld

Java — HelloWorld.java
public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

Terminal
javac HelloWorld.java
java HelloWorld

Program 2: NamePrinter (Command-Line Arguments)

Java — NamePrinter.java
public class NamePrinter {
    public static void main(String[] args) {
        if (args.length < 2) {
            System.out.println("Usage: java NamePrinter <name> <city>");
            return;
        }
        System.out.println("Name: " + args[0]);
        System.out.println("City: " + args[1]);
        System.out.println("Welcome to Java, " + args[0] + " from " + args[1] + "!");
    }
}

Terminal
javac NamePrinter.java
java NamePrinter Priya Delhi

Program 3: AdderProgram (Sum of Two Numbers via CMD Args)

Java — AdderProgram.java
public class AdderProgram {
    public static void main(String[] args) {
        if (args.length < 2) {
            System.out.println("Usage: java AdderProgram <num1> <num2>");
            return;
        }
        int a = Integer.parseInt(args[0]);
        int b = Integer.parseInt(args[1]);
        int sum = a + b;
        System.out.println("Number 1: " + a);
        System.out.println("Number 2: " + b);
        System.out.println("Sum: " + sum);
    }
}

Terminal
javac AdderProgram.java
java AdderProgram 25 17

Program 4: TypeChecker (Data Types Demo)

Java — TypeChecker.java
public class TypeChecker {
    public static void main(String[] args) {
        int age = 21;
        double cgpa = 8.75;
        char grade = 'A';
        boolean isPlaced = true;
        String name = "Rahul Sharma";

        System.out.println("=== Student Profile ===");
        System.out.println("Name: " + name);
        System.out.println("Age: " + age);
        System.out.println("CGPA: " + cgpa);
        System.out.println("Grade: " + grade);
        System.out.println("Placed: " + isPlaced);
    }
}

Terminal
javac TypeChecker.java
java TypeChecker

Program 5: SimpleCalculator (Multi-operation via CMD Args)

Java — SimpleCalculator.java
public class SimpleCalculator {
    public static void main(String[] args) {
        if (args.length < 3) {
            System.out.println("Usage: java SimpleCalculator <num1> <operator> <num2>");
            System.out.println("Operators: + - * /");
            return;
        }
        double num1 = Double.parseDouble(args[0]);
        String op = args[1];
        double num2 = Double.parseDouble(args[2]);
        double result = 0;

        if (op.equals("+")) result = num1 + num2;
        else if (op.equals("-")) result = num1 - num2;
        else if (op.equals("*")) result = num1 * num2;
        else if (op.equals("/")) {
            if (num2 == 0) {
                System.out.println("Error: Division by zero!");
                return;
            }
            result = num1 / num2;
        } else {
            System.out.println("Invalid operator: " + op);
            return;
        }
        System.out.println(num1 + " " + op + " " + num2 + " = " + result);
    }
}

Terminal
javac SimpleCalculator.java
java SimpleCalculator 100 + 250
java SimpleCalculator 500 / 4

Learn by Doing — 3-Tier Lab Structure

Terminal
java -version
javac -version

Terminal
mkdir JavaUnit1
cd JavaUnit1

Java
public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
        System.out.println("I am learning Java!");
        System.out.println("This is my first program.");
    }
}

Terminal
javac HelloWorld.java

Terminal
java HelloWorld

Java
public class Greeter {
    public static void main(String[] args) {
        System.out.println("Hello, " + args[0] + "!");
        System.out.println("You are from " + args[1]);
        System.out.println("Total arguments: " + args.length);
    }
}

Terminal
javac Greeter.java
java Greeter Ankit Pune

Terminal
java ArithmeticCalc 45 18

Terminal
java StudentIDCard "Priya Sharma" BCA-2024-042 "Computer Science"

Problem Set — 4 Categories

Category 1: Syntax & Tracing (5 Problems)

P1. Predict the Output

Java
public class Test1 {
    public static void main(String[] args) {
        System.out.println("Java" + 10 + 20);
        System.out.println(10 + 20 + "Java");
    }
}

Explanation: In line 1, "Java" + 10 concatenates to "Java10", then + 20 gives "Java1020". In line 2, 10 + 20 = 30 (arithmetic first), then + "Java" gives "30Java".

P2. Find the Compilation Error

Java
public class Test2 {
    public static void main(string[] args) {
        System.out.println("Hello");
    }
}

Error: string should be String (capital S). Java is case-sensitive. String is a class name and must be capitalised.

P3. Predict the Output

Java
public class Test3 {
    public static void main(String[] args) {
        System.out.println(args[0]);
    }
}

Run as: java Test3 (no arguments)

Answer: ArrayIndexOutOfBoundsException at runtime. The program compiles fine but crashes because args[] is empty and we're accessing args[0].

P4. Find the Error

Java
public class MyProgram {
    public static void main(String[] args) {
        System.out.println("Hello")
    }
}

Error: Missing semicolon ; after println("Hello"). Every statement in Java must end with a semicolon.

P5. Predict the Output

Java
public class Test5 {
    public static void main(String[] args) {
        System.out.print("Hello ");
        System.out.println("World");
        System.out.print("Java ");
        System.out.print("Rules");
    }
}

Explanation: print() doesn't add newline, println() does. So "Hello " and "World" are on the same line (println adds newline after), then "Java " and "Rules" are on the next line.

Category 2: Programming Exercises (8 Problems)

Easy P6. Reverse Name Printer

Write a program that accepts first name and last name as cmd args and prints them in reverse order.

Input: java ReverseName Rahul Sharma

Output: Sharma, Rahul

Easy P7. Age Calculator

Accept birth year as a cmd arg. Print the person's age (assume current year = 2025).

Input: java AgeCalc 2003

Output: Your age is: 22 years

Medium P8. Temperature Converter

Accept a temperature in Celsius as a cmd arg. Print it in Fahrenheit and Kelvin.

Formula: F = (C × 9/5) + 32, K = C + 273.15

Input: java TempConverter 37

Output: 37.0°C = 98.6°F = 310.15K

Medium P9. Percentage Calculator

Accept 5 subject marks as cmd args. Calculate total, percentage, and grade (A/B/C/D/F).

Input: java MarkSheet 85 90 78 92 88

Output: Total: 433, Percentage: 86.6%, Grade: A

Medium P10. Word Counter

Accept a sentence (multiple words) as cmd args. Print the number of words (i.e., args.length).

Input: java WordCounter Java is the best language

Output: Word count: 5

Hard P11. Argument Sorter

Accept N numbers as cmd args. Print them in sorted (ascending) order.

Input: java ArgSort 45 12 78 3 56

Output: Sorted: 3 12 45 56 78

Hard P12. Mini Invoice Generator

Accept item name, quantity, and unit price as cmd args. Print a formatted invoice with total and 18% GST.

Input: java Invoice Laptop 2 55000

Output: Item: Laptop, Qty: 2, Price: ₹55,000, Subtotal: ₹1,10,000, GST (18%): ₹19,800, Total: ₹1,29,800

Hard P13. Pattern Printer

Accept a number N as cmd arg. Print a right-angled triangle pattern of stars with N rows.

Input: java Pattern 5

Category 3: Industry Problems (Indian Context)

P14. IRCTC Ticket Printer

Build a program that accepts passenger name, train number, source, destination, and class (SL/3A/2A/1A) as cmd args. Print a formatted railway ticket. Include PNR (generate a random 10-digit number).

P15. Ola Fare Calculator

Build a program that accepts ride type (Mini/Sedan/SUV) and distance (km) as cmd args. Calculate fare using: Mini = ₹7/km, Sedan = ₹10/km, SUV = ₹14/km. Add a base fare of ₹50.

P16. GST Invoice Generator

Build a program for a small Indian shop. Accept product name, quantity, rate, and GST slab (5/12/18/28) as cmd args. Print a GST-compliant invoice with CGST and SGST split equally.

Category 4: Interview Problems

P17. Reverse a String (Without StringBuilder)

Asked at: TCS NQT, Wipro NLTH

Given a string as cmd arg, reverse it without using StringBuilder or StringBuffer.

Brute: Convert to char array, swap from both ends → O(n) time, O(n) space

Optimised: Build reversed string by iterating from end → O(n) time, O(n) space (same complexity but cleaner)

P18. Check Palindrome

Asked at: Infosys SP, Cognizant GenC

Accept a word as cmd arg. Check if it reads the same forwards and backwards.

Brute: Reverse the string and compare → O(n) time

Optimised: Compare chars from both ends, stop at middle → O(n/2) time

P19. Count Vowels and Consonants

Asked at: Capgemini, Accenture

Accept a sentence as cmd args. Count total vowels (a,e,i,o,u) and consonants.

Brute: Check each char against all 5 vowels → O(n×5)

Optimised: Use a Set/String of vowels for O(1) lookup → O(n) time

MCQ Assessment Bank — 30 Questions (Bloom's Mapped)

Remember / Identify (Q1–Q5)

Understand / Explain (Q6–Q10)

Apply / Use (Q11–Q15)

public class Test {
    public static void main(String[] args) {
        System.out.println("Hello" + 5 + 3);
    }
}

public class Test {
    public static void main(String[] args) {
        System.out.println(10 + 20 + "Hello" + 5 + 6);
    }
}

public class Test {
    public static void main(String[] args) {
        int x = Integer.parseInt(args[0]);
        System.out.println(x * 2);
    }
}

Analyze / Compare (Q16–Q20)

System.out.println(5 + 3);          // Line 1
System.out.println("5" + 3);        // Line 2
System.out.println("5" + "3");      // Line 3

Evaluate / Justify (Q21–Q25)

Create / Design (Q26–Q30)

Short Answer Questions (8 Questions)

Definition-Type (3 Questions)

Q1. Define JDK and list its main components.

Answer: JDK (Java Development Kit) is a software development environment used for developing Java applications. It contains: (1) Java compiler (javac) — converts .java to .class, (2) Java Runtime Environment (JRE) — for executing programs, (3) Java debugger (jdb) — for debugging, (4) Javadoc — for generating documentation, (5) Other tools like jar, jps, jstack. The JDK is required by developers; end-users only need the JRE.

Q2. What is bytecode in Java?

Answer: Bytecode is the intermediate, platform-independent code generated by the Java compiler (javac) from .java source files. It is stored in .class files. Bytecode is not native machine code — it cannot be directly executed by the OS. Instead, it is executed by the JVM, which translates it into platform-specific native code using the JIT compiler. This is the key mechanism behind Java's "Write Once, Run Anywhere" philosophy.

Q3. Define the term "Platform Independence" in the context of Java.

Answer: Platform independence means a Java program compiled on one operating system (e.g., Windows) can run on any other operating system (e.g., Linux, macOS) without modification or recompilation. This is achieved because Java source code is compiled into bytecode (.class files), which is platform-neutral. The platform-specific JVM installed on each OS interprets/compiles this bytecode into native instructions for that OS.

Short Code-Type (3 Questions)

Q4. Write a Java program that prints "Hello, [name]!" where name is passed as a command-line argument.

Java
public class Hello {
    public static void main(String[] args) {
        if (args.length > 0) {
            System.out.println("Hello, " + args[0] + "!");
        } else {
            System.out.println("Hello, World!");
        }
    }
}

Q5. Write a Java program that prints the number of command-line arguments passed.

Java
public class ArgCounter {
    public static void main(String[] args) {
        System.out.println("Number of arguments: " + args.length);
        for (int i = 0; i < args.length; i++) {
            System.out.println("args[" + i + "] = " + args[i]);
        }
    }
}

Q6. Write the smallest possible Java program that prints "Java" to the console.

Java
class J{public static void main(String[]a){System.out.print("Java");}}

Comparison-Type (2 Questions)

Q7. Compare System.out.print() and System.out.println().

Q8. Compare Java's compilation process with Python's execution process.

Long Answer & Case Studies

Terminal
java EmployeeIDGen "Ravi Kumar" BFS 2024 0042

Lab Programs

Industry Spotlight — A Day in the Life

Earn With It — Freelance & Income Roadmap

Chapter Summary — Tweet-Sized Bullets

Code Tweet — Smallest HelloWorld

Java — 73 characters
class Hi{public static void main(String[]a){System.out.println("☕");}}

Java Checkpoint — Self-Assessment

✅ Unit 1 complete. MCQs: 30. Ready for Unit 2: Data Types & Variables!

[QR: Link to EduArtha video tutorial — Introduction to Java]