☕ Java Fundamentals & Program Development

Q: What is the difference between JDK, JRE, and JVM?

JVM (Java Virtual Machine) executes bytecode. JRE (Java Runtime Environment) includes the JVM and standard libraries needed to run programs. JDK (Java Development Kit) includes the JRE plus development tools like the compiler (javac) and debugger, which are needed to write and compile programs.

Q: What are the three types of errors in Java?

1. Syntax Errors: Violations of Java's grammar rules detected by the compiler (e.g., missing semicolon). 2. Runtime Errors: Illegal operations that crash the program while running (e.g., dividing by zero). 3. Logical Errors: The program runs without crashing but produces an incorrect result due to flawed logic.

Q: Why is the main method declared static in Java?

The main method is declared static so that the JVM can call it directly using the class name without needing to create an object (instance) of the class first. This allows the program to start executing immediately.

Q: What is the difference between widening and narrowing in Java?

Widening (Automatic Conversion) is when Java automatically converts a smaller data type to a larger one (e.g., int to double) without data loss. Narrowing (Explicit Casting) is when you manually convert a larger type to a smaller one (e.g., double to int) using a cast, which may result in data loss.

Class 1 Complete Notes | From Basics to Mini Project

📚 About this guide: Complete, exam-friendly notes for Java Class 1: Fundamentals & Program Development. Covers Programming Basics, JDK/JRE/JVM, Variables, Data Types, Operators, Scanner Input, Type Casting, and Practical Exercises with step-by-step code explanations.

1.1 - 1.3

Programming, Algorithms & Source Code

Programming: Writing instructions that tell a computer how to perform a task.
Algorithm: A step-by-step, language-independent procedure to solve a problem (like a recipe idea).
Program: The implementation of an algorithm in a specific language (like the recipe written in English).
Source Code: Human-readable text written by the programmer (saved as .java files).

1.4 - 1.5

Compiler, JDK, JRE & JVM

Component	Contains	Used For
JVM	Bytecode interpreter	Executing bytecode ("Write Once, Run Anywhere")
JRE	JVM + Libraries	Running Java programs
JDK	JRE + Compiler (javac) + Tools	Developing and compiling programs

Flow: Main.java → (javac) → Main.class (bytecode) → (JVM) → Output

1.7 - 1.8

Syntax vs Semantics & Types of Errors

Syntax: Grammar rules of the language (e.g., missing semicolon).
Semantics: The actual meaning/logic of the code (e.g., wrong formula).

Error Type	Detected When	Example
Syntax	Compile time	Missing `;` or invalid variable name
Runtime	While running	Dividing by zero (`ArithmeticException`)
Logical	Checking output	Wrong formula (Compiles & runs, but gives wrong answer)

2.1

GUI vs CLI

GUI (Graphical User Interface): Interact using windows, icons, menus, and a mouse. (Easy for beginners).
CLI (Command Line Interface): Interact by typing text commands into a terminal. (Faster for developers, used for compiling/running).

2.3

Text Editors vs IDEs

Feature	Text Editor (Notepad)	IDE (IntelliJ, Eclipse)
Edit code	Yes	Yes
Compile & Run	Manually via CLI	Built-in button
Error detection	No	Yes, live underlining
Auto-complete	Limited	Yes, smart suggestions

2.4

Popular Java IDEs

IntelliJ IDEA: Popular, smart code completion, very beginner-friendly (Community Edition is free).
Eclipse: Long-established, free, open-source, widely used in universities.
VS Code: Lightweight editor that becomes a Java IDE after installing the Java Extension Pack.

3.0

Hello World Program

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

4.0

Line-by-Line Breakdown

public class Main: All Java code must be inside a class. If public, file name must match (Main.java).
public static void main(String[] args): The entry point. public (accessible anywhere), static (runs without creating an object), void (returns nothing), String[] args (command-line arguments).
System.out.println(): Prints text to the console and moves to a new line. (Use print() to stay on the same line).

🎯 Golden Rules: Every statement ends with ; | Java is case-sensitive | Blocks use {} | Text uses ""

5.1

Variables & Naming Rules

A variable is a named memory location that stores a value. Java is statically typed (must declare type).

Syntax: dataType variableName = value;

Rules: Can contain letters, digits, _, $. Cannot start with a digit. Cannot be a keyword. Use camelCase by convention.

5.2

Primitive vs Reference Data Types

Type	Stores	Size	Example
`int`	Whole numbers	4 bytes	`int age = 25;`
`double`	Decimals (15 digits precision)	8 bytes	`double price = 99.75;`
`float`	Decimals (7 digits precision)	4 bytes	`float temp = 36.5f;` (Needs `f`)
`char`	Single character	2 bytes	`char grade = 'A';` (Single quotes)
`boolean`	true or false	1 bit	`boolean pass = true;`
`String`	Text (Class, not primitive)	Varies	`String name = "Ali";` (Double quotes)

5.3

Constants (final keyword)

A constant's value cannot be changed after assignment. Use the final keyword. Convention: UPPERCASE_WITH_UNDERSCORES.

final double PI = 3.14159;
final int MAX_MARKS = 100;
// PI = 3.14;  ERROR: cannot assign a value to final variable

6.0

Getting User Input (Scanner)

import java.util.Scanner; // 1. Import

public class Main {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in); // 2. Create object
        
        System.out.print("Enter your age: ");
        int age = input.nextInt(); // 3. Read input
        
        System.out.println("You are " + age + " years old.");
        input.close();
    }
}

⚠️ Common Pitfall: After nextInt(), the Enter key (newline) stays in the buffer. If you use nextLine() next, it will read an empty string. Fix: Add an extra input.nextLine(); to consume the leftover newline.

7.0

Arithmetic, Relational & Logical Operators

Arithmetic: +, -, *, / (Integer division drops decimals!), % (Modulus/Remainder).
Relational: == (Equal), != (Not equal), >, <, >=, <=. (Returns boolean).
Logical: && (AND), || (OR), ! (NOT).
Increment/Decrement: x++ (post-increment: use then increase), ++x (pre-increment: increase then use).

8.2

Automatic Conversion (Widening)

Java automatically converts a smaller type to a larger type without data loss.

Order: byte → short → int → long → float → double

int a = 10;
double b = 3; // 3 becomes 3.0 automatically
double result = a / b; // 10.0 / 3.0 = 3.333...

8.3

Explicit Conversion (Narrowing / Casting)

Converting a larger type to a smaller type must be done manually using a cast. Data may be lost (decimals are truncated, not rounded).

double price = 99.99;
int rounded = (int) price; // 99 (decimal part is cut off)

// int x = 7.5; ERROR: possible lossy conversion
int y = (int) 7.5; // OK, y = 7

8.4

Casting to Fix Integer Division

If you divide two integers, Java performs integer division. Cast before dividing to get a decimal result.

int total = 17;
int count = 5;

double wrong = total / count; // 3.0 (Integer division happened first!)
double correct = (double) total / count; // 3.4 (Cast applied first)

Ex 1-4

Quick Exercise Formulas

Area of Circle: PI * radius * radius
Area of Rectangle: length * width
Simple Interest: (principal * rate * time) / 100
Celsius to Fahrenheit: (celsius * 9.0 / 5) + 32 (Use 9.0 to avoid integer division!)

Mini Project

Student Marks Calculator

Accepts name and 3 subject marks, then calculates total, average, and pass/fail result.

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);

        System.out.print("Enter student name: ");
        String name = input.nextLine();

        System.out.print("Enter marks for subject 1: ");
        int m1 = input.nextInt();
        System.out.print("Enter marks for subject 2: ");
        int m2 = input.nextInt();
        System.out.print("Enter marks for subject 3: ");
        int m3 = input.nextInt();

        int total = m1 + m2 + m3;
        double average = total / 3.0; // 3.0 forces decimal division

        // Ternary operator (short if/else)
        String result = (average >= 40) ? "Pass" : "Fail";

        System.out.println("\n------ Result Sheet------");
        System.out.println("Name   : " + name);
        System.out.println("Total  : " + total);
        System.out.printf("Average: %.2f%n", average); // 2 decimal places
        System.out.println("Result : " + result);

        input.close();
    }
}

📌 Quick Revision Summary

⭐ Core Concepts:
🔄 Flow: .java (Source) → javac → .class (Bytecode) → JVM → Output
📦 JDK ⊃ JRE ⊃ JVM
🐛 Errors: Syntax (Compile), Runtime (Crash), Logical (Wrong Output)
🚪 Entry Point: public static void main(String[] args)
🔢 Types: int, double, float, char, boolean, String
⌨️ Input: Scanner (import java.util.Scanner;)
➗ Division: int/int drops decimals. Cast to double or use .0!
🔄 Casting: Widening (Auto, Safe) | Narrowing (Manual, Data Loss)

📝 Practice Questions

Q1: Differentiate between JDK, JRE and JVM.
Q2: What is the difference between a compile-time error and a logical error? Give one example of each.
Q3: Why is main declared static?
Q4: What is the output of System.out.println(7 / 2);? How would you get 3.5?
Q5: What is the value of x and y after int x = 5; int y = x++ + ++x;?
Q6: Write a program that reads a number of days and prints the equivalent number of weeks and remaining days (Hint: use / and %).

❓ Frequently Asked Questions (FAQ)

Q: What is the difference between JDK, JRE, and JVM? A: JVM (Java Virtual Machine) executes bytecode. JRE (Java Runtime Environment) includes the JVM and standard libraries needed to run programs. JDK (Java Development Kit) includes the JRE plus development tools like the compiler (javac) and debugger, which are needed to write and compile programs.

Q: What are the three types of errors in Java? A: 1. Syntax Errors: Violations of Java's grammar rules detected by the compiler (e.g., missing semicolon). 2. Runtime Errors: Illegal operations that crash the program while running (e.g., dividing by zero). 3. Logical Errors: The program runs without crashing but produces an incorrect result due to flawed logic.

Q: Why is the main method declared static in Java? A: The main method is declared static so that the JVM can call it directly using the class name without needing to create an object (instance) of the class first. This allows the program to start executing immediately.

Q: What is the difference between widening and narrowing in Java? A: Widening (Automatic Conversion) is when Java automatically converts a smaller data type to a larger one (e.g., int to double) without data loss. Narrowing (Explicit Casting) is when you manually convert a larger type to a smaller one (e.g., double to int) using a cast, which may result in data loss.

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Monday, October 5, 2026

Java Class 1 notes computer science university programming basics, JDK/JRE/JVM, variables, data types, operators, type casting, Scanner input, and practical exercises with solutions

☕ Java Fundamentals & Program Development

Programming, Algorithms & Source Code

Compiler, JDK, JRE & JVM

Syntax vs Semantics & Types of Errors

GUI vs CLI

Text Editors vs IDEs

Popular Java IDEs

Hello World Program

Line-by-Line Breakdown

Variables & Naming Rules

Primitive vs Reference Data Types

Constants (final keyword)

Getting User Input (Scanner)

Arithmetic, Relational & Logical Operators

Automatic Conversion (Widening)

Explicit Conversion (Narrowing / Casting)

Casting to Fix Integer Division

Quick Exercise Formulas

Student Marks Calculator

📌 Quick Revision Summary

📝 Practice Questions

❓ Frequently Asked Questions (FAQ)

Expert ICT, Computer Science, Coding & Digital Marketing Classes

🚀 Learn Today. Build Tomorrow.

💡 Explore Our Training & Services

🎓 University Project Guidance

🤖 AI, Coding & Software

🏫 School ICT Classes

🌐 Web Design & Development

📢 Digital Marketing

💻 Computer & IT Skills

📚 Learn With Your Curriculum

🇱🇰 Sri Lankan Local Curriculum

📘 Pearson Edexcel

📗 Cambridge Curriculum

💻 Programming & Software Development

🛠️ Programming Languages & Technologies

🌐 Web Design & Development

🎓 University & Final Year IT Projects

🧑‍💻 Project Development

📄 Documentation & Practical

🧠 AI & Smart Applications

📊 Database, Cloud & IT Training

📱 Digital Marketing & Social Media

🖥️ Microsoft Office & Digital Skills

⭐ Why Join Our Classes?

📞 Ready to Start Learning?

🌐 Our Online Learning Platforms