📚 Java Basics – Full Notes with Explanation and Code

1. Hello World Program

This is the most basic program in Java. It prints a line of text on the console.

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

- public class HelloWorld: Declares a class named HelloWorld.
- public static void main(String[] args): Main method – the entry point of any Java application.
- System.out.println(...): Prints output to the console.

2. Data Types in Java

Java is a statically-typed language, meaning variables must be declared with a data type.

int num = 10;
float price = 5.99f;
char grade = 'A';
boolean isJavaFun = true;
String name = "Gaurav";

3. Variables and Constants

- Variable: A container to store data.
- Constant: A variable whose value cannot be changed (declared with final keyword).

int age = 25;
final double PI = 3.14159;

4. Operators

Java supports arithmetic, relational, logical, and assignment operators.

int a = 10, b = 5;
System.out.println(a + b);  // 15
System.out.println(a * b);  // 50
System.out.println(a > b);  // true
System.out.println(a == 10 && b == 5); // true

5. Conditional Statements

if-else

Checks a condition and executes code blocks based on whether it's true or false.

int age = 18;
if (age >= 18) {
    System.out.println("Adult");
}else if (age >= 1){
    System.out.println("Minor");
}else {
    System.out.println("Not Valid");
}

switch

Used instead of multiple if-else when checking for fixed values.

int day = 3;
switch (day) {
    case 1: System.out.println("Monday"); break;
    case 2: System.out.println("Tuesday"); break;
    default: System.out.println("Other Day");
}

6. Loops

for loop

Use when number of iterations is known.

for (int i = 1; i <= 5; i++) {
    System.out.println(i);
}

while loop

Use when number of iterations is unknown.

int i = 1;
while (i <= 5) {
    System.out.println(i);
    i++;
}

do-while loop

Use when you want the loop to run at least once.

int i = 1;
do {
    System.out.println(i);
    i++;
} while (i <= 5);

7. Arrays

Used to store multiple values in a single variable. Arrays are zero-indexed.

int[] marks = {90, 80, 70};
System.out.println(marks[0]); // Output: 90

8. Methods (Functions)

A method is a block of code that performs a specific task.

public class MyClass {
    public static void greet() {
        System.out.println("Hello from method!");
    }

    public static void main(String[] args) {
        greet(); // Call the method
    }
}