Java Programming Notes (हिंदी + English)

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

// Syntax example
public class SyntaxDemo {
    public static void main(String[] args) {
        int number = 10; // variable declaration
        System.out.println("Number is: " + number);
    }
}

// Data types example
public class DataTypesDemo {
    public static void main(String[] args) {
        byte b = 100;
        short s = 5000;
        int i = 100000;
        long l = 100000L;
        float f = 5.75f;
        double d = 19.99;
        char c = 'A';
        boolean bool = true;

        System.out.println("byte: " + b);
        System.out.println("short: " + s);
        System.out.println("int: " + i);
        System.out.println("long: " + l);
        System.out.println("float: " + f);
        System.out.println("double: " + d);
        System.out.println("char: " + c);
        System.out.println("boolean: " + bool);
    }
}

// Variables and constants
public class VarConstDemo {
    static int staticVar = 10;      // static variable
    int instanceVar = 20;           // instance variable
    final double PI = 3.14159;      // constant

    public static void main(String[] args) {
        int localVar = 30;           // local variable
        System.out.println("Local: " + localVar);
        System.out.println("Static: " + staticVar);

        VarConstDemo obj = new VarConstDemo();
        System.out.println("Instance: " + obj.instanceVar);
        System.out.println("Constant PI: " + obj.PI);
    }
}

// Input and output using Scanner
import java.util.Scanner;

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

        System.out.print("Enter your name: ");
        String name = sc.nextLine();

        System.out.print("Enter your age: ");
        int age = sc.nextInt();

        System.out.println("Hello " + name + ", you are " + age + " years old.");
        sc.close();
    }
}

// Operators example
public class OperatorsDemo {
    public static void main(String[] args) {
        int a = 10, b = 3;
        System.out.println("a + b = " + (a + b));
        System.out.println("a > b = " + (a > b));
        System.out.println("(a > 5) && (b < 5) = " + ((a > 5) && (b < 5)));
        a += 5;  // a = a + 5
        System.out.println("a after += 5: " + a);
        int max = (a > b) ? a : b;
        System.out.println("Max using ternary: " + max);
    }
}

// Control structures
public class ControlDemo {
    public static void main(String[] args) {
        int marks = 85;

        if (marks >= 90) {
            System.out.println("Grade A");
        } else if (marks >= 75) {
            System.out.println("Grade B");
        } else {
            System.out.println("Grade C");
        }

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

// Loops example
public class LoopsDemo {
    public static void main(String[] args) {
        // for loop
        for (int i = 1; i <= 5; i++) {
            System.out.print(i + " ");
        }
        System.out.println();

        // while loop
        int j = 1;
        while (j <= 5) {
            System.out.print(j + " ");
            j++;
        }
        System.out.println();

        // for-each loop on array
        int[] arr = {10, 20, 30, 40};
        for (int num : arr) {
            System.out.print(num + " ");
        }
        System.out.println();
    }
}

// Arrays example
public class ArraysDemo {
    public static void main(String[] args) {
        int[] numbers = {10, 20, 30, 40, 50};

        // access and print
        System.out.println("First element: " + numbers[0]);

        // loop through
        for (int i = 0; i < numbers.length; i++) {
            System.out.print(numbers[i] + " ");
        }
        System.out.println();

        // 2D array
        int[][] matrix = { {1,2}, {3,4} };
        System.out.println("Element at [1][0]: " + matrix[1][0]);
    }
}

// String example
public class StringDemo {
    public static void main(String[] args) {
        String str = "Hello Java";

        System.out.println("Length: " + str.length());
        System.out.println("Char at 0: " + str.charAt(0));
        System.out.println("Substring (6): " + str.substring(6));
        System.out.println("Uppercase: " + str.toUpperCase());

        String str2 = "hello java";
        System.out.println("Equals (ignore case): " + str.equalsIgnoreCase(str2));
    }
}

// Methods example
public class MethodsDemo {
    static int add(int a, int b) {
        return a + b;
    }

    static double add(double a, double b) { // overloaded
        return a + b;
    }

    static void greet(String name) {
        System.out.println("Hello, " + name);
    }

    public static void main(String[] args) {
        int sum = add(5, 3);
        System.out.println("Sum (int): " + sum);
        System.out.println("Sum (double): " + add(5.5, 2.3));
        greet("Alice");
    }
}

// Class and object example
class Student {
    String name;
    int age;

    void display() {
        System.out.println("Name: " + name + ", Age: " + age);
    }
}

public class OOPDemo {
    public static void main(String[] args) {
        Student s1 = new Student();
        s1.name = "Rahul";
        s1.age = 20;
        s1.display();
    }
}

// Inheritance and polymorphism
class Animal {
    void sound() {
        System.out.println("Animal makes sound");
    }
}

class Dog extends Animal {
    @Override
    void sound() {
        System.out.println("Dog barks");
    }
}

public class InheritanceDemo {
    public static void main(String[] args) {
        Animal myAnimal = new Dog();
        myAnimal.sound();  // Dog barks (polymorphism)
    }
}

// Exception handling
public class ExceptionDemo {
    public static void main(String[] args) {
        try {
            int[] arr = new int[5];
            arr[10] = 100;  // ArrayIndexOutOfBoundsException
        } catch (ArrayIndexOutOfBoundsException e) {
            System.out.println("Caught: " + e);
        } finally {
            System.out.println("Finally block always executes");
        }
    }
}

// File writing and reading
import java.io.*;

public class FileDemo {
    public static void main(String[] args) {
        // Write to file
        try (FileWriter fw = new FileWriter("test.txt")) {
            fw.write("Hello, Java File!\n");
            fw.write("This is a test.");
        } catch (IOException e) {
            e.printStackTrace();
        }

        // Read from file
        try (BufferedReader br = new BufferedReader(new FileReader("test.txt"))) {
            String line;
            while ((line = br.readLine()) != null) {
                System.out.println(line);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}