Objects & Classes
Complete Study Guide

// CLASS DEFINITION — the blueprint
class Student {
    // ─── ATTRIBUTES (instance variables / data) ───
    String name;
    int    rollNo;
    double marks;

    // ─── METHODS (behaviour) ───
    void display() {
        System.out.println("Name: " + name + "  Roll: " + rollNo + "  Marks: " + marks);
    }

    void promote() {
        if (marks >= 40)
            System.out.println(name + " is PROMOTED!");
        else
            System.out.println(name + " needs to reappear.");
    }
}

// CREATING OBJECTS (instances) in main
class Main {
    public static void main(String[] args) {
        Student s1 = new Student();  // s1 is an OBJECT / INSTANCE
        s1.name   = "Aarav";           // accessing attribute
        s1.rollNo = 101;
        s1.marks  = 92.5;
        s1.display();                   // calling method

        Student s2 = new Student();  // another independent object
        s2.name   = "Priya";
        s2.rollNo = 102;
        s2.marks  = 87.0;
        s2.display();
        s2.promote();
    }
}

class Box {
    double length, width, height;

    // ① DEFAULT CONSTRUCTOR — no parameters
    Box() {
        length = 1.0;
        width  = 1.0;
        height = 1.0;
        System.out.println("Default constructor called");
    }

    // ② PARAMETERISED CONSTRUCTOR
    Box(double l, double w, double h) {
        length = l;
        width  = w;
        height = h;
        System.out.println("Parameterised constructor called");
    }

    // ③ COPY CONSTRUCTOR — takes object of same class
    Box(Box b) {
        length = b.length;
        width  = b.width;
        height = b.height;
        System.out.println("Copy constructor called");
    }

    double volume() {
        return length * width * height;
    }
}

class Main {
    public static void main(String[] args) {
        Box b1 = new Box();          // default constructor
        Box b2 = new Box(3.0, 4.0, 5.0); // parameterised
        Box b3 = new Box(b2);         // copy constructor

        System.out.println("b1 volume: " + b1.volume()); // 1.0
        System.out.println("b2 volume: " + b2.volume()); // 60.0
        System.out.println("b3 volume: " + b3.volume()); // 60.0
    }
}

class Circle {
    double radius;
    String color;

    // Without 'this' → parameter names SAME as field names → AMBIGUITY!
    Circle(double radius, String color) {
        this.radius = radius;  // this.radius = instance var; radius = param
        this.color  = color;
    }

    double area() {
        return Math.PI * this.radius * this.radius;
    }
}

class BankAccount {
    // ATTRIBUTES
    private int    accNo;
    private String holderName;
    private double balance;

    // CONSTRUCTOR
    BankAccount(int accNo, String name, double initialBalance) {
        this.accNo      = accNo;
        this.holderName = name;
        this.balance    = initialBalance;
    }

    // METHODS (behaviour)
    void deposit(double amount) {
        if (amount > 0) balance += amount;
    }

    void withdraw(double amount) {
        if (amount > 0 && amount <= balance)
            balance -= amount;
        else
            System.out.println("Insufficient funds!");
    }

    void display() {
        System.out.println("Acc: " + accNo + "  Name: " + holderName + "  Bal: " + balance);
    }
}

class Main {
    public static void main(String[] args) {
        BankAccount acc = new BankAccount(1001, "Aarav", 5000.0);
        acc.deposit(2000.0);
        acc.withdraw(500.0);
        acc.display();   // Acc: 1001  Name: Aarav  Bal: 6500.0
    }
}

class Employee {
    int    empId;
    String name;
    double basicSalary;
    double hra, da, grossSalary;

    Employee(int id, String n, double basic) {
        empId       = id;
        name        = n;
        basicSalary = basic;
        hra         = 0.20 * basicSalary; // 20%
        da          = 0.10 * basicSalary; // 10%
        grossSalary = basicSalary + hra + da;
    }

    void displayDetails() {
        System.out.println("ID     : " + empId);
        System.out.println("Name   : " + name);
        System.out.println("Basic  : " + basicSalary);
        System.out.println("HRA    : " + hra);
        System.out.println("DA     : " + da);
        System.out.println("Gross  : " + grossSalary);
    }
}

class Main {
    public static void main(String[] args) {
        Employee e1 = new Employee(201, "Priya Sharma", 30000);
        e1.displayDetails();
    }
}

import java.util.Scanner;  // Step 1: import

class InputDemo {
    public static void main(String[] args) {

        // Step 2: Create Scanner object linked to System.in (keyboard)
        Scanner sc = new Scanner(System.in);

        // Reading different data types
        System.out.print("Enter name: ");
        String name = sc.nextLine();       // reads full line including spaces

        System.out.print("Enter age: ");
        int age = sc.nextInt();            // reads integer

        System.out.print("Enter salary: ");
        double salary = sc.nextDouble();  // reads double

        System.out.print("Enter grade: ");
        char grade = sc.next().charAt(0); // reads char (Scanner has no nextChar)

        // OUTPUT using println and printf
        System.out.println("Name: " + name);
        System.out.printf("Age: %d  Salary: %.2f  Grade: %c%n", age, salary, grade);

        sc.close(); // good practice: close scanner when done
    }
}

int age = sc.nextInt();
sc.nextLine();                  // ← consume leftover newline
String address = sc.nextLine(); // ← now reads correctly

import java.util.Scanner;
import java.util.InputMismatchException;

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

        try {
            System.out.print("Enter an integer: ");
            int n = sc.nextInt();   // may throw InputMismatchException
            System.out.println("Square = " + (n * n));
        }
        catch (InputMismatchException e) {
            System.out.println("Error: Please enter a valid integer!");
            System.out.println("Details: " + e.getMessage());
        }
        catch (Exception e) {
            // generic catch-all
            System.out.println("Unexpected error: " + e);
        }
        finally {
            sc.close();            // always executes — close resources
            System.out.println("Scanner closed.");
        }
    }
}

import java.util.StringTokenizer;

class TokenDemo {
    public static void main(String[] args) {

        // ─── EXAMPLE 1: Default delimiter (whitespace) ───
        String s1 = "Hello World Java Programming";
        StringTokenizer st1 = new StringTokenizer(s1);

        System.out.println("Total tokens: " + st1.countTokens()); // 4
        while (st1.hasMoreTokens()) {
            System.out.println(st1.nextToken());
        }
        // Output: Hello / World / Java / Programming (one per line)

        // ─── EXAMPLE 2: Custom delimiter (",") ───
        String s2 = "Aarav,Priya,Rahul,Sneha";
        StringTokenizer st2 = new StringTokenizer(s2, ",");

        System.out.println("Names count: " + st2.countTokens()); // 4
        while (st2.hasMoreTokens()) {
            System.out.print(st2.nextToken() + " | ");
        }
        // Output: Aarav | Priya | Rahul | Sneha |

        // ─── EXAMPLE 3: Multiple delimiters ───
        String s3 = "10+20-30*40";
        StringTokenizer st3 = new StringTokenizer(s3, "+-*");
        while (st3.hasMoreTokens()) {
            System.out.print(st3.nextToken() + " ");
        }
        // Output: 10 20 30 40

        // ─── EXAMPLE 4: Word count in a sentence ───
        String sentence = "ISC exams are not that hard";
        StringTokenizer st4 = new StringTokenizer(sentence);
        System.out.println("Word count = " + st4.countTokens()); // 6
    }
}

import java.util.*;

class CSVReader {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.println("Enter student data (name,roll,marks): ");
        String line = sc.nextLine();

        StringTokenizer st = new StringTokenizer(line, ",");

        String name  = st.nextToken();
        int    roll  = Integer.parseInt(st.nextToken().trim());
        double marks = Double.parseDouble(st.nextToken().trim());

        System.out.println("Name: "  + name);
        System.out.println("Roll: "  + roll);
        System.out.println("Marks: " + marks);
        sc.close();
    }
}