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Java Program to Create Class and Objects with Methods

Learn object-oriented programming basics with this Java program that demonstrates class creation, object instantiation, and method calling with practical examples.

In this program, you'll learn the fundamentals of object-oriented programming by creating classes, instantiating objects, and calling methods.

To understand this example, you should have the knowledge of the following Java programming topics:

Program to Create Class and Objects

Example 1: Basic Class and Object Creation

// Define a Student class
class Student {
    // Instance variables (attributes)
    String name;
    int age;
    String major;
    double gpa;

    // Method to display student information
    void displayInfo() {
        System.out.println("=== Student Information ===");
        System.out.println("Name: " + name);
        System.out.println("Age: " + age);
        System.out.println("Major: " + major);
        System.out.println("GPA: " + gpa);
        System.out.println();
    }

    // Method to update GPA
    void updateGPA(double newGPA) {
        gpa = newGPA;
        System.out.println(name + "'s GPA updated to: " + gpa);
    }

    // Method to check if student is eligible for honors
    boolean isEligibleForHonors() {
        return gpa >= 3.5;
    }
}

class StudentDemo {
    public static void main(String[] args) {
        // Create first student object
        Student student1 = new Student();
        student1.name = "Alice Johnson";
        student1.age = 20;
        student1.major = "Computer Science";
        student1.gpa = 3.8;

        // Create second student object
        Student student2 = new Student();
        student2.name = "Bob Smith";
        student2.age = 19;
        student2.major = "Mathematics";
        student2.gpa = 3.2;

        // Display information for both students
        student1.displayInfo();
        student2.displayInfo();

        // Update GPA and check honors eligibility
        student2.updateGPA(3.6);

        System.out.println(student1.name + " eligible for honors: " +
                          student1.isEligibleForHonors());
        System.out.println(student2.name + " eligible for honors: " +
                          student2.isEligibleForHonors());
    }
}

Output:

=== Student Information ===
Name: Alice Johnson
Age: 20
Major: Computer Science
GPA: 3.8

=== Student Information ===
Name: Bob Smith
Age: 19
Major: Mathematics
GPA: 3.2

Bob Smith's GPA updated to: 3.6
Alice Johnson eligible for honors: true
Bob Smith eligible for honors: true

Example 2: Class with Constructor

class Car {
    // Instance variables
    private String brand;
    private String model;
    private int year;
    private double price;

    // Constructor to initialize car objects
    public Car(String brand, String model, int year, double price) {
        this.brand = brand;
        this.model = model;
        this.year = year;
        this.price = price;
    }

    // Method to display car details
    public void displayDetails() {
        System.out.println("=== Car Details ===");
        System.out.println("Brand: " + brand);
        System.out.println("Model: " + model);
        System.out.println("Year: " + year);
        System.out.println("Price: $" + price);
        System.out.println();
    }

    // Method to apply discount
    public void applyDiscount(double percentage) {
        double discount = price * (percentage / 100);
        price -= discount;
        System.out.println("Discount of " + percentage + "% applied.");
        System.out.println("New price: $" + price);
    }

    // Method to check if car is vintage (older than 25 years)
    public boolean isVintage() {
        int currentYear = 2024;
        return (currentYear - year) > 25;
    }

    // Getter methods
    public String getBrand() { return brand; }
    public String getModel() { return model; }
    public int getYear() { return year; }
    public double getPrice() { return price; }
}

class CarDemo {
    public static void main(String[] args) {
        // Create car objects using constructor
        Car car1 = new Car("Toyota", "Camry", 2020, 25000.0);
        Car car2 = new Car("BMW", "X5", 1995, 15000.0);
        Car car3 = new Car("Honda", "Civic", 2023, 22000.0);

        // Display car details
        car1.displayDetails();
        car2.displayDetails();
        car3.displayDetails();

        // Apply discount to car1
        car1.applyDiscount(10);
        System.out.println();

        // Check which cars are vintage
        System.out.println("=== Vintage Car Check ===");
        Car[] cars = {car1, car2, car3};

        for (Car car : cars) {
            String vintage = car.isVintage() ? "Yes" : "No";
            System.out.println(car.getBrand() + " " + car.getModel() +
                             " (" + car.getYear() + ") - Vintage: " + vintage);
        }
    }
}

Output:

=== Car Details ===
Brand: Toyota
Model: Camry
Year: 2020
Price: $25000.0

=== Car Details ===
Brand: BMW
Model: X5
Year: 1995
Price: $15000.0

=== Car Details ===
Brand: Honda
Model: Civic
Year: 2023
Price: $22000.0

Discount of 10.0% applied.
New price: $22500.0

=== Vintage Car Check ===
Toyota Camry (2020) - Vintage: No
BMW X5 (1995) - Vintage: Yes
Honda Civic (2023) - Vintage: No

Example 3: Interactive Class with User Input

import java.util.Scanner;

class BankAccount {
    private String accountHolder;
    private String accountNumber;
    private double balance;

    // Constructor
    public BankAccount(String holder, String number, double initialBalance) {
        this.accountHolder = holder;
        this.accountNumber = number;
        this.balance = initialBalance;
    }

    // Method to deposit money
    public void deposit(double amount) {
        if (amount > 0) {
            balance += amount;
            System.out.println("Deposited: $" + amount);
            System.out.println("New balance: $" + balance);
        } else {
            System.out.println("Invalid deposit amount!");
        }
    }

    // Method to withdraw money
    public void withdraw(double amount) {
        if (amount > 0 && amount <= balance) {
            balance -= amount;
            System.out.println("Withdrawn: $" + amount);
            System.out.println("New balance: $" + balance);
        } else if (amount > balance) {
            System.out.println("Insufficient funds!");
        } else {
            System.out.println("Invalid withdrawal amount!");
        }
    }

    // Method to display account info
    public void displayAccountInfo() {
        System.out.println("=== Account Information ===");
        System.out.println("Account Holder: " + accountHolder);
        System.out.println("Account Number: " + accountNumber);
        System.out.println("Current Balance: $" + balance);
        System.out.println();
    }

    // Getter for balance
    public double getBalance() {
        return balance;
    }
}

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

        // Create a bank account
        System.out.print("Enter account holder name: ");
        String name = scanner.nextLine();

        System.out.print("Enter account number: ");
        String accNum = scanner.nextLine();

        System.out.print("Enter initial balance: ");
        double initialBalance = scanner.nextDouble();

        BankAccount account = new BankAccount(name, accNum, initialBalance);
        account.displayAccountInfo();

        // Simple banking operations
        while (true) {
            System.out.println("Choose operation:");
            System.out.println("1. Deposit");
            System.out.println("2. Withdraw");
            System.out.println("3. Check Balance");
            System.out.println("4. Exit");
            System.out.print("Enter choice: ");

            int choice = scanner.nextInt();

            switch (choice) {
                case 1:
                    System.out.print("Enter deposit amount: ");
                    double depositAmount = scanner.nextDouble();
                    account.deposit(depositAmount);
                    break;

                case 2:
                    System.out.print("Enter withdrawal amount: ");
                    double withdrawAmount = scanner.nextDouble();
                    account.withdraw(withdrawAmount);
                    break;

                case 3:
                    System.out.println("Current balance: $" + account.getBalance());
                    break;

                case 4:
                    System.out.println("Thank you for using our banking system!");
                    scanner.close();
                    return;

                default:
                    System.out.println("Invalid choice! Please try again.");
            }
            System.out.println();
        }
    }
}

Key Concepts Demonstrated

  1. Class Definition: Creating blueprints for objects
  2. Object Creation: Instantiating objects from classes
  3. Instance Variables: Storing object state
  4. Methods: Defining object behavior
  5. Constructors: Initializing objects
  6. Encapsulation: Using private variables and public methods
  7. Method Overloading: Multiple methods with different parameters

Next Steps

Try extending these programs by:

  1. Adding more methods to the classes
  2. Creating multiple objects and comparing them
  3. Implementing inheritance between classes
  4. Adding validation and error handling
  5. Creating arrays of objects