Calculator.java

import java.util.Scanner;

/**
 * A beginner-friendly console calculator in Java.
 * This class provides basic arithmetic operations: addition, subtraction, multiplication, and division.
 */
public class Calculator {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        boolean continueCalculating = true;

        System.out.println("Welcome to the Java Console Calculator!");

        while (continueCalculating) {
            displayMenu();
            int choice = getUserChoice(scanner);

            if (choice == 5) {
                continueCalculating = false;
                System.out.println("Exiting the calculator. Goodbye!");
                break;
            }

            if (choice < 1 || choice > 5) {
                System.out.println("Invalid choice. Please select an option from 1 to 5.");
                continue;
            }

            System.out.print("Enter the first number: ");
            double num1 = getValidDouble(scanner);

            System.out.print("Enter the second number: ");
            double num2 = getValidDouble(scanner);

            double result = 0;

            switch (choice) {
                case 1:
                    result = add(num1, num2);
                    System.out.println("Result: " + num1 + " + " + num2 + " = " + result);
                    break;
                case 2:
                    result = subtract(num1, num2);
                    System.out.println("Result: " + num1 + " - " + num2 + " = " + result);
                    break;
                case 3:
                    result = multiply(num1, num2);
                    System.out.println("Result: " + num1 + " * " + num2 + " = " + result);
                    break;
                case 4:
                    if (num2 == 0) {
                        System.out.println("Error: Division by zero is not allowed.");
                    } else {
                        result = divide(num1, num2);
                        System.out.println("Result: " + num1 + " / " + num2 + " = " + result);
                    }
                    break;
            }
            
            System.out.println("--------------------------------------------------");
        }

        scanner.close();
    }

    /**
     * Displays the calculator menu.
     */
    private static void displayMenu() {
        System.out.println("\nSelect an operation:");
        System.out.println("1. Addition (+)");
        System.out.println("2. Subtraction (-)");
        System.out.println("3. Multiplication (*)");
        System.out.println("4. Division (/)");
        System.out.println("5. Exit");
        System.out.print("Enter choice (1-5): ");
    }

    /**
     * Gets a valid integer choice from the user.
     */
    private static int getUserChoice(Scanner scanner) {
        while (!scanner.hasNextInt()) {
            System.out.println("Invalid input. Please enter a number (1-5).");
            scanner.next(); // Clear invalid input
        }
        return scanner.nextInt();
    }

    /**
     * Gets a valid double number from the user.
     */
    private static double getValidDouble(Scanner scanner) {
        while (!scanner.hasNextDouble()) {
            System.out.println("Invalid input. Please enter a numerical value.");
            scanner.next(); // Clear invalid input
        }
        return scanner.nextDouble();
    }

    /**
     * Adds two numbers.
     */
    public static double add(double a, double b) {
        return a + b;
    }

    /**
     * Subtracts the second number from the first.
     */
    public static double subtract(double a, double b) {
        return a - b;
    }

    /**
     * Multiplies two numbers.
     */
    public static double multiply(double a, double b) {
        return a * b;
    }

    /**
     * Divides the first number by the second.
     */
    public static double divide(double a, double b) {
        return a / b;
    }
}