Simple Java Project: Calculator (Console-Based) in Java — Detailed Guide

Building a simple calculator in Java is an excellent practice project for beginners. It blends user input, control flow, arithmetic operations, and clear program structure. Here’s a step-by-step explanation of how to create a console-based calculator in Java.

What Will This Calculator Do?

Accept two numbers from the user.

Take an arithmetic operator (+, -, *, /) from the user.

Perform the chosen operation and show the result.

Handle invalid operations and errors (such as division by zero).

Step 1: Import Scanner for User Input

The Scanner class allows your program to get input from the keyboard.

java
import java.util.Scanner;

Step 2: Main Program Structure

We’ll put all code inside the main method, which is the entry point for every Java application.

java
public class Calculator {
    public static void main(String[] args) {
        // Program code goes here
    }
}

Step 3: Create Scanner and Prompt the User

You need to create a Scanner object and use it to accept numbers and an operator:

java
Scanner scanner = new Scanner(System.in);

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

System.out.print("Enter operator (+, -, *, /): ");
char operator = scanner.next().charAt(0);

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

Step 4: Calculate the Result Using Control Flow

We’ll use a switch statement to decide the operation based on the chosen operator:

java
double result;

switch (operator) {
    case '+':
        result = num1 + num2;
        break;
    case '-':
        result = num1 - num2;
        break;
    case '*':
        result = num1 * num2;
        break;
    case '/':
        if (num2 != 0) {
            result = num1 / num2;
        } else {
            System.out.println("Error: Division by zero is not allowed.");
            scanner.close();
            return;
        }
        break;
    default:
        System.out.println("Invalid operator!");
        scanner.close();
        return;
}

Explanation:

• For each operation, the calculator computes and saves the result.
• It checks for division by zero to prevent a crash.
• If the operator is not recognized, it shows an error and exits.

Step 5: Display the Result

Finally, print the result:

java
System.out.println("Result: " + result);
scanner.close();

Complete Code: Simple Console-Based Calculator in Java

java
import java.util.Scanner;

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

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

        System.out.print("Enter operator (+, -, *, /): ");
        char operator = scanner.next().charAt(0);

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

        double result;

        switch (operator) {
            case '+':
                result = num1 + num2;
                break;
            case '-':
                result = num1 - num2;
                break;
            case '*':
                result = num1 * num2;
                break;
            case '/':
                if (num2 != 0) {
                    result = num1 / num2;
                } else {
                    System.out.println("Error: Division by zero not allowed.");
                    scanner.close();
                    return;
                }
                break;
            default:
                System.out.println("Invalid operator!");
                scanner.close();
                return;
        }

        System.out.println("Result: " + result);

        scanner.close();
    }
}

Key Concepts Practiced in This Project

• User Input: Using Scanner to accept different types of data.
• Variables and Data Types: Storing numbers as double for arithmetic.
• Control Flow: Applying switch and if-else for decisions and validation.
• Error Handling: Catching invalid operators and division by zero.
• Output: Displaying results to the user.

How to Run the Calculator

1.Copy the complete code into a file named Calculator.java.

2. Compile:

Open a terminal in the file’s folder and run:

text
javac Calculator.java

3. Run:

text
java Calculator
Enter numbers and operators as prompted to perform calculations!

Tips to Expand

• Add more operations (e.g., modulus %, power **).
• Allow calculations in a loop until the user wants to exit.
• Handle invalid input (such as letters instead of numbers) with exception handling.

Final Thoughts

Building small projects like this calculator gives you practical confidence in Java basics and forms a foundation for bigger, real-world applications. Happy coding!

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