Calculator using switch in Java

What we are building

A calculator takes two numbers and an operator (+, -, *, /) and shows the result. In Java it is a great small project that brings together Scanner input, the switch statement, and simple error handling for divide by zero.

The switch logic

double a = 10, b = 5;
char op = '+';
double result = 0;

switch (op) {
    case '+': result = a + b; break;
    case '-': result = a - b; break;
    case '*': result = a * b; break;
    case '/': result = a / b; break;
    default:  System.out.println("Invalid operator");
}
System.out.println(result);

The switch matches op to a case and runs that operation. Each case ends with break, and default catches an unknown operator.

The full program

import java.util.Scanner;

public class Calculator {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        double a = sc.nextDouble();
        char op = sc.next().charAt(0);
        double b = sc.nextDouble();
        double result = 0;

        switch (op) {
            case '+': result = a + b; break;
            case '-': result = a - b; break;
            case '*': result = a * b; break;
            case '/':
                if (b == 0) { System.out.println("Cannot divide by zero"); return; }
                result = a / b; break;
            default: System.out.println("Invalid operator"); return;
        }
        System.out.println("Result: " + result);
    }
}

We read two numbers and the operator with Scanner, choose the operation with switch, and print the result. The divide case checks for zero first.

Handling divide by zero

case '/':
    if (b == 0) {
        System.out.println("Cannot divide by zero");
    } else {
        result = a / b;
    }
    break;

Dividing by zero must be handled or the program errors. We check if (b == 0) before dividing and show a clear message. You could also catch ArithmeticException.

A menu loop

while (true) {
    // read a, op, b and compute with switch...
    System.out.print("Another? (y/n): ");
    if (!sc.next().equals("y")) break;
}
System.out.println("Goodbye");

A while (true) loop lets the user do many calculations. After each result, ask if they want another; anything other than "y" breaks the loop and ends.

Common mistakes

• Forgetting break. Without it, switch falls through to the next case.
• Not checking divide by zero. Always guard the division.
• Reading the operator wrong. Use sc.next().charAt(0) for a single char.
• No default case. Handle invalid operators with default.

Summary

• A calculator picks an operation based on the operator — perfect for switch.
• Read input with Scanner (nextDouble, next).
• Each case does one operation and ends with break.
• Guard the divide case against zero.
• Add a while loop to let the user repeat.

Next, learn the switch in depth with switch-case, or read input with Scanner.