Java Program to Create Simple Calculator

A simple Java calculator program uses Scanner to get two numbers and an operator, then uses switch to perform addition, subtraction, multiplication, or division and prints the result.

Examples

Input5 + 3

OutputResult: 8.0

Input10 / 2

OutputResult: 5.0

Input7 * 0

OutputResult: 0.0

How to Think About It

To create a simple calculator, first get two numbers and an operator from the user. Then check which operator was entered using switch or if statements. Perform the matching arithmetic operation and show the result. Handle division carefully to avoid dividing by zero.

Algorithm

1

Get the first number from the user

2

Get the second number from the user

3

Get the operator (+, -, *, /) from the user

4

Use a switch to decide which operation to perform

5

Calculate the result based on the operator

6

Print the result to the user

Code

java

import java.util.Scanner;

public class SimpleCalculator {
    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 second number: ");
        double num2 = scanner.nextDouble();
        System.out.print("Enter operator (+, -, *, /): ");
        char op = scanner.next().charAt(0);
        double result;
        switch (op) {
            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");
                    scanner.close();
                    return;
                }
                break;
            default:
                System.out.println("Invalid operator");
                scanner.close();
                return;
        }
        System.out.println("Result: " + result);
        scanner.close();
    }
}

Output

Enter first number: 5
Enter second number: 3
Enter operator (+, -, *, /): +
Result: 8.0

Dry Run

Let's trace the input '5', '3', and '+' through the code

1

Read first number

num1 = 5

2

Read second number

num2 = 3

3

Read operator

op = '+'

4

Switch on operator

case '+': result = 5 + 3 = 8

5

Print result

Output: Result: 8.0

Step	Variable	Value
1	num1	5
2	num2	3
3	op	+
4	result	8.0

Why This Works

Step 1: Input reading

The program uses Scanner to read two numbers and an operator from the user.

Step 2: Operation selection

It uses a switch statement on the operator to decide which arithmetic operation to perform.

Step 3: Calculation and output

The program calculates the result and prints it. It also checks for division by zero to avoid errors.

Alternative Approaches

Using if-else instead of switch

java

import java.util.Scanner;

public class SimpleCalculatorIfElse {
    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 second number: ");
        double num2 = scanner.nextDouble();
        System.out.print("Enter operator (+, -, *, /): ");
        char op = scanner.next().charAt(0);
        double result;
        if (op == '+') {
            result = num1 + num2;
        } else if (op == '-') {
            result = num1 - num2;
        } else if (op == '*') {
            result = num1 * num2;
        } else if (op == '/') {
            if (num2 != 0) {
                result = num1 / num2;
            } else {
                System.out.println("Error: Division by zero");
                scanner.close();
                return;
            }
        } else {
            System.out.println("Invalid operator");
            scanner.close();
            return;
        }
        System.out.println("Result: " + result);
        scanner.close();
    }
}

If-else is more flexible for complex conditions but can be longer than switch.

Using methods for each operation

java

import java.util.Scanner;

public class CalculatorWithMethods {
    public static double add(double a, double b) { return a + b; }
    public static double subtract(double a, double b) { return a - b; }
    public static double multiply(double a, double b) { return a * b; }
    public static double divide(double a, double b) {
        if (b == 0) throw new IllegalArgumentException("Division by zero");
        return a / b;
    }

    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 second number: ");
        double num2 = scanner.nextDouble();
        System.out.print("Enter operator (+, -, *, /): ");
        char op = scanner.next().charAt(0);
        double result;
        try {
            switch (op) {
                case '+': result = add(num1, num2); break;
                case '-': result = subtract(num1, num2); break;
                case '*': result = multiply(num1, num2); break;
                case '/': result = divide(num1, num2); break;
                default:
                    System.out.println("Invalid operator");
                    scanner.close();
                    return;
            }
            System.out.println("Result: " + result);
        } catch (IllegalArgumentException e) {
            System.out.println("Error: " + e.getMessage());
        }
        scanner.close();
    }
}

Using methods improves code organization and reusability but adds more lines.

Complexity: O(1) time, O(1) space

Time Complexity

The program performs a fixed number of operations regardless of input size, so it runs in constant time O(1).

Space Complexity

It uses a few variables for input and result, so space usage is constant O(1).

Which Approach is Fastest?

All approaches run in constant time; using methods adds slight overhead but improves readability.

Approach	Time	Space	Best For
Switch statement	O(1)	O(1)	Simple and clear operation selection
If-else statements	O(1)	O(1)	Flexible conditions, longer code
Separate methods	O(1)	O(1)	Better organization and reusability