Stage 5: submit a form and return the answer directly

<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <title>Arithmetic Calculator — Stage 5</title>
    <link rel="stylesheet" href="/style.css">
</head>
<body>
<main>
    <h1>Arithmetic Calculator</h1>
    <p>Enter two numbers and choose an operation.</p>

    <form action="/calculate" method="get">
        <label for="a">First number</label>
        <input id="a" name="a" type="number" step="any" required>

        <label for="b">Second number</label>
        <input id="b" name="b" type="number" step="any" required>

        <div class="buttons">
            <button type="submit" name="operation" value="add">Add (+)</button>
            <button type="submit" name="operation" value="subtract">Subtract (-)</button>
            <button type="submit" name="operation" value="multiply">Multiply (×)</button>
            <button type="submit" name="operation" value="divide">Divide (÷)</button>
            <button type="submit" name="operation" value="remainder">Remainder (%)</button>
        </div>
    </form>
    <p>The answer opens as text on the next page. Use the browser Back button to calculate again.</p>
    <p class="note">Remainder means what is left after division: 20 % 6 = 2.</p>
</main>
</body>
</html>

package com.edupoly.calculator;

import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.ResponseBody;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestParam;

@Controller
public class CalculatorController {
    @GetMapping("/calculate")
    @ResponseBody
    public String calculate(
            @RequestParam("a") double first,
            @RequestParam("b") double second,
            @RequestParam("operation") String operation) {

        if (!Double.isFinite(first) || !Double.isFinite(second)) {
            return "Please enter two finite numbers.";
        }

        double answer;
        String symbol;

        switch (operation) {
            case "add":
                answer = first + second;
                symbol = "+";
                break;
            case "subtract":
                answer = first - second;
                symbol = "-";
                break;
            case "multiply":
                answer = first * second;
                symbol = "×";
                break;
            case "divide":
                if (second == 0) {
                    return "Cannot divide by zero.";
                }
                answer = first / second;
                symbol = "÷";
                break;
            case "remainder":
                if (second == 0) {
                    return "Cannot calculate a remainder with zero as the divisor.";
                }
                answer = first % second;
                symbol = "%";
                break;
            default:
                return "Please choose one of the five operations.";
        }

        if (!Double.isFinite(answer)) {
            return "The result is too large. Please use smaller numbers.";
        }

        return first + " " + symbol + " " + second + " = " + answer;
    }
}