SmartCalc homework

package smartcalc;

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public final class SmartCalc {

    private static final Pattern OPERAND_PATTERN = Pattern.compile("\\d+(?:\\.\\d+)?");
    private static final Pattern OPERATION_PATTERN = Pattern.compile("add|subtract|multiply|divide");

    public static void main(final String[] args) {
        try (final Scanner scanner = new Scanner(System.in)) {
            String input;
            while (!(input = scanner.nextLine()).equals("quit")) {
                try {
                    final List<String> operands = allMatches(OPERAND_PATTERN, input);
                    final List<String> operations = allMatches(OPERATION_PATTERN, input);
                    if (operands.size() == 2 && operations.size() == 1) {
                        final double result = applyOperation(Double.parseDouble(operands.get(0)), Double.parseDouble(operands.get(1)), operations.get(0));
                        System.out.println("=" + result);
                        break;
                    } else {
                        throw new IllegalStateException("Illegal operand or operation size");
                    }
                } catch (final IllegalStateException e) {
                    System.out.println("You inputed more than two operands or more than one operations. Please try again or type 'quit' to stop");
                }
            }
        }
    }

    private static List<String> allMatches(final Pattern pattern, final String input) {
        final Matcher matcher = pattern.matcher(input);
        final List<String> operands = new ArrayList<>();
        while (matcher.find()) {
            operands.add(matcher.group());
        }
        return operands;
    }

    private static double applyOperation(final double firstOperand, final double secondOperand, final String operation) {
        switch (operation) {
            case "add":
                return firstOperand + secondOperand;
            case "subtract":
                return firstOperand - secondOperand;
            case "multiply":
                return firstOperand * secondOperand;
            case "divide":
                return firstOperand / secondOperand;
        }
        // TODO: throw exception
        return 0;
    }
}