问使用递归的简单阶乘程序

EN

import javax.swing.JOptionPane;

class FactorialDemo {
    public static void main(String[] args) {
        int factorialNumber = 0;
        try {
            factorialNumber = Integer.parseInt(JOptionPane.showInputDialog(null,
                "Which number should we compute the factorial of?"));
        } catch(NumberFormatException nfe) {
            JOptionPane.showMessageDialog(null,"Input must be an integer!",
                "Error", JOptionPane.ERROR_MESSAGE);

            throw new NumberFormatException("Input must be an integer.");
        }

        JOptionPane.showMessageDialog(null,
                factorialNumber + " factorial is " +recur(factorialNumber) +".",
                "Result", JOptionPane.PLAIN_MESSAGE);
    }

    public static int recur(int n) {
        int result;
        if (n < 0) {
            JOptionPane.showMessageDialog(null,"n must be >= 0 but was " + n,
                "Error", JOptionPane.ERROR_MESSAGE);

            throw new IllegalArgumentException("n must be >= 0 but was " + n);
        }
        if (n < 2) {
            return 1;
        }

        result = recur(n - 1) * n;
        return result;
    }
}