import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class Factorial extends JPanel implements ActionListener {
	private JTextField input;
	private JLabel output;

	public Factorial() {
		setPreferredSize(new Dimension(300, 50));
		input = new JTextField("0", 8);
		output = new JLabel("   1");
		input.addActionListener(this);
		setLayout(new FlowLayout());
		add(input);  add(new JLabel("の階乗は"));
		add(output); add(new JLabel("です。"));
	}

	private static int factorial(int n) { // factorial -- 階乗のこと
		int r = 1;
		for (; n > 0; n--) {
			r *= n;
		}
		return r;
	}

	public void actionPerformed(ActionEvent e) {
		try {
			int n = Integer.parseInt(input.getText());
			output.setText("  " + factorial(n));
		} catch (NumberFormatException ex) {
			input.setText("数値!");
		}
	}

	public static void main(String[] args) {
		SwingUtilities.invokeLater(() -> {
			JFrame frame = new JFrame("階乗の計算");
			frame.add(new Factorial());
			frame.pack();
			frame.setVisible(true);
			frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		});
	}
}