import java.awt.*
import javax.swing.*

fun factorial(n: Long): Long 
    = if (n == 0L) 1L else n * factorial(n - 1L)

fun main() {
    SwingUtilities.invokeLater {
        val panel  = JPanel()
        panel.setPreferredSize(Dimension(300, 50))  
        val input  = JTextField("0", 8) 
        val output = JLabel("1")
        input.addActionListener { _ ->
            val n = input.getText().toLong()
            output.setText("${factorial(n)}")
        }
        panel.add(input)
        panel.add(JLabel(" の階乗は "))
        panel.add(output)
        panel.add(JLabel(" です"))
    
        val frame = JFrame("Hello")
        frame.add(panel)
        frame.pack()
        frame.setDefaultCloseOperation(
            JFrame.EXIT_ON_CLOSE)
        frame.setVisible(true)
    }
}