import javafx.application.Application;
import javafx.geometry.Pos;
import javafx.scene.layout.HBox;
import javafx.scene.Scene;
import javafx.scene.control.TextField;
import javafx.scene.control.Label;
import javafx.stage.Stage;

public class Factorial extends Application {
    public void start(Stage stage) {
        stage.setTitle("Change Color");

        TextField input = new TextField("0");
        input.setPrefColumnCount(3);
        Label output = new Label("1");
        output.setPrefWidth(96);
        output.setAlignment(Pos.BASELINE_RIGHT);

        input.setOnAction(e -> {
            try {
                int n = Integer.parseInt(input.getText());
                output.setText("  " + factorial(n));
            } catch (NumberFormatException ex) {
                input.setText("数値!");
            }
        });
        HBox root = new HBox();
        root.setAlignment(Pos.CENTER);
        root.getChildren().addAll(input, new Label(" の階乗は "), output, new Label(" です。"));
        Scene scene = new Scene(root);
        stage.setScene(scene);
        stage.show();
    }

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

    public static void main(String... args) {
        launch(args);
    }
}