java需要JavaFX帮助写一个。txt文件

Question

我需要创建一个JavaFX项目，创建一个gui来输入名称和密码。我创造了窗格，并搭建了舞台。项目显示正确，但我不能点击按钮获取信息写入文件。它会创建文件，但不会存储数据。我需要能够有几个名字和密码，这样我就可以创建一个读者登录程序来检查。txt文件。这是我的密码。 谢谢

import javax.swing.JOptionPane;
import javafx.application.Application;
import javafx.geometry.Pos;
import javafx.geometry.HPos;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.control.TextField;
import javafx.scene.layout.GridPane;
import javafx.stage.Stage;

import java.io.*;

public class AddUser extends Application {
    private TextField tfUsername = new TextField();
    private TextField tfPassword = new TextField();
    private Button btAddUser = new Button("Add User");
    private Button btClear = new Button("Clear");

    @Override // Override the start method in the Application class
    public void start(Stage primaryStage) {
        // Create UI
        GridPane gridPane = new GridPane();
        gridPane.setHgap(5);
        gridPane.setVgap(5);
        gridPane.add(new Label("Username:"), 0, 0);
        gridPane.add(tfUsername, 1, 0);
        gridPane.add(new Label("Password:"), 0, 1);
        gridPane.add(tfPassword, 1, 1);
        gridPane.add(btAddUser, 1, 3);
        gridPane.add(btClear, 1, 3);

        // Set properties for UI
        gridPane.setAlignment(Pos.CENTER);
        tfUsername.setAlignment(Pos.BOTTOM_RIGHT);
        tfPassword.setAlignment(Pos.BOTTOM_RIGHT);

        GridPane.setHalignment(btAddUser, HPos.LEFT);
        GridPane.setHalignment(btClear, HPos.RIGHT);

        // Process events
        btAddUser.setOnAction(e -> writeNewUser());

        btClear.setOnAction(e -> {
            tfUsername.clear();
            tfPassword.clear();
        });   

        // Create a scene and place it in the stage
        Scene scene = new Scene(gridPane, 300, 150);
        primaryStage.setTitle("Add User"); // Set title
        primaryStage.setScene(scene); // Place the scene in the stage
        primaryStage.show(); // Display the stage
    }

    public void writeNewUser() {      
        PrintWriter fw = null;

        try {
            fw = new PrintWriter("users.txt");
            BufferedWriter bw = new BufferedWriter(fw);
            bw.write(tfUsername.getText());
            bw.newLine();
            bw.write(tfPassword.getText());
            fw.close();
        } catch (IOException e) {
            e.printStackTrace();

        }
    }

    /**
     * The main method is only needed for the IDE with limited
     * JavaFX support. Not needed for running from the command line.
     */
    public static void main(String[] args) {
        launch(args);
    }
}