Gong.java

import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Date;
import javafx.animation.AnimationTimer;
import javafx.application.Application;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.Label;
import javafx.scene.input.MouseEvent;
import javafx.scene.layout.StackPane;
import javafx.scene.media.AudioClip;
import javafx.scene.paint.Color;
import javafx.scene.text.Font;
import javafx.stage.Stage;
import javafx.stage.StageStyle;

public class Gong extends Application {

    private static DateFormat formatter = new SimpleDateFormat("m:ss");
    private AnimationTimer timer;
    private Label label;
    
    private static final long M = 1_000_000; 
    private static final long FIVE = 300_000;
    private long start = 0;
    private long diff = 0;
    
    @Override
    public void start(Stage stage) {
        StackPane root = new StackPane();
        root.setPrefSize(1024, 768);
        root.setOnMousePressed(new EventHandler<MouseEvent>() {
            @Override
            public void handle(MouseEvent t) {
                if (diff == 0) {
                    timer.start();
                }
            }
        });

        label = new Label("5:00");
        label.setFont(Font.font("Arial Black", 380));
        label.setScaleY(2.0);
        label.setStyle("-fx-text-fill: white;");
        root.getChildren().add(label);
        
        Scene scene = new Scene(root, 1024, 768);
        scene.setFill(Color.BLACK);
        
        stage.setScene(scene);
        stage.initStyle(StageStyle.UNDECORATED);
        stage.show();

        final AudioClip clip = new AudioClip(getClass().getResource("gong.wav").toString());

        timer = new AnimationTimer() {
            @Override
            public void handle(long now) {
                if (start == 0) {
                    start = now;
                }
                
                if ((now - start)/M - diff > 1_000) {
                    diff += 1_000;
                    String time = formatter.format(new Date(FIVE - diff));
                    label.setText(time);
                }
                
                if (diff == FIVE) {
                    clip.play();
                    timer.stop();
                    diff = 0;
                    start = 0;
                }
            }
        };
    }

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