控制时钟）修改程序清单14-21以在类中加入动画，添加两个方法start()和stop()以启动和停止时钟。编写程序，让用户使用Start和Stop按钮来控制始终

以下是修改后的程序清单14-21：

import javafx.animation.KeyFrame;
import javafx.animation.Timeline;
import javafx.beans.property.DoubleProperty;
import javafx.scene.Group;
import javafx.scene.effect.DropShadow;
import javafx.scene.paint.Color;
import javafx.scene.shape.Circle;
import javafx.scene.shape.Line;
import javafx.scene.text.Font;
import javafx.scene.text.Text;
import javafx.util.Duration;

public class ControlledClock extends Group {
    private final Circle face = new Circle(80, 80, 80);
    private final Text[] text = new Text[12];
    private final Line[] tick = new Line[60];
    private final DoubleProperty seconds;
    private final DoubleProperty minutes;
    private final DoubleProperty hours;
    private Timeline animation;

    public ControlledClock() {
        // Draw clock face
        face.setFill(Color.WHITE);
        face.setStroke(Color.BLACK);
        getChildren().add(face);

        // Draw numbers
        for (int i = 1; i <= 12; i++) {
            text[i - 1] = new Text(i + "");
            text[i - 1].setFont(Font.font(20));
            double angle = i * Math.PI / 6;
            double x = 80 + 60 * Math.sin(angle);
            double y = 80 - 60 * Math.cos(angle);
            text[i - 1].setX(x - text[i - 1].getLayoutBounds().getWidth() / 2);
            text[i - 1].setY(y + text[i - 1].getLayoutBounds().getHeight() / 2);
            getChildren().add(text[i - 1]);
        }

        // Draw ticks
        for (int i = 0; i < 60; i++) {
            tick[i] = new Line(0, 0, 0, 0);
            tick[i].setStrokeWidth(i % 5 == 0 ? 3 : 1);
            tick[i].setStroke(Color.BLACK);
            tick[i].setEffect(new DropShadow(5, Color.BLACK));
            double angle = i * Math.PI / 30;
            double x1 = 80 + 70 * Math.sin(angle);
            double y1 = 80 - 70 * Math.cos(angle);
            double x2 = 80 + 80 * Math.sin(angle);
            double y2 = 80 - 80 * Math.cos(angle);
            tick[i].setStartX(x1);
            tick[i].setStartY(y1);
            tick[i].setEndX(x2);
            tick[i].setEndY(y2);
            getChildren().add(tick[i]);
        }

        // Draw hands
        Line hourHand = new Line(0, 0, 0, -50);
        hourHand.setStrokeWidth(5);
        hourHand.setStroke(Color.BLACK);
        hourHand.setEffect(new DropShadow(5, Color.BLACK));
        hourHand.setTranslateX(80);
        hourHand.setTranslateY(80);
        getChildren().add(hourHand);
        hours = hourHand.rotateProperty();

        Line minuteHand = new Line(0, 0, 0, -70);
        minuteHand.setStrokeWidth(3);
        minuteHand.setStroke(Color.BLACK);
        minuteHand.setEffect(new DropShadow(5, Color.BLACK));
        minuteHand.setTranslateX(80);
        minuteHand.setTranslateY(80);
        getChildren().add(minuteHand);
        minutes = minuteHand.rotateProperty();

        Line secondHand = new Line(0, 0, 0, -80);
        secondHand.setStrokeWidth(1);
        secondHand.setStroke(Color.RED);
        secondHand.setEffect(new DropShadow(5, Color.BLACK));
        secondHand.setTranslateX(80);
        secondHand.setTranslateY(80);
        getChildren().add(secondHand);
        seconds = secondHand.rotateProperty();
        
        // Setup animation
        KeyFrame keyFrame = new KeyFrame(Duration.seconds(1), e -> {
            seconds.set(seconds.get() + 6);
            minutes.set(minutes.get() + 0.1);
            hours.set(hours.get() + 1.0 / 120);
        });
        animation = new Timeline(keyFrame);
        animation.setCycleCount(Timeline.INDEFINITE);
    }
    
    public void start() {
        animation.play();
    }
    
    public void stop() {
        animation.pause();
    }
}

在这个修改后的版本中，我们添加了两个方法 `start()` 和 `stop()` 以启动和停止时钟的动画。这个方法使用 JavaFX 中的 `Timeline` 类来实现动画效果。

我们可以编写以下代码来创建一个带有 "Start" 和 "Stop" 按钮的窗口，并控制时钟的开始和停止：

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class ControlledClockDemo extends Application {
    @Override
    public void start(Stage primaryStage) throws Exception {
        ControlledClock clock = new ControlledClock();
        Button startButton = new Button("Start");
        startButton.setOnAction(e -> clock.start());
        Button stopButton = new Button("Stop");
        stopButton.setOnAction(e -> clock.stop());
        VBox root = new VBox(clock, startButton, stopButton);
        Scene scene = new Scene(root);
        primaryStage.setScene(scene);
        primaryStage.show();
    }
    
    public static void main(String[] args) {
        launch(args);
    }
}

这个程序创建了一个带有时钟和两个按钮的窗口。当用户点击 "Start" 按钮时，时钟动画开始播放；当用户点击 "Stop" 按钮时，时钟动画停止。