Java黑皮书15.3（移动小球）

package sample;

import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.Pane;
import javafx.scene.shape.Circle;
import javafx.scene.control.Button;
import javafx.scene.layout.HBox;
import javafx.scene.paint.Color;
import javafx.stage.Stage;

public class Main extends Application {
    public static void main(String[] args) {
        launch(args);
    }

    Ball ball = new Ball(200, 100, 30);

    @Override
    public void start(Stage primaryStage) {
        Board board = new Board(ball);
        frame frame = new frame();
        BorderPane borderPane = new BorderPane();
        borderPane.setCenter(board);
        borderPane.setBottom(frame);
        Scene scene = new Scene(borderPane, 600, 300);
        primaryStage.setTitle("移动小球");
        primaryStage.setScene(scene);
        primaryStage.show();
        frame.btLeft.setOnAction(new Left());
        frame.btRight.setOnAction(new Right());
        frame.btUp.setOnAction(new Up());
        frame.btDown.setOnAction(new Down());
    }

    class Left implements EventHandler {
        @Override
        public void handle(ActionEvent e) {
            ball.Left();
            ball.checkBall();
        }
    }

    class Right implements EventHandler {
        @Override
        public void handle(ActionEvent e) {
            ball.Right();
            ball.checkBall();
        }
    }

    class Up implements EventHandler {
        @Override
        public void handle(ActionEvent e) {
            ball.Up();
            ball.checkBall();
        }
    }

    class Down implements EventHandler {
        @Override
        public void handle(ActionEvent e) {
            ball.Down();
            ball.checkBall();
        }
    }

    class Ball extends Circle {
        public Ball() {
        }

        public Ball(double x, double y, double r) {
            super(x, y, r);
            setStroke(Color.BLACK);
            setFill(Color.WHITE);
        }

        public void Left() {
            setCenterX(getCenterX() - 10);
        }

        public void Right() {
            setCenterX(getCenterX() + 10);
        }

        public void Up() {
            setCenterY(getCenterY() - 10);
        }

        public void Down() {
            setCenterY(getCenterY() + 10);
        }

        public void checkBall() {
            if (getCenterX() < 0) Right();
            if (getCenterX() > 400) Left();
            if (getCenterY() < 0) Down();
            if (getCenterY() > 220) Up();
        }
    }

    class Board extends Pane {
        public Board() {
        }

        public Board(Ball ball) {
            getChildren().add(ball);
        }
    }

    class frame extends HBox {
        public Button btLeft = new Button("左");
        public Button btRight = new Button("右");
        public Button btUp = new Button("上");
        public Button btDown = new Button("下");

        public frame() {
            this.getChildren().add(btLeft);
            this.getChildren().add(btRight);
            this.getChildren().add(btUp);
            this.getChildren().add(btDown);
            setAlignment(Pos.CENTER);
        }
    }
}