扩展javafx.scene.shape.Circle [duplicate]的类的InvocationTargetException

Question

这个问题在这里已有答案：

• What is a NullPointerException, and how do I fix it? 12个答案

我正在制作一个用于测试鼠标准确性的游戏（如AimBooster.com上所示），这是我的第一个JavaFX侧项目。这个游戏的主要元素是目标，我在类Target.java中描述的行为，它扩展了类Circle。这是我的类定义（在块“public class Target extends Circle”中）：

Circle target;
double centerX;
double centerY;
double radius;

StackPane game;

public Target(double x, double y, double rad) {

    centerX = x;
    centerY = y;
    radius = rad;

    target.setCenterX(centerX); // InvocationTargetException here
    target.setCenterY(centerY);
    target.setRadius(radius);


}

这一切看起来都非常简单，但是当我打电话时

target = new Target(200, 200, 10);

我得到一个InvocationTargetException。

我已就此异常做过一些研究，但我发现没有任何适用于JavaFX的内容。我究竟做错了什么？

编辑：完整堆栈跟踪：

Exception in Application start method
Exception in thread "main" java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at sun.launcher.LauncherHelper$FXHelper.main(LauncherHelper.java:767)
Caused by: java.lang.RuntimeException: Exception in Application start method
    at com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:917)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication$154(LauncherImpl.java:182)
    at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.NullPointerException
    at Target.<init>(Target.java:19)
    at GraphicsDemo.start(GraphicsDemo.java:168)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$161(LauncherImpl.java:863)
    at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$174(PlatformImpl.java:326)
    at com.sun.javafx.application.PlatformImpl.lambda$null$172(PlatformImpl.java:295)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.application.PlatformImpl.lambda$runLater$173(PlatformImpl.java:294)
    at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.lambda$null$147(WinApplication.java:177)
    ... 1 more

Answer 1

首先，这应该有助于未初始化的Circle。

Circle target = new Circle(); //create an instance.
double centerX;
double centerY;
double radius;

StackPane game;

public Target(double x, double y, double rad) {

    centerX = x;
    centerY = y;
    radius = rad;

    target.setCenterX(centerX); // InvocationTargetException here
    target.setCenterY(centerY);
    target.setRadius(radius);


}