我看到有人问这个，但那是两年前的事了，他们一直没有得到答案。我正试图运行《java game development with libgdx》一书中的一个示例程序，但我遇到了一个奇怪的错误。错误如下：

java.lang.ClassNotFoundException: Rock
at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:606)
at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:168)
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:522)
at java.base/java.lang.Class.forName0(Native Method)
at java.base/java.lang.Class.forName(Class.java:377)
at com.mygdx.game.BaseActor.getList(BaseActor.java:319)
at com.mygdx.game.StarfishCollector.update(StarfishCollector.java:35)
at com.mygdx.game.GameBeta.render(GameBeta.java:24)
at com.badlogic.gdx.backends.lwjgl.LwjglApplication.mainLoop(LwjglApplication.java:232)
at com.badlogic.gdx.backends.lwjgl.LwjglApplication$1.run(LwjglApplication.java:127)
Exception in thread "LWJGL Application" java.lang.NullPointerException: Cannot invoke "java.lang.Class.isInstance(Object)" because "theClass" is null
at com.mygdx.game.BaseActor.getList(BaseActor.java:326)
at com.mygdx.game.StarfishCollector.update(StarfishCollector.java:35)
at com.mygdx.game.GameBeta.render(GameBeta.java:24)
at com.badlogic.gdx.backends.lwjgl.LwjglApplication.mainLoop(LwjglApplication.java:232)
at com.badlogic.gdx.backends.lwjgl.LwjglApplication$1.run(LwjglApplication.java:127)
11:45:50 PM: Task execution finished 'DesktopLauncher.main()'.

我正在使用intellij，源代码可以找到：https://github.com/apress/java-game-dev-libgdx/blob/master/ch03%20starfish%20collector/baseactor.java 还有https://github.com/apress/java-game-dev-libgdx/blob/master/ch03%20starfish%20collector/starfishcollector.java
该方法的代码为：

public static ArrayList<BaseActor> getList(Stage stage, String className)
{
    ArrayList<BaseActor> list = new ArrayList<BaseActor>();

    Class theClass = null;
    try
    {  theClass = Class.forName(className);  }
    catch (Exception error)
    {  error.printStackTrace();  }

    for (Actor a : stage.getActors())
    {
        if ( theClass.isInstance( a ) )
            list.add( (BaseActor)a );
    }

    return list;
}

和

new Starfish(400,400, mainStage);
    new Starfish(500,100, mainStage);
    new Starfish(100,450, mainStage);
    new Starfish(200,250, mainStage);

    new Rock(200,150, mainStage);
    new Rock(100,300, mainStage);
    new Rock(300,350, mainStage);
    new Rock(450,200, mainStage);

    turtle = new Turtle(20,20, mainStage);

    win = false;
}

public void update(float dt)
{
    for (BaseActor rockActor : BaseActor.getList(mainStage, "Rock"))
        turtle.preventOverlap(rockActor);

    for (BaseActor starfishActor : BaseActor.getList(mainStage, "Starfish"))
    {
        Starfish starfish = (Starfish)starfishActor;
        if ( turtle.overlaps(starfish) && !starfish.collected )
        {
            starfish.collected = true;
            starfish.clearActions();
            starfish.addAction( Actions.fadeOut(1) );
            starfish.addAction( Actions.after( Actions.removeActor() ) );

            Whirlpool whirl = new Whirlpool(0,0, mainStage);
            whirl.centerAtActor( starfish );
            whirl.setOpacity(0.25f);
        }
    }

似乎找不到类名rock，所以类变量保持为null并导致错误。我希望有人能找到解决办法，否则这是一本好书。
谢谢