EnumTest类
package java;

public class EnumTest {

        public static void main(String[] args) {
                WeekDay weekDay = WeekDay.MON;
                System.out.println(weekDay.nextDay());
        }

}
WeekDay 类
package java;

public class WeekDay {
        private WeekDay(){}
       
        public final static WeekDay SUN = new WeekDay();
        public final static WeekDay MON = new WeekDay();
       
        public WeekDay nextDay()
        {
                if(this==SUN)
                        return MON;
                else
                        return SUN;
        }
        public String toString()
        {
                return this==SUN?"SUM":"MON";
        }

}
错误类型
java.lang.SecurityException: Prohibited package name: java
        at java.lang.ClassLoader.preDefineClass(Unknown Source)
        at java.lang.ClassLoader.defineClass(Unknown Source)
        at java.security.SecureClassLoader.defineClass(Unknown Source)
        at java.net.URLClassLoader.defineClass(Unknown Source)
        at java.net.URLClassLoader.access$000(Unknown Source)
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClassInternal(Unknown Source)
Exception in thread "main"

好纠结啊，出来了原来是包名有问题，谢谢楼上了

包名，文件在这个里面

package java;
为什么要写这一句？

运行时出现 could not find the main class :java.EnumTest.Program will exit

看不到错误，我试着运行了一下，无误，控制台返回SUM；
你看看你不是点到别的工程或者类上了。