"error: main method not found"，即使它在那里

我是Java的初学者，我正在尝试一些变量类型的练习。我刚刚复制了以下练习，我正试图用NetBeans 7.4(在Windows计算机上)运行它:

public class ClassVariables {
    public static class Employee{  
        private static double salary;
        // 'DEPARTMENT' is a constant:
        public static final String DEPARTMENT = "Development ";

        public static void main(String[] args){
            salary = 1000;
            System.out.println(DEPARTMENT + "average salary: " + salary);
        }
    } 
}

没有错误或警告显示在编辑器中，但当我运行代码时，我得到一个错误消息，说:

"Error: Main method not found in class classvariables.ClassVariables, please define the main method as:
    public static void main(String[] args)"

我已经根据需要定义了我的主要方法，但我一直得到这个错误消息!