密封类

JDK15的预览特性，jdk17转正
sealed类，被sealed修饰的类可以指定子类,使用permits指定子类
non-sealed修饰的类可以被任何类继承，没有sealed的限制
要求指定的子类必须是final的或被sealed修饰的或被non-sealed修饰的

public sealed class Person permits Student,Teacher,Worker {
}
public final class Student extends Person{}
public sealed class Teacher extends Person{}
public sealed class Worker extends Person{}