Understanding Sealed Classes in Java JDK 15

JDK 15 introduced sealed classes, allowing a class to restrict which other classes may extend it by using the sealed modifier together with a permits clause.

When a class is declared sealed, each subclass must be declared as final , sealed , or non-sealed ; otherwise the compiler reports an error.

Below is a typical inheritance hierarchy without sealed classes:

public class Person { }
class Teacher extends Person { } // teacher
class Student extends Person { } // student
class MiddleSchoolStudent extends Student { } // middle school student
class GraduateStudent extends Student { } // graduate student
class Worker extends Person { } // worker
class RailWayWorker extends Worker { } // railway worker

Attempting to mark Person as sealed without specifying permitted subclasses results in a compilation error:

public sealed class Person { }
class Teacher extends Person { } // teacher
class Student extends Person { } // student
class MiddleSchoolStudent extends Student { } // middle school student
class GraduateStudent extends Student { } // graduate student
class Worker extends Person { } // worker
class RailWayWorker extends Worker { } // railway worker

To correctly inherit from a sealed class, the permits clause must list all allowed subclasses, and each listed subclass must be declared final , sealed , or non-sealed :

public sealed class Person permits Teacher, Student, Worker { }
final class Teacher extends Person { } // teacher
sealed class Student extends Person permits MiddleSchoolStudent, GraduateStudent { }
final class MiddleSchoolStudent extends Student { } // middle school student
final class GraduateStudent extends Student { } // graduate student
non-sealed class Worker extends Person { } // worker
class RailWayWorker extends Worker { } // railway worker

Sigh, sealed classes can be inherited now, it overturns my code‑learning worldview.