sealed

• https://www.baeldung.com/java-sealed-classes-interfaces
• Introduced in Java 15+
• introduces keywords: sealed, permits and non-sealed

sealed Interfaces/Classes

• only permit certain classes to implement the class/interface
• sealed classes/interfaces constraints:
  • permitted class should be in same module as the sealed class
  • permitted class should explicitly extend the sealed class
  • permitted class must define a modifier:
    • final: class cannot be inherited
    • sealed: class is sealed
    • non-sealed: open for extension, other classes can inherit this class

sealed interface Vehicle permits Car, Truck {
    // ...
}
 
// must use `final` or `sealed` or `non-sealed`
final class Truck extends Vehicle {
    // ...
}

• permits must be used after extends or implements

sealed class Parent extends GrandParent permits Child {
    // ...
}

• Although possible, it is not recommended to omit permits clause.
• If it is omitted then all classes in the same compilation unit can inherit the class