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Java 16 - Sealed Classes



Java 15 introduces a sealed classes as preview feature which provides a fine grained control over inheritance. Java 16 provides some minor enhancements and keep this feature as Preview. Following are salient points to consider for a sealed class −

  • Sealed class is declared using sealed keyword.

  • Sealed classes allow to declare which class can be a subtype using permits keyword.

  • A class extending sealed class must be declared as either sealed, non-sealed or final.

  • Sealed classes helps in creating a finite and determinable hiearchy of classes in inheritance.

Example

Consider the following example −

ApiTester.java

public class APITester {
   public static void main(String[] args) {
      Person manager = new Manager(23, "Robert");
      manager.name = "Robert";
      System.out.println(getId(manager));
   }
   public static int getId(Person person) {
      if (person instanceof Employee) {
         return ((Employee) person).getEmployeeId();
      } 
      else if (person instanceof Manager) {
         return ((Manager) person).getManagerId();
      }
      return -1;
   }
}
abstract sealed class Person permits Employee, Manager {
   String name;
   String getName() {
      return name;
   }
}
final class Employee extends Person {
   String name;
   int id;
   Employee(int id, String name){
      this.id = id;
      this.name = name;
   }
   int getEmployeeId() {
      return id;
   }
}
non-sealed class Manager extends Person {
   int id;
   Manager(int id, String name){
      this.id = id;
      this.name = name;
   }
   int getManagerId() {
      return id;
   }
}

Compile and Run the program

$javac -Xlint:preview --enable-preview -source 16 APITester.java
$java --enable-preview APITester

Output

23