Java 15 introduced Sealed Classes, which provide a way to restrict the inheritance hierarchy of a class to a finite set of subclasses. Sealed classes are declared using the sealed modifier, and their permitted subclasses are declared using the permits keyword.
Example:
public sealed class Shape permits Circle, Rectangle, Triangle {
// ...
}
public final class Circle extends Shape {
// ...
}
public final class Rectangle extends Shape {
// ...
}
public final class Triangle extends Shape {
// ...
}