sealed-classes.md

Sealed classes

Java 15 added a preview feature - sealed classes which existed in Scala for a long time.

An example of sealed class.

sealed interface Shape permits Shape.Circle, Shape.Rectangle {

    record Circle(Point center, int radius) implements Shape {
    }

    record Rectangle(Point lowerLeft, Point upperRight) implements Shape {
    }
}

The sealed keyword can be applied on classes or an interface. Like Enum and Record, the subclasses are implicitly final.

You can explicitly add final, sealed or non-sealed to the subclasses.

Currently you can use instanceof pattern matching in if/else conditional statement.

double area = 0.0f;
if (shape instanceof Shape.Circle c) {
    area = Math.PI * c.radius() * c.radius();
} else if (shape instanceof Shape.Rectangle r) {
    area = Math.abs((r.upperRight().y() - r.lowerLeft().y())
                    * (r.upperRight().x() - r.lowerLeft().x()));
}

System.out.println("area:" + area);

Ideally sealed classes should work seamlessly in the multicases of switch statement, like Scala or Kotlin, but currently it is NOT included yet.