Java 17: Sealed classes restrict the hierarchy

A sealed type permits only a fixed set of subclasses, enabling exhaustive handling.

Code

sealed interface Shape permits Circle, Square {}
record Circle(double r) implements Shape {}
record Square(double side) implements Shape {}

Shape s = new Circle(2);
double area = switch (s) {
    case Circle c -> Math.PI * c.r() * c.r();
    case Square sq -> sq.side() * sq.side();
};
System.out.printf("area=%.2f%n", area);

Output

area=12.57

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Published 2026-07-26