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🏷 Sealed Classes 2 patterns

Topic: Sealed Classes

All Java patterns related to Sealed Classes — java.evolved

Sealed classes for type hierarchies

Old // Anyone can extend Shape public abstract class Shape { } public class Circle extends Shape { } public class Rect extends Shape { } // unknown subclasses possible

Modern public sealed interface Shape permits Circle, Rect {} public record Circle(double r) implements Shape {} public record Rect(double w, double h) implements Shape {}

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JDK 17+ learn more →

Exhaustive switch without default

Old // Must add default even though // all cases are covered double area(Shape s) { if (s instanceof Circle c) return Math.PI * c.r() * c.r(); else if (s instanceof Rect r) return r.w() * r.h(); else throw new IAE(); }

Modern // sealed Shape permits Circle, Rect double area(Shape s) { return switch (s) { case Circle c -> Math.PI * c.r() * c.r(); case Rect r -> r.w() * r.h(); }; // no default needed! }

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JDK 21+ learn more →