---

id: 6

slug: "sealed-classes"

title: "Sealed classes for type hierarchies"

category: "language"

difficulty: "intermediate"

jdkVersion: "17"

oldLabel: "Java 8"

modernLabel: "Java 17+"

oldApproach: "Open Hierarchy"

modernApproach: "sealed permits"

oldCode: |-

// Anyone can extend Shape

public abstract class Shape { }

public class Circle extends Shape { }

public class Rect extends Shape { }

// unknown subclasses possible

modernCode: |-

public sealed interface Shape

permits Circle, Rect {}

public record Circle(double r)

implements Shape {}

public record Rect(double w, double h)

implements Shape {}

summary: "Restrict which classes can extend a type — enabling exhaustive switches."

explanation: "Sealed classes define a closed set of subtypes. The compiler knows all\

\ possible cases, enabling exhaustive pattern matching without a default branch.\

\ Combined with records, they model algebraic data types."

whyModernWins:

- icon: "🔐"

title: "Controlled hierarchy"

desc: "Only permitted subtypes can extend — no surprise subclasses."

- icon: "✅"

title: "Exhaustive matching"

desc: "The compiler verifies switch covers all cases, no default needed."

- icon: "📐"

title: "Algebraic data types"

desc: "Model sum types naturally — sealed + records = ADTs in Java."

support:

state: "available"

description: "Widely available since JDK 17 LTS (Sept 2021)"

prev: "language/records-for-data-classes"

next: "language/record-patterns"

related:

- "language/switch-expressions"

- "language/module-import-declarations"

- "language/pattern-matching-switch"

docs:

- title: "Sealed Classes (JEP 409)"

href: "https://openjdk.org/jeps/409"