What are sealed classes in Java?

Sealed classes and interfaces (Java 17) restrict which other classes can extend or implement them. public sealed class Shape permits Circle, Rectangle, Triangle { } — only the explicitly listed classes are allowed to be subclasses. Each permitted subclass must be declared final, sealed itself, or non-sealed (re-opens the hierarchy). Sealed classes enable exhaustive pattern matching in switch expressions — the compiler can verify all subtypes are handled without a default case. This brings algebraic data type (sum type) modeling to Java, similar to Kotlin sealed classes. They are particularly useful for modeling domain states, results, and discriminated unions.