From Neilmadden · · 50s
Java sealed classes and exhaustive pattern matching
Java sealed classes prevent runtime errors but require recompiling to avoid breaking code.
In brief
Java sealed classes prevent runtime errors but require recompiling to avoid breaking code. Sealed classes ensure exhaustive pattern matching at compile time but break when new subtypes are added without recompiling. Originally reported by Neilmadden.
Java 17 sealed classes
Java 17 introduced sealed classes to explicitly list allowed sub-types of interfaces or base classes.
Sealed interface example
A sealed interface with records TypeA and TypeB allows switch expressions without default clauses.
Exhaustiveness checking
Sealed classes enable compile-time error detection for missing switch cases.
Adding new subtypes
Adding a new subtype to a sealed class breaks consumers without recompiling.
Runtime exception risk
Separate compilation causes runtime exceptions when new subtypes are added.
Internal vs public API
Sealed classes are most useful internally where recompiling everything is possible.
What it means
Sealed classes ensure exhaustive pattern matching at compile time but break when new subtypes are added without recompiling.
