GoKawiilJava's sealed classes add compile-time exhaustiveness checks, new breaking-change risk
A technical explainer describes how sealed classes, introduced in Java 17, let developers restrict which subtypes can implement an interface or extend a class, enabling the compiler to verify that switch expressions handle every possible case without a default branch. The piece notes that forgetting a case triggers a compile-time error rather than a runtime failure, demonstrating this with a toy SealedType example using records.
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The analysis warns that this exhaustiveness checking has a tradeoff: adding a new permitted subtype to an already-published sealed type can break any code that pattern-matches on it, since previously exhaustive switches would no longer compile. This suggests that library maintainers exporting sealed types in public APIs should treat adding new subtypes as a breaking change requiring a major version bump under semantic versioning.
- Sealed classes/interfaces, added in Java 17, restrict which types may implement or extend them.
- The compiler can verify switch statements over sealed types are exhaustive, removing the need for a default case.
- Adding a new subtype to an already-public sealed type can break consumer code at compile time, implying a SemVer major version bump.
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Published there as: “Java sealed classes and exhaustive pattern matching”
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