When do sealed classes or interfaces improve a Java model?

Uses sealed types for deliberately closed variants and connects them to exhaustive reasoning, evolution, and open extension boundaries.

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A sealed class or interface restricts which types may directly extend or implement it. That is useful for a deliberately closed model such as a payment result with Approved, Declined, and Unavailable variants. The permitted subtypes must declare whether they are final, sealed, or non-sealed. Because the set is known, pattern matching and switch expressions can reason exhaustively about the variants. A normal open interface is better when third parties or future modules are expected to add implementations freely.

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Closed-world modeling, permitted subtypes, exhaustive handling, and versioning trade-offs.

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Sealed types are useful when the domain owns a known set of variants and wants the compiler to help keep that set controlled.
A sealed class or interface restricts which types may directly extend or implement it. That is useful for a deliberately closed model such as a payment result with Approved, Declined, and Unavailable variants. The permitted subtypes must declare whether they are final, sealed, or non-sealed. Because the set is known, pattern matching and switch expressions can reason exhaustively about the variants. A normal open interface is better when third parties or future modules are expected to add implementations freely.
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Using sealed as another word for immutable.
Sealing a public extension point intended for plugins.
Creating variants that still rely on nullable fields and type codes.
Ignoring the impact of adding a new subtype.

When do sealed classes or interfaces improve a Java model?

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