Designing with Sealed Classes
summary
Sealed classes in modern Java close inheritance for plugin-style APIs so core abstractions can be public but not extensible, replacing brittle hacks like package-private constructors, runtime instanceof checks, or custom annotation processors. Example: a sealed TrafficLight with three permitted leaves (Red, Yellow, Green) compiles to bytecode that records permitted subclasses and the compiler rejects unauthorized subclasses at compile time; reflection exposes getPermittedSubclasses, and using sealed + final yields exhaustiveness for pattern matching. Modules widen where permitted subclasses may live.
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chapters (8)
- 0:00Introduction to sealed classes
- 7:27Why restrict inheritance
- 16:45Hacky solutions for inheritance
- 26:38Sealed classes explained
- 38:09Practical sealed class example
- 45:54Non-sealed classes and use cases
- 57:05Reflection and pattern matching
- 1:00:16Summary and best practices
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