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Sealed Classes
Prerequisites: Records
A sealed class or interface (Java 17+) lists exactly which classes
are allowed to extend or implement it — closing off the set of
subtypes so the compiler can reason about all of them.
sealed interface Shape permits Circle, Square, Triangle {}
record Circle(double radius) implements Shape {}
record Square(double side) implements Shape {}
record Triangle(double base, double height) implements Shape {}
Paired with pattern matching (next lesson), a switch over a sealed
type can be checked exhaustively by the compiler — if a new
subtype is added later and a switch doesn't handle it, that switch
fails to compile instead of silently misbehaving at runtime.