Java Sealed Class

public sealed class Shape permits Circle, Rectangle, Triangle {
    // Class implementation
}

final class Circle extends Shape {}
final class Rectangle extends Shape {}
final class Triangle extends Shape {}

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Subclass Modifier Rules

1. final: Terminates the hierarchy. The subclass cannot be extended further.
2. sealed: Continues the restricted hierarchy. The subclass restricts its own extensions (either via an explicit permits clause or implicitly if its own subclasses are declared in the same source file).
3. non-sealed: Re-opens the hierarchy. The subclass behaves like a standard Java class and can be extended by any unknown class.

public sealed interface Expression permits ConstantExpr, PlusExpr, EvalExpr, ValueExpr {}

// 1. Terminates the hierarchy
final class ConstantExpr implements Expression {}

// 2. Continues the restricted hierarchy
sealed class PlusExpr implements Expression permits FastPlusExpr {}
final class FastPlusExpr extends PlusExpr {}

// 3. Re-opens the hierarchy
non-sealed class EvalExpr implements Expression {}
class LazyEvalExpr extends EvalExpr {} // Allowed because EvalExpr is non-sealed

// Implicitly final (no explicit modifier required)
record ValueExpr(int value) implements Expression {}

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Compilation Constraints

• Direct Extension: Every type listed in the permits clause must directly extend or implement the sealed type.
• Module and Package Locality:
  • If the sealed class belongs to a named module, all permitted subclasses must belong to the same module.
  • If the sealed class belongs to an unnamed module, all permitted subclasses must reside in the same package.
• Interface Compatibility: The sealed and non-sealed modifiers, as well as the permits clause, apply identically to interfaces. A sealed interface restricts which interfaces can extend it and which classes or records can implement it.

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Implicit Permitting (Same-File Declarations)

public sealed class Result {
    // The compiler implicitly infers: permits Success, Failure
}

final class Success extends Result {}
final class Failure extends Result {}

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Reflection API Support

• isSealed(): Returns true if the class or interface is sealed.
• getPermittedSubclasses(): Returns an array of Class<?> objects representing all permitted subclasses, including both those explicitly declared in a permits clause and those implicitly inferred by the compiler via same-file declarations. If the class or interface is not sealed, this method returns null (not an empty array), which must be handled to avoid NullPointerExceptions.