> ## Documentation Index
> Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Java Non-Sealed Class
A `non-sealed` class is a subclass within a sealed inheritance hierarchy that explicitly terminates the extension restrictions imposed by its `sealed` superclass. Introduced in Java 17, the `non-sealed` modifier reverts the class to Java's default, open inheritance model, allowing any arbitrary, undeclared class to extend it.
When a standard class or interface inherits from a `sealed` parent, the Java compiler mandates that the subclass explicitly declare its ongoing inheritance posture using one of three modifiers. This explicit declaration rule does not apply to `record` or `enum` types, as records are implicitly `final` and enums are implicitly `final` or `sealed`. For standard classes, the modifiers are:
1. `final`: Completely prevents further extension.
2. `sealed`: Continues the restricted hierarchy by requiring its own `permits` clause.
3. `non-sealed`: Opens that specific branch of the hierarchy to unrestricted extension.
## Syntax and Implementation
The `non-sealed` modifier must be placed before the `class` or `interface` keyword. It is the first hyphenated keyword in the Java language.
The following example demonstrates a valid single-file compilation unit. The subclasses omit the `public` modifier so they can legally reside in the same file as the public root class.
```java theme={"dark"}
// 1. Root sealed class explicitly permitting three subclasses
public sealed class Shape permits Circle, Rectangle, Polygon {}
// 2. Terminal node: Cannot be extended
final class Circle extends Shape {}
// 3. Continued sealed node: Maintains restricted inheritance
sealed class Rectangle extends Shape permits Square {}
final class Square extends Rectangle {}
// 4. Non-sealed node: Terminates the restriction for this branch
non-sealed class Polygon extends Shape {}
// 5. Unrestricted extension: Any class can now extend Polygon
// without needing permission from Shape or Polygon
class Hexagon extends Polygon {}
class Octagon extends Polygon {}
```
## Technical Characteristics
* **Hierarchy Requirement:** The `non-sealed` modifier is only valid on a class or interface that directly extends a `sealed` class or implements a `sealed` interface. Applying it to a standalone class results in a compilation error.
* **Pattern Matching Exhaustiveness:** When using pattern matching for `switch` expressions on the root `sealed` class, the compiler considers the `non-sealed` subclass as a single exhaustive branch. The compiler does not require (or expect) you to handle the infinite potential subclasses of the `non-sealed` class.
* **Interface Application:** Interfaces cannot be marked `final`. Therefore, an interface extending a `sealed` interface must be marked either `sealed` or `non-sealed`.
* **Reflection:** At the bytecode level, a `non-sealed` class does not possess the `PermittedSubclasses` attribute. Invoking `Class::isSealed` on a `non-sealed` class evaluates to `false`.
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