Sealed Classes have been added to the Java Language. Sealed classes and interfaces restrict which other classes or interfaces may extend or implement them.

Sealed Classes were proposed by [JEP 360](https://openjdk.java.net/jeps/360) and delivered in JDK 15 as a preview feature. They were proposed again, with refinements, by [JEP 397](https://openjdk.java.net/jeps/397) and delivered in JDK 16 as a preview feature. Now in JDK 17, Sealed Classes are being finalized with no changes from JDK 16.

For further details, see [JEP 409](https://openjdk.java.net/jeps/409).