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JEP 409: Sealed Classes
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 and delivered in JDK 15 as a preview feature. They were proposed again, with refinements, by JEP 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.[1]