FULL PRODUCT VERSION :
java version "9"
Java(TM) SE Runtime Environment (build 9+181)
Java HotSpot(TM) 64-Bit Server VM (build 9+181, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows [Version 6.1.7601]

A DESCRIPTION OF THE PROBLEM :
The JShell ignores the keyword 'final' in top-level declarations. This makes sense for methods and variables, but final top-level classes should be perfectly legal.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
jshell> final class A {}
|  Warning:
|  Modifier 'final'  not permitted in top-level declarations, ignored
|  final class A {}
|  ^---^
|  created class A

jshell> class B extends A {}
|  created class B

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The JShell should accept the keyword final for class A and should print an error for the declaration of class B such as "cannot inherit from final A".
ACTUAL -
The JShell ignores the keyword final for class A and allows class B to inherit from A.

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
final class A {}
class B extens A {}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
You can experiment with final classes in the JShell inside a local context:

jshell> {
   ...>   final class A{}
   ...>   class B extends A{}
   ...> }
|  Error:
|  cannot inherit from final A
|    class B extends A{}
|                    ^