What are the rules for external declarations in JShell in Java 9?

JShell is a command-line tool introduced in Java 9, and it is Java's first official REPL tool to create a simple programming environment that reads the user's inputs, evaluates it, and prints the result.

The declarations outside a class or interface (and declarations of classes and interfaces by themselves) have been created under the following rules.

Rules for External Declarations:

1) Access modifiers like public, protected, and private can be ignored. All declaration snippets can be accessible to all other snippets.

jshell> private int i = 10;
i ==> 10

jshell> System.out.println(i);
10

2) The modifier final can be ignored. Changes and inheritance are allowed.

jshell> final class A {void m() {} }
|   Warning:
|   Modifier 'final' not permitted in top-level declarations, ignored
|   final class A {void m() {} }
|   ^---^
|   created class A

3) The modifier static can be ignored because there is not a container class.

jshell> static char letter = 'A;
|   Warning:
|   Modifier 'static' not permitted in top-level declarations, ignored
|   static char letter = 'A';
|   ^----^
letter ==> 'A'

4) The modifier's default and synchronized are not allowed.

jshell> synchronized void method() {}
|   Error:
|   Modifier 'synchronized' not permitted in top-level declarations
|   synchronized void method() {}
|   ^----------^

5) The modifier abstract is allowed only in classes.

jshell> abstract void method();
|   Error:
|   Modifier 'abstract' not permitted in top-level declarations
|   abstract void method();
|   ^------^