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JShell is a Javaread-eval-print loop which was first introduced in the JDK 9. It is tracked by JEP 222 jshell: The Java Shell (Read-Eval-Print Loop). One reason why JShell was proposed for Java 9 is the lack of a standard interactive environment for the language; the de facto library to use a Java REPL was often BeanShell, which has been dormant since 2003, and arbitrarily diverged from the Java language.
Example
jshell> int a = { 0, 1, 3, 5, 8 }
a ==> int { 0, 1, 3, 5, 8 }
jshell> int fact(int n) {
...> return n < 2 ? 1 : n * fact(n - 1);
...> }
| created method fact(int)
jshell> for (int i = 0; i < a.length; ++i)
...> a = fact(a);
jshell> a
a ==> int { 1, 1, 6, 120, 40320 }
"JEP 222: jshell: The Java Shell (Read-Eval-Print Loop)". Oracle Corporation. 2016-10-10. Retrieved 2016-11-01. Provide an interactive tool to evaluate declarations, statements, and expressions of the Java programming language, together with an API so that other applications can leverage this functionality
"JEP 222: jshell: The Java Shell (Read-Eval-Print Loop)". Oracle Corporation. 2016-10-10. Retrieved 2016-11-01. Another alternative is to maintain the status quo: Use another language or use a third-party REPL such as BeanShell, though that particular REPL has been dormant for many years, is based on JDK 1.3, and makes arbitrary changes to the language.
"JShell, the Java 9 REPL – What does it do?". jaxenter.com. 2015-09-03. Archived from the original on 2015-09-05. Retrieved 2016-11-01. BeanShell (just like Groovy, too by the way) made an attempt of standardisation by the Java Community Process in JSR 274 – a JSR that did not produce any notable output, in spite of the fact that (or perhaps because?) two major companies, Sun and Google, had joined the expert group. Under the JCP.next initiative this JSR was declared "Dormant".
