JShell: The Java Shell and the Read-Eval-Print Loop

Let's talk about JShell. We can explore it with the JDK 9 Early Access Release. As of now, the general availability of JDK9 is scheduled for 27 July, 2017, and the JShell feature was proposed as part of JEP 222. The motivation behind it is to provide interactive command line tools to quickly explore the features of Java.

From what I've seen, it is a very useful tool to get a glimpse of Java features very quickly, which is especially useful for new learners. Already, Java is incorporating functional programming features from Scala. Consider the move to a REPL (Read-Eval-Print Loop) interactive shell for Java, just like Scala, Ruby, JavaScript, Haskell, Clojure, and Python.

JShell will be a command-line tool with features like a history of statements with editing, tab completion, automatic addition of needed terminal semicolons, and configurable predefined imports.

After downloading JDK 9, set the PATH variable to access JShell. Below is a simple program using made using JShell. See how we don't need to write a class with the public static void main(String[] args) method to run a simple hello world application?

C:\Users\ABC>jshell
|  Welcome to JShell -- Version 9-ea
|  For an introduction type: /help intro

jshell> System.out.println("Say hello to jshell!!!");
Say hello to jshell!!!

jshell>

Now we will write a method that will add two variables and invoke a method via JShell.

jshell> public class Sample {
   ...> public int add(int a, int b) {
   ...> return a+b;
   ...> }
   ...> }
|  created class Sample
jshell> Sample s = new Sample();
s ==> Sample@49993335

jshell> s.add(10,9);
$4 ==> 19
jshell>

Now, we will create a static method with StringBuilder class without importing it, as jshell does that for you.

jshell> public class Sample {
   ...> public static void join() {
   ...> StringBuilder sb = new StringBuilder();
   ...> sb.append("Smart").append(" ").append("Techie");
   ...> System.out.println("The string is " + sb.toString());
   ...> }
   ...> }
|  created class Sample

jshell> Sample.join();
The string is Smart Techie

jshell>

I hope you've enjoyed this glance at JShell. In my next article, we will see another JDK 9 feature. Until then, stay tuned!