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jshell - The java REPL

When I had started with Scala the first thing that I loved was the Scala REPL(read–eval–print loop). It’s a great tool to learn Scala and test code snippets.

While back in the java world we had to write a class with main method to try something. But with Java 9 release this will change. JShell will be our shiny new java REPL - http://openjdk.java.net/jeps/222 .

To get started with java 9 currently you can download and install the early access version from here - https://jdk9.java.net/download/.

Starting the REPL

Jshell is part of the jdk bin directory so if u have that in your path you can just run the jshell command to start the REPL.

bash-3.2$ jshell
|  Welcome to JShell -- Version 9-ea
|  For an introduction type: /help intro

jshell>

There is a verbose mode which is feel will be very helpful for beginners. To start in verbose mode you can run : jshell –v.

To enter or exit verbose mode once you are in the repl you can use – /set feedback verbose and /set feedback normal.

jshell> /set feedback verbose
|  Feedback mode: verbose

jshell> String a = "A"
a ==> "A"
|  created variable a : String

jshell> /set feedback normal
|  Feedback mode: normal

jshell> String b = "B"
b ==> "B"

The verbose mode will be a good way to learn java.

Expressions

You can write any valid java expression and jshell will execute it and assign it to a variable.

jshell> 3 * 3
$1 ==> 9
|  created scratch variable $1 : int

jshell> $1
$1 ==> 9
|  value of $1 : int

Variables

Variables can be defined just like they are defined in a java program. Once a variable is defined it is present in the scope.

jshell> String str = "Hello World!"
str ==> "Hello World!"
|  created variable str : String

jshell> str
str ==> "Hello World!"
|  value of str : String

Did u notice no semicolons!

Methods

jshell> void printHelloWorld(){
   ...> System.out.println("Hello World!");
   ...> }
|  created method printHelloWorld()

jshell> printHelloWorld()
Hello World!

Class

jshell> class SampleClass {
   ...> public static void hello(){
   ...> System.out.println("hello");
   ...> }
   ...> }
|  created class SampleClass

jshell> SampleClass.hello()
hello

Commands

Jshell provides some helpful commands for you to get more information. Type /help and you will know all the list of commands. Here I have it for you to browse -

Networking

Jshell is not just for testing local code snippets. It gives you networking access which opens all sorts of possibilities.

jshell> URL url = new URL("http://kkanojia.me")
url ==> http://kkanojia.me
|  created variable url : URL

jshell> url.openConnection().getContentType()
$4 ==> "text/html; charset=utf-8"
|  created scratch variable $4 : String

Exceptions

One of my favourite things about jshell is it wraps checked exceptions in the background.

Here I read and print a file without catching IOEXception.

jshell> Path path = Paths.get("/Users/kunalkanojia/file.txt")
path ==> /Users/kunalkanojia/file.txt
|  created variable path : Path

jshell> Stream<String> lines = Files.lines(path)
lines ==> java.util.stream.ReferencePipeline$Head@78e67e0a
|  created variable lines : Stream<String>

jshell> lines.forEach(System.out::println)
this is sample

Forward reference

Another good thing is you dont have to worry about defining your methods and variables in correct order. The forward reference support is pretty good.

jshell> double area(double radius){
   ...> return PI * square(radius);
   ...> }
|  created method area(double), however, it cannot be invoked until variable PI, and method square(double) are declared

JavaDOC

Pressing shift tab will get you the javadoc

jshell> java.nio.file.Paths
java.nio.file.Paths
<press shift-tab again to see javadoc>

jshell> java.nio.file.Paths
java.nio.file.Paths
This class consists exclusively of static methods that return a Path by
converting a path string or URI .

Conclusion

The repl is definitely something which I think people will find useful.

Some features I would love to see:

  1. Paste Mode: to copy multiline commands
  2. Syntax Highlighting: Who doesn’t want that
  3. Multiline editing.

Having said that, I really like jshell. The tab auto-completion is quite good and features like forward reference & exception handling really make it usable. The ability to open external editor using /edit is cool.

It’s time to get rid of that TestMain.java class we all have in our workspace.