JShell in Five Minutes

This post builds on my My Top Java 9 Features post by looking more in depth at these features. Here we show you how you can learn JShell in five minutes, and improve your Java 9 development experience.

Getting Started

Assuming you have downloaded and installed Java 9 then you can start the shell by typing:

jshell

Or if you want verbose:

C:\jdk9TestGround>jshell -v
| Welcome to JShell -- Version 9
| For an introduction type: /help intro

jshell>

Variables

Simply type the variable, with or without semi-colons:

jshell> int i = 1;
i ==> 1
| created variable i : int

Unassigned values are automatically assigned to a variable beginning with $:

jshell> "Hello World"
$1 ==> "Hello World"
| created scratch variable $1 : String

This means we can reuse the value later:

jshell> System.out.println($1);
Hello World

Control Flows

The next step in JShell is to use control flows (for, if, while, …). We can do this by entering our condition, using return for each new line:

jshell> if ("Hello World".equals($1)) {
 ...> System.out.println("Woohoo my if condition works");
 ...> }
Woohoo my if condition works

A quick tip is to use TAB for code completion.

Methods

We can declare a method in a similar way as Flow control, and press for each new line:

jshell> String helloWorld() {
 ...> return "hello world";
 ...> }
| created method helloWorld()

Then call it:

jshell> System.out.println(helloWorld());
hello world

We can also change methods in our shell, and have methods calling methods that aren't defined yet:

jshell> String helloWorld() {
 ...> return forwardReferencing();
 ...> }
| modified method helloWorld(), however, it cannot be invoked until method forwardReferencing() is declared
| update overwrote method helloWorld()

Now we fix the method:

jshell> String helloWorld() {
 ...> return forwardReferencing();
 ...> }
| modified method helloWorld(), however, it cannot be invoked until method forwardReferencing() is declared
| update overwrote method helloWorld()

Classes

We can also define classes in JShell:

jshell> class HelloWorld {
 ...> public String helloWorldClass() {
 ...> return "helloWorldClass";
 ...> }
 ...> }
| created class HelloWorld

And assign and access them:

jshell> HelloWorld hw = new HelloWorld();
hw ==> HelloWorld@27a5f880
| created variable hw : HelloWorld

jshell> System.out.println(hw.helloWorldClass());
helloWorldClass

Imports

The current classes in the workspace can be seen using the /imports command.

Additional classes can be added through the –class-path, or –module-path:

jshell --class-path ...

Or you can use /env within the shell:

/env

Useful Commands

Now we’ve got the basics here are some quick commands:

Conclusion

The real strength of the JShell REPL editor is you can test out snippets and API’s. This is really useful for saving time, and experimenting when learning Java9. In fact, even if you are not moving to Java 9 straight away its worth installing JDK9 to use JShell as a testing ground.