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 worksA 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 arent 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 forwardReferencing() {
...> return "forwardReferencing";
...> }
| created method forwardReferencing()
| update modified method helloWorld()Classes
We can also define classes in jshell –
jshell> class HelloWorld {
...> public String helloWorldClass() {
...> return "helloWorldClass";
...> }
...> }
| created class HelloWorldAnd assign and access them –
/env
Useful Commands
Now we’ve got the basics here are some quick commands –
| Tab | Code completion |
| /vars | list of variables in the current shell |
| /methods | list of methods in current shell |
| /list | All code snippets in jshell session |
| /imports | Current imports in the shell |
| /methods | list of methods in current shell |
| /types | Current classes defined in the shell, in the case above we would see “class HelloWorld” |
| /edit | Lets you edit your session in an editor(default to JEditPad) |
| /exit | close session |
| Published on Java Code Geeks with permission by Martin Farrell, partner at our JCG program. See the original article here: Introduction to Java Virtual Machine (JVM) Opinions expressed by Java Code Geeks contributors are their own. |
