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.
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>
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
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
We can declare a method in a similar way as Flow control, and press
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()
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
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
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 |
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.