Java Pocket Guide, 4th Edition

Chapter 20. JShell: the Java Shell

JShell, originally called Project Kulla, is an interactive command-line read-eval-print-loop (REPL) tool introduced in the Java 9 SDK. Similar in functionality to such interpreters as Python’s ipython and Haskell’s ghci, JShell allows users to evaluate and test fragments of code in real time without the trouble of creating a test project or a class housing a main function.

The code in this chapter was tested against JShell version 9-ea.

Getting Started

JShell can be launched from the menu of the NetBeans IDE (Tools→Java Platform Shell), from the Windows command line by running jshell.exe from the /bin/ directory of your JDK installation, or in POSIX environments with the jshell command.

When the environment has loaded, you will be greeted with a prompt:

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


jshell>

From here, you will be able to enter, execute, or modify code snippets, or interact with the JShell environment through its built-in commands.

Snippets

JShell operates upon units called snippets, which are code fragments entered by the user at the jshell> prompt. Each snippet must take a form defined in the JLS, as summarized in Table 20-1:

Table 20-1. Permitted snippet forms

Java Language Specification Production	Example
Primary	10 / 2
Statement	if (value == null) { numWidgets = 0; }
ClassDeclaration	class Foo { }
MethodDeclaration	void sayHello () { System.out.println(“Hello”); }
FieldDeclaration ...