Section 1.8 The JShell REPL
JShell is an interactive commandline tool introduced in Java 9 as part of the Java Development Kit (JDK). As mentioned in Section 1.6, JShell provides a Read-Eval-Print Loop (REPL) environment for evaluating and executing Java code snippets interactively, and is an excellent tool to explore Java syntax while learning the language. JShell runs in a terminal on top of your standard terminal shell program.
Several key features and characteristics of JShell include the following.
Interactive Java Execution: JShell allows developers to enter Java code directly into the command-line interface and immediately see the results without the need for writing a complete Java program. It eliminates the need to compile and run a separate Java file.
Code Exploration and Experimentation: JShell enables developers to explore and experiment with Java features, APIs, and libraries in an interactive manner. They can quickly test code snippets, try out new language constructs, and experiment with different approaches before incorporating them into a full-fledged Java application.
Instant Feedback: As you enter code in JShell, it evaluates and executes each line immediately, providing instant feedback on the results. It displays the output, errors, and any generated exceptions right after each statement is executed.
Tab Completion: JShell provides tab completion functionality, allowing developers to easily explore available classes, methods, variables, and packages. By typing a partial name and pressing the tab key, JShell suggests and completes the remaining portion, helping reduce typing errors and providing a convenient way to discover available options.
Variable Retention: JShell remembers and retains previously defined variables and their values, allowing developers to refer to and use them in subsequent code snippets. This feature facilitates incremental development and experimentation, as you can build upon previously defined variables without redefining them.
Multiline Input: JShell supports multiline input, enabling the entry of code blocks or longer code snippets spread across multiple lines. This is useful for writing more complex Java statements, loops, or methods interactively.
To start JShell, first open a terminal and enter the command
jshell. You will see the JShell prompt, similar to the following. At the JShell prompt you may enter any valid Java expression. We’ll explore this more in Chapter 2.>jshell | Welcome to JShell -- Version 17.0.6 | For an introduction type: /help intro jshell> 2 + 2 $1 ==> 4 jshell>
To exit JShell enter the command
/exit.While learning Java, it is useful to start JShell in verbose mode. To accomplish that, add the
-v option to the initial jshellcommand, in other words enter jshell -v into the terminal. Once in verbose mode, the output from executing Java expressions is more verbose, including the output type.>jshell -v | Welcome to JShell -- Version 17.0.6 | For an introduction type: /help intro jshell> 2 + 2 $1 ==> 4 | created scratch variable $1 : int jshell>
JShell includes many additional commands, all of which begin with the forward slash, like
/exit (see below). We will explore several of these additional commands in Chapter 2.jshell> /help | Type a Java language expression, statement, or declaration. | Or type one of the following commands: | /list [<name or id>|-all|-start] | list the source you have typed | /edit <name or id> | edit a source entry | /drop <name or id> | delete a source entry | /save [-all|-history|-start] <file> | Save snippet source to a file | /open <file> | open a file as source input | /vars [<name or id>|-all|-start] | list the declared variables and their values | /methods [<name or id>|-all|-start] | list the declared methods and their signatures | /types [<name or id>|-all|-start] | list the type declarations | /imports | list the imported items | /exit [<integer-expression-snippet>] | exit the jshell tool | /env [-class-path <path>] [-module-path <path>] [-add-modules <modules>] ... | view or change the evaluation context | /reset [-class-path <path>] [-module-path <path>] [-add-modules <modules>]... | reset the jshell tool | /reload [-restore] [-quiet] [-class-path <path>] [-module-path <path>]... | reset and replay relevant history -- current or previous (-restore) | /history [-all] | history of what you have typed | /help [<command>|<subject>] | get information about using the jshell tool | /set editor|start|feedback|mode|prompt|truncation|format ... | set configuration information | /? [<command>|<subject>] | get information about using the jshell tool | /! | rerun last snippet -- see /help rerun | /<id> | rerun snippets by ID or ID range -- see /help rerun | /-<n> | rerun n-th previous snippet -- see /help rerun
JShell is particularly helpful for learning Java, prototyping code, and exploring the language’s features in an interactive manner.
