Java REPL (JShell): Interactive Java Programming at Your Fingertips

Discover Java's REPL (JShell), introduced in Java 9, which allows you to run Java code snippets interactively without the need for compiling a full program. Learn how JShell simplifies testing, evaluating expressions, and experimenting with code directly in the console.

Java - REPL (JShell)

Introduction to REPL (JShell)

REPL stands for Read Evaluate Print Loop. JShell, introduced in Java 9, is an interactive console that allows you to run arbitrary snippets of Java code directly in the console without needing to save and compile a Java file. This feature is particularly useful for quickly testing code, such as evaluating regular expressions or checking the formatting of strings and dates.

JShell reads each line you enter, evaluates it, prints the result, and then gets ready for the next input.

Advantages of Using JShell

JShell offers several advantages for developers:

• No editor is required to write a Java program; JShell itself functions as the editor and executes the Java code.
• There is no need to save a Java file, compile, and execute in a cycle. You can test code directly in JShell without saving.
• Compilation is unnecessary before executing the code.
• If any compile-time or runtime error occurs, you can start fresh without any hassle.

Running JShell

To start JShell, open the command prompt and type:

D:\test>jshell
|  Welcome to JShell -- Version 20.0.2
|  For an introduction type: /help intro

Welcome to JShell -- Version 20.0.2
For an introduction type: /help intro

Create and Invoke Method in JShell

Here's an example of a simple "Hello World" program in JShell. We will create a method greet() that prints "Hello World!" and then invoke it.

jshell> void greet() { System.out.println("Hello World!");}
|  created method greet()

jshell> greet()

Hello World!

Creating Variables in JShell

The following snippet demonstrates how to create variables in JShell. The semicolon is optional. You can also create objects. If a variable is not initialized, it gets a default value or null if it is an object reference. Once a variable is created, you can use it as shown in the last statement where we print the value of a string variable.

jshell> int i = 20
i ==> 20

jshell> String name = "John";
name ==> "John"

jshell> Date date = new Date()
date ==> Fri Feb 02 14:52:49 IST 2024

jshell> long l
l ==> 0

jshell> List list
list ==> null

jshell> name
name ==> "John"

20
"John"
Fri Feb 02 14:52:49 IST 2024
0
null
"John"

Evaluate Expression in JShell

The snippet below shows how to evaluate an expression in JShell. Here, we use a statement that returns a formatted string. JShell automatically creates a String variable $9 and assigns it the result.

jshell> String.format("%d pages read.", 5);
$9 ==> "5 pages read."

jshell> $9

"5 pages read."

JShell Built-In Commands

JShell provides various commands to list created variables, methods, and imports. Some important JShell commands include:

• /drop – Drops code snippets identified by name, ID, or ID range.
• /edit – Opens an editor.
• /env – Displays the environment settings.
• /exit – Exits from the tool.
• /history – Displays the history of commands.
• /help – Displays help for commands.
• /imports – Displays current active imports.

Example: Demonstrating /help Command

You can view all commands using the /help option.

jshell> /help
|  Type a Java language expression, statement, or declaration.
|  Or type one of the following commands:
|  /list [|-all|-start]
|       list the source you have typed
|  /edit 
|       edit a source entry
|  /drop 
|       delete a source entry
|  /save [-all|-history|-start] 
|       Save snippet source to a file
|  /open 
|       open a file as source input
|  /vars [|-all|-start]
|       list the declared variables and their values
|  /methods [|-all|-start]
|       list the declared methods and their signatures
|  /types [|-all|-start]
|       list the type declarations

Example: Demonstrating /vars Command

In the following example, we use the /vars command to print the variables declared during the session.

C:\Users\John>jshell
|  Welcome to JShell -- Version 20.0.2
|  For an introduction type: /help intro

jshell> int i = 20
i ==> 20

jshell> String name="John"
name ==> "John"

jshell> /vars
|    int i = 20
|    String name = "John"

Example: Demonstrating /imports Command

You can use the /imports command to check the imports available in JShell, as shown below:

jshell> /imports
|    import java.io.*
|    import java.math.*
|    import java.net.*
|    import java.nio.file.*
|    import java.util.*
|    import java.util.concurrent.*
|    import java.util.function.*
|    import java.util.prefs.*
|    import java.util.regex.*
|    import java.util.stream.*

Exiting JShell

We can use the /exit command to exit JShell, as shown below:

jshell> /exit
|  Goodbye

Goodbye