Java 9 JShell – REPL


Java 9 provides an interactive REPL tool to test code snippets rapidly without a test project or main method. So we can learn or evaluate Java features easily. In this tutorial, we’re gonna look at how to use Java 9 JShellREPL.

I. Start JShell
1. Run

We can run JShell by jshell command available at ${JAVA_HOME}\bin directory:

This is how the JShell command line tool looks like:

Now we don’t need to create Java Project or define a public static void main(String[] args) method for testing code. Just write and run immediately.

2. Some commands

After executing /help command, we can see other useful commands:

For example, /help /list shows us some deeper commands:

II. JShell features
1. Default import types

To know set of common imports, just use /import command:

For example,* was imported by default, so the code below runs without error:

If we want to test other classes or interfaces of another package, we have to import it:
import java.nio.CharBuffer

*Note: For simple statements, we don’t need to use “semicolon”.

2. Expressions

A valid java expression will be returned a value and assigned to a variable.

In the example, $1 and $2 are assigned the values.

3. Variables

We can declare and initialize our variables:

4. Method

We can also define methods with JShell:

If rewrite the method with same name, it will be replaced.

5. Tab-Completion

Tab-Completion detection uses javac lexer, custom and table-driven code. So this helps us save time when typing only part of all characters.

For example, just type add, then press key Tab, JShell will complete the code: addTwoNumber(.

If we define another method:

Type add, then press key Tab, JShell shows :

That means we just type some more characters to make it unique, then Jshell will complete it.

6. List

– list variables using /vars

– list methods using /methods

– list all things we type using /list

Using /help command, we can see more functions.

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