Master Java’s Interactive Shell: Quick JShell Guide for Rapid Prototyping

What is an interactive programming environment? In such an environment each line of code you enter receives immediate feedback, making it ideal for quick verification and simple calculations, though not suited for complex engineering tasks.

JShell Quick Start

Open a terminal and run jshell:

➜  ~ jshell
|  Welcome to JShell -- version 9
|  Type /help intro for a brief overview.

jshell>

Running calculations

jshell> 1+2
$1 ==> 3

Defining variables

jshell> int a=1, b=2;
a ==> 1
b ==> 2

jshell> int c = a + b
c ==> 3

Defining functions

jshell> int sum(int a, int b){
    return a + b;
}
|  Created method sum(int,int)

jshell> int c = sum(1, 2)
c ==> 3

Defining classes

jshell> public class Calculate {
    public int a;
    public int b;
    public int sum(){
        return a + b;
    }
}
|  Created class Calculate

jshell> Calculate c = new Calculate();
c ==> Calculate@4fca772d

jshell> c.a = 1
$9 ==> 1

jshell> c.b = 2
$10 ==> 2

jshell> c.sum()
$11 ==> 3

You can also use Java 16 records for simpler data carriers.

Viewing commands: /help

Use /help to list available commands such as /list, /edit, /drop, /save, /open, /vars, /methods, /imports, /types, /reset, /exit, etc.

jshell> /help
| Type a Java expression, statement, or declaration.
| Or type one of the following commands:
| /list [name|id|-all|-start]
| /edit <name|id>
| /drop <name|id>
| /save [-all|-history|-start] <file>
| /open <file>
| /vars [name|id|-all|-start]
| /methods [name|id|-all|-start]
| /imports
| /types
| /reset [-class-path <path>] ...
| /exit [integer-expression-snippet]
| ...

Viewing defined methods: /methods

jshell> /methods
|    int sum(int,int)

Viewing defined variables: /vars

jshell> /vars
|    int $1 = 3
|    int b = 2
|    Calculate c = Calculate@4fca772d
|    int $9 = 1
|    int $10 = 2
|    int $11 = 3

Listing source entries: /list

jshell> /list

   1 : 1+2
   3 : int a=1, b=2;
   5 : int sum(int a, int b){
           return a + b;
       }
   7 : public class Calculate {
           public int a;
           public int b;
           public int sum(){
               return a + b;
           }
       }
   8 : Calculate c = new Calculate();
   9 : c.a = 1
  10 : c.b = 2
  11 : c.sum()
  12 : Map a = new HashMap();

Editing source entries: /edit

Use /edit 7 to modify entry 7; an editor window appears.

Dropping source entries: /drop

Use /drop 12 to delete the variable defined at entry 12.

jshell> /drop 12
|  Deleted variable a

Saving to a file: /save

Use /save aaa.txt to write the current session to a file.

Opening a file: /open

Use /open aaa.txt to reload a previously saved session.

Resetting JShell: /reset

Use /reset to clear all previous snippets.

jshell> /reset
|  Resetting state.

Viewing imports: /imports

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.*

Viewing defined types: /types

jshell> /types
|    class Calculate

Exiting JShell: /exit

jshell> /exit
| Goodbye