Unlike Ruby, Python, and other JVM language, such as Groovy, Scala, Java itself lacks an interactive shell for a long time. Some 3rd party projects tried to fill the blank, such as Apache Bean Shell. Java 9 brings a built-in shell.
Open your terminal(cmd under Windows or Gnome Terminal under Linux/Gnome) and type jshell(make sure it is in your PATH environment variable or use the full path) to start up JShell console.
#jshell
| Welcome to JShell -- Version 9
| For an introduction type: /help intro
jshell>
It shows the welcome info.
Type /help intro to get a simple introduction to JShell.
jshell> /help intro
|
| intro
|
| The jshell tool allows you to execute Java code, getting immediate results.
| You can enter a Java definition (variable, method, class, etc), like: int x = 8
| or a Java expression, like: x + x
| or a Java statement or import.
| These little chunks of Java code are called 'snippets'.
|
| There are also jshell commands that allow you to understand and
| control what you are doing, like: /list
|
| For a list of commands: /help
Type /help to get the all help info of jshell.
Try add some code pieces to taste Java language features.
jshell> int i=1+1;
i ==> 2
jshell>
jshell> System.out.println(i);
2
Test the new Immutable Collections features we have discussed in the last section.
jshell> import java.util.*;
jshell> Set.of("One", "Two", "Three", "Four");
$4 ==> [Four, One, Three, Two]
jshell> System.out.println("set ::" + $4);
set ::[Four, One, Three, Two]
jshell> $4.add("Five");
| java.lang.UnsupportedOperationException thrown:
| at ImmutableCollections.uoe (ImmutableCollections.java:70)
| at ImmutableCollections$AbstractImmutableSet.add (ImmutableCollections.java:280)
| at (#6:1)
jshell> $4.remove("One");
| java.lang.UnsupportedOperationException thrown:
| at ImmutableCollections.uoe (ImmutableCollections.java:70)
| at ImmutableCollections$AbstractImmutableSet.remove (ImmutableCollections.java:283)
| at (#7:1)
jshell> Set<String> set= $4;
set ==> [Four, One, Three, Two]
jshell> set.add(null);
| java.lang.UnsupportedOperationException thrown:
| at ImmutableCollections.uoe (ImmutableCollections.java:70)
| at ImmutableCollections$AbstractImmutableSet.add (ImmutableCollections.java:280)
| at (#10:1)
As you see, the created collections do not allow to add or remove items.
jshell> /vars
| int i = 2
| Set<String> $4 = [Four, One, Three, Two]
| boolean $6 = false
| boolean $7 = false
| boolean $8 = false
| Set<String> set = [Four, One, Three, Two]
| boolean $10 = false
Every step JShell creates a variable to mark it, if you do not define an explicit variable, it will assign one for you. You can refer any defined before in your current code snippets.
jshell> /exit
| Goodbye
Type /exit to quit JShell console.