REPL(Read Eval Print Loop)意为交互式的编程环境。
JShell 是 Java 9 新增的一个交互式的编程环境工具。它允许你无需使用类或者方法包装来执行 Java 语句。它与 Python 的解释器类似，可以直接 输入表达式并查看其执行结果。
执行 JSHELL
$ jshell
| Welcome to JShell -- Version 9-ea
| For an introduction type: /help intro
jshell>
查看 JShell 命令
输入 /help 可以查看 JShell相关的命令：
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 referenced by name or id
| /drop <name or id>
| delete a source entry referenced by name or id
| /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 declared types
| /imports
| list the imported items
执行 JShell 命令
/imports 命令用于查看已导入的包：
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.*
jshell>
JShell 执行计算
以下实例执行 JShell 简单计算：
jshell> 3+1
$1 ==> 4
jshell> 13%7
$2 ==> 6
jshell> $2
$2 ==> 6
jshell>
JShell 创建与使用函数
创建一个函数 doubled() ，将传入的整型参数乘于 2 后返回：
jshell> int doubled(int i){ return i*2;}
| created method doubled(int)
jshell> doubled(6)
$3 ==> 12
jshell>
退出 JShell
输入 /exit 命令退出 jshell：
jshell> /exit
| Goodbye