REPL （JShell）

REPL代表Read-Eval-Print Loop。 使用JShell，java具有REPL功能。 使用REPL，我们可以编写和测试基于java的逻辑，无需使用javac进行编译，直接查看计算结果。

运行JShell

打开命令提示符并键入jshell。

$ jshell
|  Welcome to JShell -- Version 9-ea
|  For an introduction type: /help intro
jshell>

Viewing JShell commands

一旦jshell命令开始运行，输入/ help。

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

Running JShell command

一旦jshell命令开始运行就输入/ import，并查看使用的导入。

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>

在JShell中运行计算。

尝试在JShell中运行简单的计算。

jshell> 3+1
$1 ==> 4
jshell> 13%7
$2 ==> 6
jshell> $2
$2 ==> 6
jshell>

在JShell中创建和使用函数

创建一个doubled（）函数来获取int并返回其doubled值。

jshell> int doubled(int i){ return i*2;}
|  created method doubled(int)
jshell> doubled(6)
$3 ==> 12
jshell>

退出JShell

输入/退出。

jshell> /exit
| Goodbye