参考教程：https://www3.ntu.edu.sg/home/ehchua/programming/howto/JDK_HowTo.html

1.准备工作&基础命令

//    display the current JDK version 

    javac -version

//    display the jre version

    java -version

//    display the location of java compiler

    which javac

//    display the location of java runtime

    which java

2.（如果java已经预安装，则跳过此步）jdk安装

    http://www.oracle.com/technetwork/java/javase/downloads/index.html

3.    写一个hello world