uninstall OpenJDK9

sudo apt-get autoremove openjdk-9-jre openjdk-9-jdk    ///要慎用auto命令，会把所有的软件包删掉

https://www.linuxidc.com/Linux/2017-11/148941.htm
https://askubuntu.com/questions/335457/how-to-uninstall-openjdk