1.查看jdk的版本，centos7自带的jdk
[root@yuanmomo ~]# java -version
openjdk version "1.8.0_181"
OpenJDK Runtime Environment (build 1.8.0_181-b13)
OpenJDK 64-Bit Server VM (build 25.181-b13, mixed mode)
2.查看已经安装的jdk
[root@yuanmomo ~]# rpm -qa |grep java
java-1.7.0-openjdk-1.7.0.191-2.6.15.5.el7.x86_64
python-javapackages-3.4.1-11.el7.noarch
java-1.7.0-openjdk-headless-1.7.0.191-2.6.15.5.el7.x86_64
java-1.8.0-openjdk-headless-1.8.0.181-7.b13.el7.x86_64
tzdata-java-2018e-3.el7.noarch
javapackages-tools-3.4.1-11.el7.noarch
java-1.8.0-openjdk-1.8.0.181-7.b13.el7.x86_64
3.卸载（将1.8和1.7的jdk全部卸载掉），有两个版本的jdk，都要卸载了
[root@yuanmomo ~]# rpm -e --nodeps java-1.8.0-openjdk-headless
[root@yuanmomo ~]# rpm -e --nodeps java-1.7.0-openjdk-headless
[root@yuanmomo ~]# java -version
bash: /usr/bin/java: 没有那个文件或目录 ------------>说明卸载成功，然后就可以安装自己的jdk了