apt-get install software-properties-common
sudo add-apt-repository ppa:webupd8team/java

sudo apt-get update
sudo apt-get install oracle-java8-installer

设置系统默认jdk

JDk7

sudo update-java-alternatives -s java-7-oracle
JDK8

sudo update-java-alternatives -s java-8-oracle
如果即安装了jdk7,又安装了jdk8,要实现两者的切换,可以:

　　jdk8 切换到jdk7

sudo update-java-alternatives -s java-7-oracle
　　jdk7 切换到jdk8

sudo update-java-alternatives -s java-8-oracle
测试jdk 是是否安装成功:

java -version

javac -version