方法一：

sudo add-apt-repository ppa:webupd8team/java
sudo apt-get update

sudo apt-get installOracle-java8-installer

方法二：

sudo add-apt-repository ppa:openjdk-r/ppa

sudo apt-get update
sudo apt-get install openjdk-8-jdk

openjdk version "1.8.0_01-internal"
OpenJDK Runtime Environment (build 1.8.0_01-internal-b04)
OpenJDK 64-Bit Server VM (build 25.40-b08,mixed mode)

sudo update-alternatives --config java

sudo update-alternatives --config javac

java -version