OpenJDK Java 8 has been made into official Ubuntu repositories for 14.10 Utopic and higher. For Ubuntu 14.04, Ubuntu 12.04, and Linux Mint 17 users here’s how to install it from PPA(Personal Pakage Archives)个人软件包档案.它是Ubuntu Launchpad网站提供的一项服务，允许个人用户上传软件源代码，通过Launchpad进行编译并发布为2进制软件包，作为apt/新立得源供其他用户下载和更新。在Launchpad网站上的每一个用户和团队都可以拥有一个或多个PPA。

OpenJDK 8 was released in March 2014. It’s available in Ubuntu Software Center for Ubuntu 14.10 and Ubuntu 15.04. Someone has reported in launchpad asking for packaging openjdk-8 in Ubuntu 14.04, but no luck so far.

As a workaround(工作区), you can install OpenJDK 8 from a PPA repository:

1. Open terminal from the Dash or by pressing Ctrl+Alt+T. When it opens, run the command below to add PPA:

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

OpenJDK 8 PPA

Type in user password when it asks and hit Enter to continue.

2. After that, update system package cache and install OpenJDK 8:

sudo apt-get update

sudo apt-get install openjdk-8-jdk

3. If you have more than one Java versions installed on your system. Run below command set the default Java:

sudo update-alternatives --config java
Type in a number to select a Java version.

set default java version

And set default Java Compiler by running:

sudo update-alternatives --config javac

4. Finally check out current Java version by running:

java -version
It outputs something like this:

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)