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)個人軟件包檔案.它是Ubuntu Launchpad網站提供的一項服務，允許個人用戶上傳軟件源代碼，通過Launchpad進行編譯並發布為2進制軟件包，作為apt/新立得源供其他用戶下載和更新。在Launchpad網站上的每一個用戶和團隊都可以擁有一個或多個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(工作區), 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)