在unbuntu14.04系統上安裝 Open JDK 8 時遇到了如下問題：

$ sudo apt-get install openjdk-8-jdk
Reading package lists... Done
Building dependency tree       
Reading state information... Done
E: Unable to locate package openjdk-8-jdk

解決方法：

As you can see I only have java 1.7 installed (on a Ubuntu 14.04 machine).

$ update-java-alternatives -
 
l
java-1.7.0-openjdk-amd64 1071 /usr/lib/jvm/java-1.7.0-openjdk-amd64

To install Java 8, I did,

$ sudo add-apt-repository ppa:openjdk-r/ppa
$ sudo apt-get update
$ sudo apt-get install openjdk-8-jdk

Afterwards, now I have java 7 and 8,

$ update-java-alternatives -l
java-1.7.0-openjdk-amd64 1071 /usr/lib/jvm/java-1.7.0
 
-openjdk-amd64
java-1.8.0-openjdk-amd64 1069 /usr/lib/jvm/java-1.8.0-openjdk-amd64

 How to switch between different versions?

   run the follwing command from the terminal:

$ sudo update-alternatives --config java
There are 2 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                            Priority   Status
 
------------------------------------------------------------
  0            /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java   1071      auto mode
  1            /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java   1071      manual mode
* 2            /usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java   1069      manual mode

Press enter to keep the current choice[*], or type selection number:

As you can see I'm running open jdk 8. To switch to to jdk 7, press 1 and hit the Enter key. Do the same for javac,javap,javadoc as well with,:

$ sudo update-alternatives --config javac
$ sudo update-alternatives --config javap
$ sudo update-alternatives --config javadoc