1. install default JRE/JDK
   1. Installing Java with apt-get is easy. First, update the package index:
      • sudo apt-get update
   2. check if Java is not already installed:
      • java -version
   3. If it returns "The program java can be found in the following packages", Java hasn‘t been installed yet, so execute the following command:
      • sudo apt-get default-jre(
        This will install the Java Runtime Environment (JRE). )
   4. If you instead need the Java Development Kit (JDK), which is usually needed to compile Java applications (for example Apache Ant, Apache Maven, Eclipse and IntelliJ IDEA execute the following command:
      • sudo apt-get install default-jdk
   5. That is everything that is needed to install Java.All other steps are optional and must only be executed when needed.
2. Installing Oracle JDK (optional)
   1. The Oracle JDK is the official JDK; however, it is no longer provided by Oracle as a default installation for Ubuntu.
   2. You can still install it using apt-get.
   3. To install any version, first execute the following commands:
      • sudo apt-get install python-software-properties
      • sudo add-apt-repository ppa:webupd8team/java
      • sudo apt-get update
   4. Then, depending on the version you want to install, execute one of the following commands:
      1. Oracle JDK6:This is an old version but still in use.
         • sudo apt-get install oracle-java6-installer
      2. Oracle JDK7:This is the latest stable version.
         • sudo apt-get install oracle-java7-installer
      3. Oracle JDK8:This is a developer preview, the general release is scheduled for March 2014. This external article about Java 8 may help you to understand what it‘s all about.
         • sudo apt-get install oracle-java8-installer
3. Managing Java (optional)(多版本環境下設置默認使用的版本)
   1. When there are multiple Java installations on your Droplet, the Java version to use as default can be chosen. To do this, execute the following command:
      • sudo update-alternatives --config java
   2. It will usually return something like this if you have 2 installations (if you have more, it will of course return more):
      • 
   3. You can now choose the number to use as default. This can also be done for the Java compiler (javac):
      • sudo update-alternatives --config javac
   4. Setting the "JAVA_HOME" environment variable（取得Java安裝路徑，設置Java環境變量）
      1. To set the JAVA_HOME environment variable, which is needed for some programs, first find out the path of your Java installation:
         • sudo update-alternatives --config java
      2. It returns something like:
         1. 
      3. The path of the installation is for each:
         • /usr/lib/jvm/java-7-oracle
         • /usr/lib/jvm/java-6-openjdk-amd64
         • /usr/lib/jvm/java-7-oracle
      4. Copy the path from your preferred installation and then edit the file /etc/environment:
         • sudo nano /etc/environment
      5. In this file, add the following line (replacing YOUR_PATH by the just copied path):
         • JAVA_HOME="YOUR_PATH"
      6. That should be enough to set the environment variable. Now reload this file:
         • source /etc/environment
      7. Test it by executing:
         • echo $JAVA_HOME（If it returns the just set path, the environment variable has been set successfully. If it doesn‘t, please make sure you followed all steps correctly. ）
4. 通過URL鏈接安裝jdk1.8
   • wget --no-check-certificate --no-cookies --header "Cookie: oraclelicense=accept-securebackup-cookie" http://download.oracle.com/otn-pub/java/jdk/8u91-b14/jdk-8u91-linux-x64.tar.gz
     sudo mkdir /usr/java
     sudo tar zxf jdk-8u91-linux-x64.tar.gz -C /usr/java
     sudo echo "export JAVA_HOME=/usr/java/jdk1.8.0_91" >> /etc/bashrc
     sudo "export PATH=$PATH:$JAVA_HOME/bin" >> /etc/bashrc
     sudo echo "export CLASSPATH=$JAVA_HOME/lib/dt.jar:$JAVA_HOME/lib/tools.jar" >> /etc/bashrc
     source /etc/bashrc
5. 
   

(2)Linux Java環境變量安裝