HADOOP INSTALL SERIES: 1. Install Java

Hadoop is written in Java, so you will need a recent Java Development Kit (JDK) installed
on the Ubuntu host. Perform the following steps to check the prerequisites:

1. First, check what's already available by opening up a terminal and typing
the following:
$ javac
$ java -version

2. If either of these commands gives a no such file or directory or similar
error, or if the latter mentions "Open JDK", it's likely you need to download the full
JDK. Grab this from the Oracle download page at http://www.oracle.com/
technetwork/java/javase/downloads/index.html; you should get the
latest release.

3. Once Java is installed, add the JDK/bin directory to your path and set the
JAVA_HOME environment variable with commands such as the following,
modified for your specific Java version:

$ export JAVA_HOME=/usr/local/java/jdk
$ export PATH=$JAVA_HOME:$JAVA_HOME/bin:$PATH