Installing default JRE/JDK   This is the recommended and easiest option. This will install OpenJDK 6 on Ubuntu 12.04 and earlier and on 12.10+ it will install OpenJDK 7.   Installing Java with  apt-get  is easy. First, update the package index:  sudo apt-get update   Then, check if Java is not already installed:  java -version   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 install default-jre   This will install the Java Runtime Environment (JRE). 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   That is everything that is needed to install Java.   All other steps are optional and must only be executed when needed.     Installing OpenJDK 7 (optional)   T...