Introduction

This page is based on experience of setting up Java 8 on Ubuntu 14.04.  Much of it may be applicable to other Java versions and distros.

Architecture

First we check which architecture we are running on.

Open a terminal and type the command :

arch

The output could be i686 or i386 for a 32 bits system or x86_64 for a 64 bits system.

Environment

On Ubuntu 14.04 and for historical reasons the Java commands are under the /usr/bin directory.

However in order to manage multiple version of Java the update-alternatives mechanism has been introduced. It allows multiple "providers" to be installed on the same machine, and configures which is the default. 

This mechanism consists of two directories, /etc/alternatives who contains the symlinks and /var/lib/dpkg/alternatives who contains update-alternatives state information.

Let's see where is the java command:

user@machine$ ls -al /usr/bin/java
lrwxrwxrwx 1 root root 22 Mar 12 2013 /usr/bin/java -> /etc/alternatives/java

As you can see the java command from /usr/bin will be resolved against the one contained in /etc/alternatives.

user@machine$ file /etc/alternatives/java
/etc/alternatives/java: symbolic link to `/usr/lib/jvm/ramdisk-java-8-oracle/jre/bin/java'

The previous command shows us where lies the java command.

Now take a look in the file /var/lib/dpkg/alternatives/java:

manual
/usr/bin/java
java.1.gz
/usr/share/man/man1/java.1.gz
/usr/lib/jvm/java-8-openjdk-i386/jre/bin/java
1069
/usr/lib/jvm/java-8-openjdk-i386/jre/man/man1/java.1.gz
/usr/lib/jvm/java-8-oracle/jre/bin/java
1070
/usr/lib/jvm/java-8-oracle/man/man1/java.1.gz
/usr/lib/jvm/ramdisk-java-8-oracle/jre/bin/java
1073

That file teaches us that there are several java command installed on this machine