1. find /usr/lib/jvm/java-1.x.x-openjdk

2. vim /etc/profile

   Prepend sudo if logged in as not-privileged user, ie. sudo vim

3. Press 'i' to get in insert mode

4. add:

   export JAVA_HOME="path that you found"
   
   export PATH=$JAVA_HOME/bin:$PATH
   

5. logout and login again, reboot, or use source /etc/profile to apply changes immediately in your current shell

You can set your JAVA_HOME in /etc/profile as Petronilla Escarabajo suggests. But the preferred location for JAVA_HOME or any system variable is /etc/environment.

Open /etc/environment in any text editor like nano or gedit and add the following line:

JAVA_HOME="/usr/lib/jvm/open-jdk"

(java path could be different)

Use source to load the variables, by running this command:

source /etc/environment

Then check the variable, by running this command:

echo $JAVA_HOME

Update

Usually most linux systems source /etc/environment by default. If your system doesn't do that add the following line to ~/.bashrc (Thanks @pje)

source /etc/environment

Your PATH variable is messed up and I don't know why you should need JRE_HOME, JAVA_BINDIR, JDK_HOME and JAVA_ROOT. Generally you will only need the java binary in your PATH and the JAVA_HOME variable. But maybe your setup requires some of these, so I would suggest you leave them until your environment works as desired.

Allow me to dissect your PATH, remove the entries with comments:

PATH=/home/me/adb-fastboot/platform-tools:
/home/me/bin:
/usr/local/bin:
/usr/bin:
/bin:
/usr/bin/X11:
/usr/games:
/home/me/bin: # remove, already defined above
/usr/lib64/jvm/jre-1.8.0-openjdk/bin/java: # remove, java is already in /usr/bin
/usr/lib64/jvm/jre-1.8.0-openjdk/jre/bin/java: # remove, java is already in /usr/bin
/usr/bin/ # remove, already defined above

Your new PATH is:

PATH=/home/me/adb-fastboot/platform-tools:/home/me/bin:/usr/local/bin:/usr/bin:/bin:/usr/bin/X11:/usr/games

JAVA_HOME/JRE_HOME should be:

JAVA_HOME=/usr/lib64/jvm/jre-1.8.0-openjdk
JRE_HOME=$JAVA_HOME/jre

Edit: Your variables are probably exported in your ~/.bashrc or ~/.profile. Make the changes where you find them (with export before the variable name).

In general you should use System.getProperty() or System.getEnv().

import java.lang.*;

public class SystemDemo {

   public static void main(String[] args) {

     // prints Java Runtime Version before property set
     System.out.print("Previous : ");
     System.out.println(System.getProperty("java.runtime.version" ));
     System.setProperty("java.runtime.version", "Java Runtime 1.6.0");

     // prints Java Runtime Version after property set
     System.out.print("New : ");
     System.out.println(System.getProperty("java.runtime.version" ));
   }
}

import java.util.Map;

public class EnvMap {
    public static void main (String[] args) {
        Map<String, String> env = System.getenv();
        for (String envName : env.keySet()) {
            System.out.format("%s=%s%n",
                              envName,
                              env.get(envName));
        }
    }
}

If you are using maven there is another option. You can access environment variables from maven by using ${env.VARIABLE_NAME}.

You shouldn't refer to the jre/ subfolder, as Maven will need a Java compiler and potentially other JDK-only tools.

So just use:

 JAVA_HOME=/home/z222189/jdk1.8.0_31

EDIT: (making sure that a correct, working JDK is installed at that location, of course...)

For obtaining only one System Variable use the following code:

 String sysEnvStr = System.getenv("JAVA_HOME");

If it returns null then make changes in your .bashrc file. Try exporting that particular variable.

Using 12.04, placing the information in /etc/environment worked for me:

tku@buster:~$ cat /etc/environment 
PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games"
[...]
LC_MEASUREMENT="de_DE.UTF-8"
JAVA_HOME=/home/tku/work/jdk1.7.0_05