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

Eclipse is famous for Java Integrated Development Environment (IDE), but it works with C/C++ and PHP  too.

How you can change the default work space in Eclipse? It is very easy!

1. Go to eclipse\configuration\
2. Open the file "config.ini"
3. Modify the line
   

   osgi.instance.area.default="F:/Workspace/Java"
   

   where "F:/Workspace/Java" should be your default workspace!

If this is not working,  go to inside the configuration/.settings folder of your Eclipse installation, there is a file called org.eclipse.ui.ide.prefs. Open this file in a text editor and change the property SHOW_WORKSPACE_SELECTION_DIALOG from false to true to select the workspace at the startup.