How do I add execution environment 1.8 to Eclipse Luna?

Older versions of Eclipse might not work with javaSE-1.8

You need to find out what version of Eclipse you have by going into its help menu and clicking on About Eclipse.

If your version is older than the one at https://www.eclipse.org/downloads/ then you need to uninstall your older version and install the latest version.

I had the same problem with my Eclipse on Linux Mint. And the only way I could solve it was by following above instructions.

If the eclipse version is old as @mike stated in his answer then you would be unable to view the JavaSE-1.8 on the Execution Environments list in your eclipse; You can do the following to use it even on older versions:

• Help --> Install New Software
• Place the following URL --> http://download.eclipse.org/eclipse/updates/4.3-P-builds/
• Install the plugins and restart eclipse.
• Now if you have JDK 1.8 on your computer and added it as an installed JRE you'd be able to view and use JavaSE-1.8

Or just download and use the latest eclipse

Open Preferences and look in 'Java > Installed JREs'. If you don't have a Java 8 shown there click 'Add...' to add it. Close the preferences.

Open Preferences again and look in 'Java > Installed JREs > Execution Environments', select 'JavaSE-1.8' and make sure your Java 1.8 is checked as the Compatible JRE