I'm writing this because I found the other answers hard to understand.
Essentially your JRE is not updated and/or Eclipse is not configured to use the most recent JRE.
On Windows, go to Control Panel -> Programs -> Java -> update and proceed to update java
or if you don't have Java, go to Oracle's website and download the most recent JRE.
Once this is done, go into eclipse, and under the project view, right click on your project, select Java Build Path, double click on JRE System Library, then Select Workspace Default JRE 14.1.
If a recent version of Java doesn't show up here, it probably isn't installed. Check you JRE(NOT JDK) version and make sure it's recent. If it is, try restarting the computer then trying this again.