If you have upgraded Mac OSX 10.10 Yosemite, you may have noticed that the previously installed JRE is not available.
As there is no JRE installed, Eclipse would throw the following error,
Installed JRE location no longer exists. JRE will be removed.
So to resolve this issue, you need to download JRE dmg (from oracle website) and re-configure JRE with Eclipse.
Configuring JRE with Eclipse
Go to Eclipse -> Preferences
Select Java-> Installed JREs -> Add
Now Select Standard VM
Now enter "/System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home" in JRE Home text field. If the location is correct you will see all JRE system libraries being listed.
Click Finish and we are done with it ! ..