You may want to check your Run Configurations setting if you're using Eclipse v4.4 (Luna) and have already completed all steps mentioned above.
There could be several possibilities that cause this error. The root cause is a mismatch of the project require compilation in JDK1.8/JRE8 while the environment compiler is JDK1.7/JRE7.
You can check my blog post to go through all your settings are correct.
