How to Resolve Incompatible Java version Error in Eclipse

I have installed both java 1.6 and 1.7 on my ubuntu developer machine and when i am trying to do some GUI designing using Google WindowBuilder extension for eclipse i ran in to a problem where eclipse fail to load the GUI design view and gave me this error

"Incompatible Java Version

"Eclipse is running under 1.6, but this java project has a 1.7 java compliance level, so windowBuilder will not be able to classes from this project. use a lower level of java for the project, or run eclipse using a newer java version"

so in this tutorial, you can find how to resolve this problem and hopefully this will help you to resolve any other java version compatibility issues with eclipse