This compilation unit is not on the build path of a Java project

Problem: Eclipse Showing error message - "This compilation unit is not on the build path of a Java project"

If you import a project as a General Project, it wont have the java nature and that will be a problem. In my case this was true . And rectifying the same will be as follows,

 Add the below lines in the .project file of your workspace and refresh.

      org.eclipse.jdt.core.javanature