I ran into this really strange error the other day while using Eclipse. I had been creating some JUnit tests, and everything was going just fine. Then I tried to create another one, and I saw the dialog below with the error message, Superclass does not implement the ‘junit.framework.Test’ interface.
The JUnit JAR file was in my classpath, so I was befuddled for a minute or two. After searching around I discovered that I had accidentally deleted the JRE System Library from my project Java Build Path. After I restored that, everything worked fine.