Android : testing android with eclipse and junit: Why the test Project contents no java classes

first of all please excuse my english.

I´m doing a course to learn how to program apps with android. I´ve got a question about testing android applications with junit and eclipse. When I start a new Test project, which is linked to a other project, the test project contains no java classes in the src folder. One of my exercises is to explain, why eclipse isn´t able to build such a class in this folder compared to a normal project.

If you need more information, just ask and I try to explain it better. Thank you for your help.

Best regards Patrick