is it possible to have .class and .java classes inside the same source code package?

while going throw some open source programs source code, I realized that some projects have .class and .java inside thier source code files! I understand that .class are is already compiled .java file, and that .class is binary but is it possible to have compiled and uncompiled files in a project? if yes, then why to do that? what are the benefits?

long story short: am trying to study test classes in different projects, where I realized that some programs have test files under the (build) folder, where they have .class as extension! Do these classes differ in behave than test classes located under test package with regular .java extenstion? is there anyway to decompile them?

thanks