At the root of your project:
/src (at leat as default)
How to know if
/src is the sources dir?
When you create a new Java class, it is contained in a package (normally it is called as the same name of the dir where it is created). So, in your class declarion you can see something like this:
In default IDE settings, the class should found under
/src/foo/MyClass.java. As you can see, in this scenario
/src acts as root sources dir.