A class that is not in a named package is in an unnamed package. Thus the full class name is Main. Such classes cannot be used from a named package, except via reflection. The JLS says that: Unnamed packages are provided by the Java SE platform principally for convenience when developing small or temporary applications … Read more
From the Java language spec: It is a compile time error to import a type from the unnamed package. You’ll have to access the class via reflection or some other indirect method.
You can’t use classes in the default package from a named package. (Technically you can, as shown in Sharique Abdullah’s answer through reflection API, but classes from the unnamed namespace are not in scope in an import declaration) Prior to J2SE 1.4 you could import classes from the default package using a syntax like this: … Read more