A java source code file contains atmost one public class 
or interface defined with in the file.Java source code 
files takes the name of the public class or interface that 
is defined with in the source code file.If no public class 
or interface is  defined with in the source code file then 
file must take on a name that is different than its public 
class or interface.

java source code names will be named withthe name of the
class that is declare as publicwith .java extension

If a  java source code file contain public class then the 
source code file name should be public class name other 
wise compiler error.


if a java source code does not contain  the public class 
then the name may be class name or anything ,not a problem.