within the file. A source code file may contain at most one public class or interface. If a public class or interface is defined A Java source code file takes the name of a public class or interface that is defined within a source code file, then the source code file must take the name of the public class or interface.

If no public class or interface is defined within a source code file, then the file must take on a name that is different than its classes and interfaces. Source code files use the .java extension.