Can a .java file contain more than one java classes (or interfaces)?

Yes, a .java file can contain more than one java classes (or interfaces). But there are two restrictions:

• A .java source file can contain at most one top-level class (or interface) defined by the public modifier.
• The .java file name must be the same as the public class' (or interface's) name.

Even an empty .java file is a valid source file too.