source file can't have more than one public class

Question

we can declare only one public file in a source file and file name must match the public class name

is there any reason to this restriction....

Answer 1

The reason is, that this is the convention. Also the classloader expects a class in a specific file. You can write your own classloader to avoid this restriction, but there is no good reason to do this. Everyone looking on your code will get confused. ;)

However, you can create "multiple" classes in one file by creating inner classes. I know, its not the same, but usually you should think about more important things than why there is only one class in one file.