Question

For example I have a file name call “A.java”. Here its structure:

public class A{
}

private class B{
   // no error
}
public class C{
   //error : must declare at different file
}

I think this is Java rules. But, I really want to know: WHY java need to do that.

Thanks 🙂

Answer 1

public class A{
}

private class B{
   // no error
}

This shouldn’t even work (see Java Language Specification):

It is a compile-time error if a top level type declaration contains
any one of the following access modifiers: protected, private, or
static

Are you sure you haven’t declared class B as an inner class?

It doesn’t make sens to declare a top-level class as private, since private restricts the visibility of members to the enclosing top-level class/interface. If your class is a top-level class (whether or not in its own file), you’d at least want package-level visibility (no modifier at all).