Q:

What are auxiliary classes?

I know these questions may sound stupid, but in Java, what are Auxiliary classes, how does some one write one, and how does the compiler know that something is an Auxiliary class?
Edit:
The reason I ask this is because the compiler is generating a warning regarding an object in an external library, and I want to know why.
Edit 2:
Here is the compiler warning for those who want it:
warning: auxiliary class Pattern in jregex/Pattern.java should not be accessed from outside its own source file

A:

As descried in Java specification here, you can specify more than one class in one .java file. The class which name matches .java file name will be the main class which can be declared public and be visible to other classes. All other classes in the file therefore are "auxilary" classes. Auxilary class can NOT be declared public and (as @trashgod rightfully pointed out) therefore they only be declared with package-private access. For instance for AClass.java file:
public class AClass {
    private AuxilaryClass a;
}
class AuxilaryClass {
    private int b;
}

AuxilaryClass class can't be public and is not visible outside this AClass.java file.
However, using auxilary classes considered extremely bad style and against Java Code Convention. Please use separate or inner classes if really needed.
Edit: The term "Auxilary" is not Oracle/Sun official terminology. It has been introduced (or used) here: http://www.youtube.com/watch?v=miTM9rY3He0 and/or here: http://doc.sumy.ua/prog/java/langref/ch05_03.htm

