title: What are auxiliary classes?
body: <p>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?</p>
  
  <p>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.</p>
  
  <p>Edit 2:</p>
  
  <p>Here is the compiler warning for those who want it:</p>
  
  <pre><code>warning: auxiliary class Pattern in jregex/Pattern.java should not be accessed from outside its own source file
  </code></pre>