title: what does the Square brackets before a class declaration mean?
body: <p>the scala class is declared as below:</p>
  
  <pre><code>private[kafka] abstract class AbstractServerThread(connectionQuotas: ConnectionQuotas) extends Runnable with Logging { 
    ....
  }
  </code></pre>
  
  <p>I don't know what's the meaning of <code>[]</code>.</p>
  
  <p>Anyone could give me the answer?</p>