what does the Square brackets before a class declaration mean?

Question

the scala class is declared as below:

private[kafka] abstract class AbstractServerThread(connectionQuotas: ConnectionQuotas) extends Runnable with Logging { 
  ....
}

I don't know what's the meaning of [].

Anyone could give me the answer?

Answer 1

This means that this class is private to the kafka package (can only be accessed from within that package).