title: Why System class declared as final and with private constructor?
body: <p>As per my understanding</p>
  
  <h2>Final class</h2>
  
  <blockquote>
    <p>A final class is simply a class that can't be extended. </p>
  </blockquote>
  
  <h2>A class with single no argument private constructor</h2>
  
  <blockquote>
    <p>A class with private constructors cannot be instantiated except form inside that same class. This make it useless to extend it from another class. But it does not mean it cannot be sub classed at all, among inner classes we can extend and call the private constructor.</p>
  </blockquote>
  
  <p>So my understanding is, <em>if we create a class with single no argument private constructor, their is no meaning to declare that class as final</em>. Then <strong>why System class in Java, declared as final class although it has single no argument private constructor?</strong></p>
  
  <p>I heard that <strong>making a class final has some performance gain</strong>. Is this correct and is this the only reason to declare the System class as final? Please clarify me why Java implemented the System class like this.</p>