title: What is the colon in the type parameter of a scala class
body: <p>I know when a class is parameterized, it could be declared as</p>
  
  <pre><code>class A[T]
  </code></pre>
  
  <p>I see declaration of RDD of Spark begins with:</p>
  
  <pre><code>abstract class RDD[T: ClassTag]
  </code></pre>
  
  <p>I don't know what does the <code>: ClassTag</code> mean.</p>