title: How to write custom method ++= for my custom mutable.HashMap
body: <p>I hvae written map like below, my implementation of mutable.HashMap.</p>
  
  <pre><code>class SampleMap() extends mutable.HashMap[String, (Any, BigInt)]
  </code></pre>
  
  <p>And overriden += method of map and rest of the methods are used from super class not overriden. Below works perfectly.</p>
  
  <pre><code>override def +=(kv: (String, (Any, BigInt))): this.type = {/*compiled code*/}
  </code></pre>
  
  <p>Now I want to write my custom implementation of ++= method which is inherited from growable class by Map. when I write overriden method ++=, Compiler complains nothing to override.</p>
  
  <pre><code>override def ++=(currentMap: MergeMap): this.type  = {
  </code></pre>
  
  <p>How to write custom implementation of this method for my custom Map.</p>