title: Putting element into HashMap with Map interface
body: <p>I'm trying Kotlin and I've encountered a small problem that I can't resolve.
  When I have the following construction I can put elements into the map:</p>
  
  <pre><code>val map = HashMap&lt;String, String&gt;()
  map["asd"] = "s"
  map.put("34", "354")
  </code></pre>
  
  <p>However when I create a map with the <code>Map</code> interface I can only read them, what I'm doing wrong ?</p>
  
  <pre><code>val map: Map&lt;String, String&gt; = HashMap&lt;String, String&gt;();
  map.put("24", "34") //error
  map["23"] = "23" //error
  </code></pre>
  
  <p>Or maybe I'm confusing something about interfaces in Kotlin ?</p>