title: Converting java.util.Properties to HashMap<String,String>
body: <pre><code>Properties properties = new Properties();
  Map&lt;String, String&gt; map = new HashMap&lt;String, String&gt;(properties);// why wrong?
  </code></pre>
  
  <p><a href="https://docs.oracle.com/javase/9/docs/api/java/util/Properties.html" rel="noreferrer"><code>java.util.Properties</code></a> is a implementation of <a href="https://docs.oracle.com/javase/9/docs/api/java/util/Map.html" rel="noreferrer"><code>java.util.Map</code></a>, And <a href="https://docs.oracle.com/javase/9/docs/api/java/util/HashMap.html#HashMap-java.util.Map-" rel="noreferrer"><code>java.util.HashMap</code>'s constructor</a> receives a <code>Map</code> type param. So, why must it be converted explicitly? </p>