Java NullPointerException on concatenating Double types but not on String types : <pre><code>     Double d1 = null;
     Double d2 = null;
     System.out.println(d1+d2);//throw NullPointerException
     String s1 = null;
     String s2 = null;
     System.out.println(s1+s2);//doesn't throw any exception prints nullnull
</code></pre>

<p>Since both Double and String are of type Object then why Double types throw exception??</p>
