title: why "abc" + null results abcnull
body: <p>why <code>"abc" + null</code>  results <code>abcnull</code></p>
  
  <pre><code>String s1 = "abc";
  String s2 = null; 
  String s3 = s1+ s2;
  
  System.out.println(s3);
  </code></pre>
  
  <p>Result: <code>abcnull</code></p>