title: some confusions about [NSNull null]，nil
body: <blockquote>
    <p><strong>Possible Duplicate:</strong><br>
    <a href="https://stackoverflow.com/questions/836601/whats-the-difference-between-nsnull-null-and-nil">What&#39;s the difference between [NSNull null] and nil?</a><br>
    <a href="https://stackoverflow.com/questions/6814427/what-are-the-differences-between-nil-null-and-nsnull-nil">What are the differences between nil, NULL and [NSNULL nil]?</a>  </p>
  </blockquote>
  
  
  
  <p>1.</p>
  
  <pre><code>id dicValue = [aDictionary objetForKey:@"aKey"];
  
  if(dicValue != nil)
  {
       blablala...
  }
  </code></pre>
  
  <p>or</p>
  
  <p>2.</p>
  
  <pre><code>if(dicValue != [NSNull null]) 
  {
       blablala...
  }
  </code></pre>
  
  <p>should I choose the first one,or the second one?</p>
  
  <p>or when it comes like this :</p>
  
  <p>3.</p>
  
  <pre><code>if ([aDictionary objetForKey:@"aKey"] != nil)
  </code></pre>
  
  <p>or</p>
  
  <p>4.</p>
  
  <pre><code>if ([aDictionary objetForKey:@"aKey"] != [NSNull null])
  </code></pre>
  
  <p>and what again ?</p>
  
  <p>1.2.3.4. which is correct and recommended?</p>