title: Which method of checking to see if a NSDictionary contains a particular key is faster?
body: <p>I can test for the presence of a key in an NSDictionary in two ways:</p>
  
  <pre><code>BOOL containsKey = [[dictionary allKeys] containsObject:foo];
  
  BOOL containsKey = ([dictionary objectForKey:foo] != nil);
  </code></pre>
  
  <p>which method is faster, and why?</p>