title: Objective-C Sort NSMutableArray of Objects by a specific Object attribute
body: <p>I have an object called Station in my system with these attributes:</p>
  
  <pre><code>@interface Station : NSObject {
  NSString *stationID;
  NSString *callsign;
  NSString *stationState;
  }
  </code></pre>
  
  <p>I also have an NSMutableArray containing 20 'Station' objects as defined above.</p>
  
  <p>I need to define a method which can can sort this array in 2 ways:
  1) By stationID
  2) By callsign</p>
  
  <p>Can someone please explain how I can do this?</p>