title: Android: How to know if an activity is finished?
body: <p>I'm working on an app that plays mp3 files automatically and in succession from within a given activity. it knows when the current mp3 play is completed by listening for an "onCompletion" event from the MediaPlayer. </p>
  
  <p>However, if I return to the start display by pressing the back button while media is playing, apparently the activity is still running. When the "onCompletion" event is triggered, it tries to access the view within a list activity and crashes on an null pointer exception.</p>
  
  <p>What is the best way to determine that the activity is no longer "active" and therefore be able to prevent the call to play the next audio? </p>