gistfile1.java

/////////////////////////////
// An API method  ///////////
/////////////////////////////

/**
 * Reads the player's rank in each Skill from the database and runs the
 * callback on the main thread.
 */
public void readRankAsync(String playerName, Function<Map<SkillType, Integer>, Void> callback);

/////////////////////////////
// API implementation  //////
/////////////////////////////

public void readRankAsync(String playerName, Function<Map<SkillType, Integer>, Void> callback) {
  Bukkit.getScheduler().runTaskAsynchronously(new ReadRankAsyncTask(playerName, database, callback));
}
public class ReadRankAsyncTask implements Runnable {
  public ReadRankAsyncTask(String player, Database database, Function<Map<SkillType, Integer>, Void> callback) {
    // (assign those to instance vars)
  }
  
  public void run() {
    Map<SkillType, Integer> ret = database.slowRankReadingOperation(player);
    Bukkit.getScheduler().runCallback(MyPlugin.getInstance(), callback, ret);
  }
}

/////////////////////////////
// API Usage ////////////////
/////////////////////////////

public void onCommand(....) {
  final CommandSener fSender = sender;
  readRankAsync(targetPlayer, new Function<Map<SkillType, Integer>, Void>() {
      @Override
      public Void apply(Map<SkillType, Integer> rankData) {
        fSender.sendMessage("Your Ranks:");
        // blah blah loop the data and send the message
      }
    });
  return true;
}