Sorting ArrayList of Hashtables

[Tim Harrison]

I have an ArrayList of Hashtables that I need to sort. Ideally I would like to specify which key-value pair to sort against. What would be the best way to implement this?
Thanks in advance,
Tim Harrison

[Jim Yingst]

First, just on general principle - Hashtables are evil. Use HashMaps instead. This applies to all the legacy collection classes. Use ArrayList and Iterator rather than Vector and Enumeration. Peter den Haan's last post here nicely summarizes why. Thanks.
For your question, you probably will want to do something like this:

class KeyValueComparator implements Comparator
{
  private Object key;
 
  public KeyValueComparator(Object key) 
  {
    this.key = key;
  }
 
  public int compare(Object obj1, Object obj2)
  {
    Map m1 = (Map) ob1;
    Map m2 = (Map) ob2;
    Comparable v1 = (Comparable) m1.get(key);
    Comparable v2 = (Comparable) m2.get(key);
    return v1.compareTo(v2);
  }
}
 
class Sorter
{
  private static final Comparator nameComparator =  new KeyValueComparator("name");
  private static final Comparator priceComparator =  new KeyValueComparator("price");
  private static final Comparator idComparator =  new KeyValueComparator("id");
  public static void sortByName(List list)
  {
    Collections.sort(list, nameComparator);
  }
 
  public static void sortByPrice(List list)
  {
    Collections.sort(list, priceComparator);
  }
 
  public static void sortById(List list)
  {
    Collections.sort(list, idComparator);
  }
 
  public static void sortByKey(List list, Object key)
  {
    Collections.sort(list, new KeyValueComparator(key));
  }
}

Here I assume that each List will contain Maps (such as HashMap or Hashtable) which will contain the expected key values - and that throwing ClassCastException or NullPointerException are appropriate if these assumptions are not met. Also this is set up to sort by many different keys. Each sort may not be as fast as you'd like - the problem is that you're going to be invoking the get() method a lot of times for each search. It would probably be faster to replace the Maps with some other custom class which caches the required search criteria in an instance field, rather than having to look it up in a table each time. But then you probably need a separate field for each key/value you want to be able to search on, and a separate Comparator to use this field. This may be worthwhile to set up, but you lose the ability to sort on an arbitrary key. So it depends what your requirements are.