[轉貼] Java Lists: Group by element property (as Ruby group_by method)

This is a very funny function, similar to group_by from Ruby.
It takes a collection and it transforms to a hash, having keys the property *groupBy*, and values the objects having that property.
A usage example:

collectionToHash(users, "name"); //returns a hash where the keys are the distinct user names, and the values are lists of users having that property.

public static SortedMap<String, ArrayList<Object>> collectionToHash(Collection list, String groupBy){
 TreeMap<String, ArrayList<Object>> hash = new TreeMap<String, ArrayList<Object>>();
 for(Object obj : list){
 Class<?> klass = obj.getClass();
 String groupByGetter = groupBy;
 try { // dynamic method invocation
   Method m = klass.getMethod(groupByGetter);
   Object result = m.invoke(obj);
   String resultAsKey = result.toString();
   ArrayList<Object> arrayList;
   if(hash.containsKey(resultAsKey)){
     arrayList = hash.get(resultAsKey);
   } else{
     arrayList = new ArrayList<Object>();
     hash.put(resultAsKey, arrayList);
   }
   arrayList.add(obj);
 }catch (SecurityException e){
   .....
 }catch (NoSuchMethodException e){
   ....
 }
 return hash;
} 轉貼自
http://vladzloteanu.wordpress.com/2009/05/20/java-lists-group-by-element-property-as-ruby-group_by-method/