Multiple indexes for a Java Collection - most basic solution?

Question

I\'m looking for the most basic solution to create multiple indexes on a Java Collection.

Required functionality:

• When a Value is removed, all index en

Answer 1

My first thought would be to create a class for the thing being indexed, then create multiple HashMap's to hold the indexes, with the same object added to each of the HashMaps. For an add, you'd then simply add the same object to each HashMap. A delete would require searching each HashMap for the reference to the destination object. If deletes need to be fast, you might want to create two HashMaps for each index: one for index-to-value and the other for value-to-index. Of course I'd wrap whatever you do in a class with a clearly-defined interface.

Doesn't seem like this would be hard. If you know the numbers and types of the indexes and the class of the widget up front, it would be pretty easy, like:

public class MultiIndex
{
  HashMap<String,Widget> index1=new HashMap<String,Widget>();
  HashMap<String,Widget> index2=new HashMap<String,Widget>();
  HashMap<Integer,Widget> index3=new HashMap<Integer,Widget>();

  public void add(String index1Value, String index2Value, Integer index3Value, Widget widget)
  {
    index1.put(index1Value, widget);
    index2.put(index2Value, widget);
    index3.put(index3Value, widget);
  }
  public void delete(Widget widget)
  {
    Iterator i=index1.keySet().iterator(); 
    while (i.hasNext())
    {
      String index1Value=(String)i.next();
      Widget gotWidget=(Widget) index1.get(index1Value);
      if (gotWidget.equals(widget))
        i.remove();
    }
    ... similarly for other indexes ...
  }
  public Widget getByIndex1(String index1Value)
  {
    return index1.get(index1Value);
  }
  ... similarly for other indexes ...

  }
}

If you want to make it generic and accept any object, have variable number and types of indexes, etc., it's a little more complicated, but not much.

Answer 2

Take a look at CQEngine (Collection Query Engine), it's an exact fit for this kind of requirement, being based around an IndexedCollection.

Also see related question How do you query object collections in Java (Criteria/SQL-like)? for more background.