Java set add to put a custom add impl

Question

I need to be using an implementation of Set interface for storing objects of type MyObject.

MyObject has two fields: id and score. For equality on MyObject, only id is used. One of the functionality I need to use is to be able to support to be able to add scores when two MyObjects (with same ids) are inserted into this set.

Currently, I am using TreeSet and its isn't working the way I want it to work.

    Set<MyObject> container = new TreeSet<MyObject>();
    MyObject obj1 = new MyObject(12, 0.345);
    container.add(obj1);

    MyObject obj2 = new MyObject(12, 0.1);
    container.add(obj2);

I want container to have Myobject(12,0.445) but it being a set, the container has MyObject(12, 0.1).

Is there something I can use which will give me what I am looking for?

barfuin · Accepted Answer · 2011-12-21 22:19:44Z

1

You should use a Map for this:

Map<Integer, Double> container = new TreeMap<Integer, Double>();

or

Map<Integer, MyObject> container = new TreeMap<Integer, MyObject>();

Then you can call get on the container to see if the object is already there, and increment its score.

A Set will not work for you because there is no efficient way of getting the obj1 from the Set.

answered Dec 21, 2011 at 22:19

barfuin

17.6k11 gold badges90 silver badges136 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Aravind Yarram · Accepted Answer · 2011-12-21 22:22:31Z

0

Change the code/classes to follow the below semantics

if(container.contains(newObj))
{
  MyObject oriiginalObj = container.get(newObj);
  originalObj.incrementScore(newObj.getScore());
}

There are other ways of doing this like extending the TreeSet and overriding the add() method or by delegation (suggest using Guava's forwarding collection) but the solution I provided is simple and explicit.

Correction: Use TreeMap instead of TreeSet as suggested by Thomas

edited Dec 21, 2011 at 22:22

answered Dec 21, 2011 at 22:13

Aravind Yarram

80.5k49 gold badges239 silver badges335 bronze badges

Comments

jambriz · Accepted Answer · 2011-12-21 22:46:24Z

0

The add method for the TreeSet implementation looks like this

private transient SortedMap<E,Object> m; // The backing Map
public boolean add(E o) {
return m.put(o, PRESENT)==null;
}

where PRESENT is a dummy object used to indicate the object added already exists. you could subclass TreeSet to use the score sum instead of a dummy object

or, if possible, get rid of the Set altogether and instead use directly the Map as @Thomas says

answered Dec 21, 2011 at 22:46

jambriz

1,3031 gold badge11 silver badges26 bronze badges

Collectives™ on Stack Overflow

Java set add to put a custom add impl

3 Answers 3

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related