Java: Most efficient way to find matching object values in two ArrayLists?

Question

I have two ArrayLists of objects: modelList which contains Model objects, and entityList which contains Entity objects. Both Model and Entity objects have a property called id.

My goal is to loop through each Model in modelList and, if there is an Entity in entityList with the same id value, call the method merge().

Currently, I am doing this:

for (Model model : modelList) {
    for (Entity entity : entityList) {
        if (model.getId().equals(entity.getId())) merge(entity, model);
    }
}

This doesn't seem very efficient, especially with a large dataset. What would be a better way of achieving the desired result?

Thanks in advance!

As Rene answered, hashing is usually faster than traversing a list. — sdasdadas, Oct 28 '13 at 17:42
Thank you. The retainAll() method will not work in this case, since Model and Entity are two different object classes. — Shaun Scovil, Oct 28 '13 at 18:09

score 4 · Answer 1 · answered Oct 28 '13 at 17:41

4

Use a Map<IdType, Entity> for the Entity objects that maps the id to the Entity.

Maybe you can change the code that produces the List<Entity> to return a Map

answered Oct 28 '13 at 17:41

René Link

48,224
13
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140

This puts me on the right track, I think... – Shaun Scovil Oct 28 '13 at 18:25

score 0 · Answer 2 · answered Oct 28 '13 at 17:42

0

Maybe you could build a Map models of your Model and search in it.

for(Entity entity: entityList) {
    Model model = models.get(entity.getId());
    if (model != null) {
        merge(entity, model);
    }
}

This work if you have much less model than entity!

answered Oct 28 '13 at 17:42

Tyco

131
4

Thank you. In this case, entity.getId() would not return a value that works as a parameter for modelList.get(). – Shaun Scovil Oct 28 '13 at 18:10

Shaun Scovil · Answer 3 · 2013-10-29T10:08:46.790

Based on René's answer, I went with this:

Map<String, Model> modelMap = new HashMap<String, Model>();
for (Model model : modelList)
    if (!modelMap.containsKey(model.getKey())) modelMap.put(model.getKey(), model);

Iterator it = entityList.iterator();
while (it.hasNext()) {
    Entity entity = (Entity) it.next();
    String key = entity.getKey();
    if (modelMap.containsKey(key)) merge(entity, modelMap.get(key));
    else it.remove();
}

Still open to improved answers...

Java: Most efficient way to find matching object values in two ArrayLists?

3 Answers3