Question

What method should I use to keep items in a list if :

1. I have a an object which contains a list of items and can do action on those items
2. Those items are ordered inside a list
3. I need to be able to change the position of items inside the list and add or remove an item based on its id.
4. The list will be saved into a database eventually (and items will need to keep their order inside this list)
5. It doesn’t make sense to have a property “order” on the orderd items (these objects can be in multiple kind of lists).

The choices that I have (is there any other choices?) :

1. Use a List object so it will automatically keep the order. But I’ll have to loop through all objects (at worse) to find an object by Id.
2. Use a map with a Key / Value (object id / object), so it’ll be easy to retrieve the object (and fast) and keep another map that just keeps the order of the object (object Id / object order)

Solution #1) This seems like the easiest solution with a performance drawback.

Solution #2) This seems like the fastest solution but with a lot more code when reordering list.

Is there an object that I don’t know about that would be ordered, have Key / Value pairs and have methods like object.add(index, key, value) or object.getByIndex(index) or object.getByKey(key)?

Answer 1

As Roni suggested you need to use your own data structure. I believe that is the only option viable to fill in your requirement.

I have done a sample to fill in your requirement as a starting point and you can build on it.

public class MapAndList {

    private List<String> keys = new ArrayList<String>();
    private Map<String, Object> keyValuePair = new HashMap<String, Object>();

    public void addItem(String key, Object item) {
        // TODO if the key is already present in the list throw exception
        keys.add(key);
        keyValuePair.put(key, item);
    }

    public void removeItem(String key) {
        keys.remove(key);
        keyValuePair.remove(key);
    }

    public void removteItem(int index) {
        removeItem(keys.get(index));
    }

    public void addItem(String key, Object item, int index) {
        keys.add(index, key);
        keyValuePair.put(key, item);
    }

}

Hope this helps you….