Any easy way to split an Arraylist into multiple lists based on a property of the objects?

[forums UseR]

Hi,

I have an ArrayList of similar objects.

Now I have to split the lists into multiple lists based on a property of these objects.

Here is the sample data set
List:
A{1, A}
A{2, A}
A{3, B}
A{4, B}
A{5, B}
A{6, C}

Result should be

List 1:
A{1, A}
A{2, A}

List2:
A{3, B}
A{4, B}
A{5, B}

List3:
A{6, C}

Any body has a easier way doing this...

[Costi Ciudatu]

Perhaps commons-collections could help you with such a task.

But I would try to put those all in a map, using that splitting/filtering property as key and a List as value:

Map<YourSplitterProperty, List<YourItem>> map = new HashMap<YourSplitterProperty, List<YourItem>>();
for (YourItem item : yourItems) {
  List<YourItem> list = map.get(item.getSplitterProperty());
  if (list == null) {
    list = new ArrayList<YourItem>();
    map.put(item.getSplitterProperty(), list);
  }
  list.add(item);
}

After this block, map.values() will return a Collection of List objects containing what you are looking for.
In case the splitter property is of a custom type, make sure that type has a proper equals() and hashCode() implementations.

[forums UseR]

I can only use Java 1.4

Not sure if your code will work in my case.

[Costi Ciudatu]

For Java 4, just strip down the generics from my code.

I actually have to split the ArrayList into an array.

You need to split the ArrayList into an array of lists ? If so, map.values().toArray().

[forums UseR]

Costi Ciudatu wrote:Perhaps commons-collections could help you with such a task.

But I would try to put those all in a map, using that splitting/filtering property as key and a List as value:

Map<YourSplitterProperty, List<YourItem>> map = new HashMap<YourSplitterProperty, List<YourItem>>();
for (YourItem item : yourItems) {
  List<YourItem> list = map.get(item.getSplitterProperty());
  if (list == null) {
    list = new ArrayList<YourItem>();
    map.put(item.getSplitterProperty(), list);
  }
  list.add(item);
}

Can you please explain whats going on here..., I have not worked much with Maps.., and I have only used Java 1.4

[David Newton]

Map map = new HashMap();

for (Iterator iter = items.iterator(); iter.hasNext();) {
    Item item = (Item) iter.next();

    List list = (List) map.get(item.getSplitterProperty());
    if (list == null) {
        list = new ArrayList();
        map.put(item.getSplitterProperty(), list);
    }
    
    list.add(item);
}

(Although I'd refactor out that middle chunk of the iteration.)

[David Newton]

(Or, if feeling particularly refactory:)

private static class ItemHashMap extends HashMap {
    public Item add(Item item) {
        get(item).add(item);
        return item;
    }

    public List get(Item key) {
        List list = (List) get(createItemKey((Item) key));
        return list == null ? createItemEntry((Item) key) : list;
    }

    private List createItemEntry(Item item) {
        List list = new ArrayList();
        put(createItemKey(item), list);
        return list;
    }

    private Object createItemKey(Item item) {
        return item.getSplitterProperty();
    }
}

public static void main(String[] args) {
    ItemHashMap itemMap = new ItemHashMap();
    List items = Arrays.asList(new Object[]{new Item("A"), new Item("B"), new Item("C")});
    for (Iterator iter = items.iterator(); iter.hasNext();) {
        Item item = (Item) iter.next();
        itemMap.add(item);
    }
}

(Dealing with people unfamiliar with refactoring for a job, so aways coming up with examples to show them :)