ArrayList groupby in java android based on same attributes without java 8 stream and lambda

Question

My object looks something like below

 class Item{
    String color;
    int price;
    int size;
}

Now my arraylist contains objects of type item

I want to create sublist of items having same price.

Want to group items into sublist with same color.

Want to create sublist of items with same size.

Since I am implementing this in android and want to support all android version I can't use Lambda and Stream

I want to use CollectionUtils by apache or Guava by google but don't know how to do it any help?

Take a look at [this](https://stackoverflow.com/questions/8463543/grouping-elements-of-a-list-into-sublists-maybe-by-using-guava) — StatelessDev, Jun 21 '18 at 19:22
it is just for String I am looking for objects based on properties — amodkanthe, Jun 21 '18 at 19:24

score 3 · Accepted Answer · answered Jun 22 '18 at 07:45

3

Using Guava, you can create Multimap in which keys are your desired property (ex. price) and values are items for each group by using Multimaps#index(Iterable, Function).

Note that without lambdas functions are very cumbersome. See a definition of a function for getting price (could be inlined):

private static final Function<Item, Integer> TO_PRICE =
  new Function<Item, Integer>() {
    @Override
    public Integer apply(Item item) {
      return item.price;
    }
  };

Create your grouped multimap:

ImmutableListMultimap<Integer, Item> byPrice = Multimaps.index(items, TO_PRICE);

Sample data:

ImmutableList<Item> items = ImmutableList.of(
    new Item("red", 10, 1),
    new Item("yellow", 10, 1),
    new Item("green", 10, 2),
    new Item("green", 42, 4),
    new Item("black", 4, 4)
);

Usage:

System.out.println(byPrice);
// {10=[Item{color=yellow, price=10, size=1}, Item{color=green, price=10, size=2}], 42=[Item{color=green, price=42, size=4}], 4=[Item{color=black, price=4, size=4}]}
System.out.println(byPrice.values());
// [Item{color=yellow, price=10, size=1}, Item{color=green, price=10, size=2}, Item{color=green, price=42, size=4}, Item{color=black, price=4, size=4}]
System.out.println(byPrice.get(10));
//[Item{color=yellow, price=10, size=1}, Item{color=green, price=10, size=2}]

answered Jun 22 '18 at 07:45

Grzegorz Rożniecki

27,415
11
90
112

I have List items how can I convert them to ImmutableListMultimap – amodkanthe Jun 23 '18 at 15:31
@apk `ImmutableListMultimap byPrice = Multimaps.index(items, TO_PRICE);` as mentioned in answer, where `TO_PRICE` is the `Function` you define (either inline or constant, as written above). – Grzegorz Rożniecki Jun 23 '18 at 15:46
ohh thanks I thought I will have to create ImmutableList first – amodkanthe Jun 23 '18 at 15:48
`Multimaps#index` accepts any `Iterable`, so any `List`, `Set` or even `Collection` is OK. – Grzegorz Rożniecki Jun 23 '18 at 15:49
heyy sorry one more query how can I access ImmutableListMultimap by index – amodkanthe Jun 23 '18 at 16:27
`multimap.get(10)` will get you a list of all `Items` with price `10`. – Grzegorz Rożniecki Jun 24 '18 at 07:54

score 2 · Answer 2 · answered Jun 21 '18 at 20:19

2

Try this

Map<String, List<Item>> map = new HashMap<>();
for (Item item : items) {
   List<Item> list;
   if (map.containsKey(item.getColor())) {
      list = map.get(item.getColor());
   } else {
      list = new ArrayList<>();
   }
   list.add(item);
   map.put(item.getColor(), list);
}
map.values(); // this will give Collection of values.

answered Jun 21 '18 at 20:19

kTest

367
1
11

Looking for more optimised way – amodkanthe Jun 22 '18 at 03:01

ArrayList groupby in java android based on same attributes without java 8 stream and lambda

2 Answers2