ArrayList to HashMaps in Java

Question

I am trying to get values from an ArrayList that is sorted and want to store it in a HashMap , where the values of the ArrayList become keys of the HashMap . Will the order of the values in the HashMap still be the same as that of ArrayList ?

Answer 1

No. Use a TreeMap instead. This will preserve the order of insertion.

Answer 2

HashMap makes no guarantees as to the order the mappings are stored or iterated, so simply running through the ArrayList and putting them into the HashMap as keys will very likely result in unordered iterations.

As others have pointed out, LinkedHashMap does preserve insertion order for iterations. An additional run of insertions will result in unordered iterations again, though. Both HashMap and LinkedHashMap support constant time lookup - LinkedHashMap pays for its extra feature in space (by maintaining pointers between the keys).

As others have also pointed out, TreeMap preserves order after updates, so this might be a better option for you, or not. Of course, if the ArrayList is sorted with a specific Comparator, you must feed that same Comparator to the TreeMap on construction for the sorting to be the same. Note that TreeMap does not have constant time lookup, due to being implemented as a Red-Black search tree.

Answer 3

As your ArrayList has been ordered, no need to use a TreeMap because this will compare to order again and it's not necessary. You should use a LinkedHashMap that will keep the exact order of your ArrayList when you put your value in.

Answer 4

Check This: Insert Values of ArrayList into HashMap

HashMap<String, Item> itemMap = new HashMap<String, Item>();

for (Item item : itemList) 
{
   itemMap.put(item.getitemCode(), item);
}