Question

I have a two lists. List A and List B.

Each element of list A has a list of elements from List B. I have populated the two lists but population of the List within the element of List A is still to be done.

They both have a property on which the linkage can be done; say

an element from List B; if has the field 1 as the id of the element from List A, then it gets added to that element from List A.

Now i am currently doing it using a for loop within another for loop. Somethign like this

for(each A)
{
for(each B)
{
if(fieldsmatch)
{
add B to the List of the element from A
}
}

Will it be better if I Hash all the elements from List A and then perform lookups while going through each element from List B.

Thanks.

Can i use HashMap for this.

Answer 1

I’d reverse the logic. Create a hashmap from B that maps each id to a list of elements that have that id. Then for each element of A, look up the list by id and assign it to the field in the A element.

Map<IdType, List<B>> map = new HashMap<IdType, List<B>>();
for(each b in B) {
    List<B> list = map.get(b.id);
    if (list == null) {
        list = new ArrayList<B>();
        map.put(b.id, list);
    }
    list.add(b);
}
for(each a in A) {
    a.b_list = map.get(a.id);
    // if you need an empty list instead of null:
    if (a.b_list == null) {
        a.b_list = new ArrayList<B>();
    }
}