How to map two lists of objects by id efficiently

Question

I have a List of objects ( List 1 ) that I'm looping through. Each object has an id string associated with it. I have another list ( List 2 ) containing a different type of object. I want to map the objects in List 2 to my List 1 objects. List 1 has a getId() function and List 2 has a getList1Id() function, that gives the id of the object its supposed to map to in List 1 . How do I do this in the most efficient way possible?

Answer 1

I would iterate over both lists and build a map using the id as key and the object as value. Eg for the first list:

Map<Integer, Object> map1 = new HashMap<>();
for (Object o : list1) {
    map1.put(o.getId(), o);
}

Do the same for the second list:

Map<Integer, Object> map2 = new HashMap<>();
for (Object o : list2) {
    map2.put(o.getId(), o);
}

Now you can relate an object in one list to an object in the other by calling get with a given id.