Should I use a Map storing the id, or iterate over entries to find an object?

I’m wondering which of the following approaches I should use when finding an entry with an id from a collection?

Using a map

Map<String, MyObject> map = new HashMap<>();

public MyObject getById(String id) {
  return map.get(id);
}

Using a list/set

List<MyObject> list = new ArrayList<>();

public MyObject getObjectById(String id) {
    for (MyObject object : list) {
        if (object.getId().equals(id)) {
            return object;
        }
    }

    return null;
}

Without having a specific use case, which would be recommended to use?

>Solution :

If you can use a HashMap, you absolutely should. Retrieving an item from a HashMap using its key takes a constant amount of time, but iterating through an array takes time proportional to the number of elements in the array. In more technical terms, the lookup using a HashMap is O(1), while the lookup using an ArrayList is O(n).

What that means is the code that uses a HashMap will always be fast, but the code that uses the ArrayList will slow down significantly as you add more elements to it.