Subtract two Lists in Kotlin using a custom equal function

Question

I have two Lists and I want to get the List containing only the elements in the first list that are not in the second one. The problem is that I need to specify a custom equal when subtracting. Let's suppose I want to use one of the fields in the entries of the lists. Let's say the id.

I implemented it this way:

list1.filter { log -> list2.none { it.id == log.id } }

or

val projection = logEntries.map { it.id }
list1.filter { it.id !in projection }

Is there a better way to do it? Please take into account that I can't set a new equal method for the class.

Answer 1

The way you do it is ok, but when the lists become bigger you might want to do that:

You could make the process more efficient by turning your reference list (list2) to a set first.

val referenceIds = list2.distinctBy { it.id }.toSet()

list1.filter { it.id !in referenceIds }

Background:

An ArrayList which you are most likely using has a time complexity of O(n) when you check if an element is contained. So, if the list gets bigger, it will take longer.

A HashSet on the other hand has a time complexity of O(1) when checking if an element is contained. So, if list2 becomes bigger it won't get slower.