Count number of items with property

Question

I have list List where Custom is like

class Custom{
    public int id;
    public String name;
}

How to

Answer 1

The way it is defined now will always require looping over the list.

Creating a secondary index with a map of names to list of ids is one good idea.

One more option would be to make sure the list is ordered by name, in which case all "Tom"s would be stored next to each other. Then you could find the fist "Tom" in O(log(n)) time with a binary search, and just keep counting from there until you reach a non-"Tom" or end of the list. The insert operation would have O(n) complexity as you need to move all elements after the insert location by one position, so consider this carefully :-)