grammar - What is correct form of writing: "users names" or "user names"?

Let me ask you a question which I've derived from my programming practice:

Let's assume, I have a number of users represented by their names: John, Pete, Stanislaw, Marc, ...

What words should I use to name such collection of names of users: user names, or users names?

Thanks.

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To provide more context, here is similar example for programmers:

JSON of objects: [{"id": 1, ...}, {"id": 2, ...}, {"id": 3, ...}, ...]

How should I name a variable representing a collection of identificators (ids) of given objects:

var objectIds; // or
var objectsIds;

var objectIdentificators; // or

var objectsIdentificators;

?

Answer

What words should I use to name such collection of names of users: user names, or users names?

In the second example, users names sounds like user's names. You are already pluralizing names; I believe you are modifying names with the adjective user.

Using another adjective-noun pair might help clarify the situation:

sentence structures vs. sentenses structures

The latter is very odd. Structures is being modified by sentence. That is enough information.

Edited by request: There is too much information imparted in the second example. The thing, the noun, we want to emphasize is the head (or final) word: structures. The word sentence here is an adjective. It doesn't need to be plural.

Again, if we were writing about a more than one horse farm, we would say horse farms to indicate that the most important word (which is farm) is plural. It would just confuse things to say horses farms. It would only make sense if the farms belonged to the all of the horses. Obviously they do not, or there would be an apostrophe: horses' farms.

Your 'names' do not belong to all of the users collectively. Each has his own user name. Together , they are user names.