How do I access the first non null term of a List in Java?

Question

So I have a method (of which I can't change the parameters, or else this could have been made easier through HashMaps...more on this later), that passes an item as a parameter. Now I have a list of instances from this other class, of which one of its attributes is of the same type of this item, and I want to find the instances in the list which correspond to this item (of which there should be only one). This is what I did to find this:

List<Instance> instances = ...

public static void checkItems(Item i) {

    List<Instance> n = new ArrayList<>();
    instances.forEach(p -> n.add(p.i == i ? p : null));

    Instance currentInstance = n.get(0); 
    //Instance currentInstance = instances.stream().filter(p -> p.i == i).collect(Collectors.toList()).get(0);

}

You'll probably notice two things straight up:

1. I used a conditional operator which adds a null value to the list when the condition isn't passed
2. My commented code which was another attempt to solve this issue

So in the first case, I put null because it requires you to put something, and a null value is probably easier to work with, which is why the question arises: How do I access the first non-null value in a list (without resorting to iterating over the entire list to find it...)?

You may notice that I just assign the first value of the list with n.get(0) to currentInstance, because I know that only one value will have passed the test. However, due to some other code that I apply to currentInstance, this value cannot be null.

Just a note on the second point: the way I tried to solve it with streams actually works exactly as planned, except that for some reason the list of instances recovered is not a direct copy of the original instances. This resulted in the values of some of the attributed to have been reset to default values, therefore rendering this method useless.

EDIT: I just wanted to mention that the streams method wasn't working because of some silly mistake that I made in another class, there was nothing wrong with the code so I'll be using that bit to solve my problem :D

Answer 1

If you know that only one p passes the test, I don't know what the point of creating a list with a load of null values plus p is.

Your problem seems to stem from wanting to use forEach. In my opinion, you should almost always use a for loop in preference to forEach. With a simple for loop you can just use break when the item is found.

In detail:

Instance p = null;
for (Instance q : instances) {
    if (q.i == i) {
        p = q;
        break;
    }
}
if (p == null)
    throw new IllegalStateException();   // It wasn't there.
// Do something with p.