运行时错误，原因我无法弄清楚

Question

This code is supposed to take 2 integer array lists of length 3 such as (3, 3, 4) and (1, 2, 3), compare each index, and find which one is greater.

It then awards a point to whichever is greater, so 3>1 means array list 1 gets a point for total score 1-0, 3>2 means array list 1 gets a point for total score 2-0, and 4>3 means array list 1 gets a point for total score 3-0. It's then supposed to create a new array list of length 2 and set the first index to 3, the score of the first array list, then set the second index to 0, the score of the second array list.

For some reason I'm getting a runtime error and I cannot figure out what it is.

Any help is appreciated, and if someone could explain what a runtime error is in laymen's terms that would be much appreciated.

static List<Integer> compareTriplets(List<Integer> a, List<Integer> b) {
        int bobscore = 0;
        int alicescore = 0;
        int throwaway = 0;
        
    
        for (int i = 0; i < a.size(); i ++){
            if (a.get(i) == b.get(i)){
                throwaway++;
            }       
            
            if (a.get(i) > b.get(i)){
                alicescore++;
            }       
            if (a.get(i) < b.get(i)){
                bobscore++;
            }       
            
        }
        
        List<Integer> scores = new ArrayList<>(2);
        scores.set(0, alicescore);
        scores.set(1, bobscore);
        return scores;
    }

Answer 1

It's this part:

List<Integer> scores = new ArrayList<>(2);
        scores.set(0, alicescore);
        scores.set(1, bobscore);

That constructor still makes an empty list , so scores.set(0 fails because there is no 1st element to set. The '2' is a 'capacity hint': It is conveying to the arraylist constructor that it should start off by having room for 2 elements.

ArrayLists automatically resize themselves if you keep adding elements to them; however, such a resize does involve an under-the-hood copy of an array, and conversely, arraylists (usually) have more room than you're using, which is fine as well, but in rare circumstances, you want to optimize this; if you know beforehand how large it's gonna be, you can pass that in and the arraylist will be almost inperceptibly faster.

That's why that constructor exists. You should not be using it here.

Most likely you want:

List<Integer> scores = new ArrayList<>();
scores.add(aliceScore);
scores.add(bobScore);

or even:

int[] scores = new int[2];
scores[0] = aliceScore;
scores[1] = bobScore;

Note that arraylist and arrays are completely different beasts. ArrayLists start out empty and can grow. arrays have a set size that you must provide upon creation and can never be any smaller or any larger; a new int[2] is size 2. And will always be. (The name 'ArrayList' refers to the notion that it uses arrays internally to do its work).

Answer 2

Use the simple static function List.of(aliceScore, bobScore) or Arrays.asList(aliceScore, bobScore) to return the list result.