Create an ArrayList of ArrayList<Integer>

Question

I'm trying to use the code below to create a multidimensional ArrayList. My code populates the inner ArrayList (localSolutions) just fine, but when I try an add that ArrayList to the outer ArrayList (solutions), something goes wrong, and it adds empty ArrayLists instead.

public class MathCapstone {

public static void main(String[] args) {
    ArrayList<ArrayList<Integer>> list = entireList(10);

    for(int q = 0;q<list.size();q++) {
        System.out.println(list.get(q));
    }

public static ArrayList<ArrayList<Integer>> entireList(int max) {
    ArrayList<ArrayList<Integer>> solutions = new ArrayList<ArrayList<Integer>>();
    ArrayList<Integer> localSolutions = new ArrayList<Integer>();

    for(int i = 1; i <= max; i++) {
        for(int j = 1; j < i; j++) {
           //System.out.println(j + "mod" + i + "=" + (j*j)%i);
            if ((j*j)%i == 1) {
                localSolutions.add(j);
            }
        }
        //System.out.println(localSolutions.toString());
        solutions.add(localSolutions);
        localSolutions.clear();
    }
    return solutions;
}

On a final note: would it be better to use a HashMap of ArrayLists (eventually I'm going to be creating CDF's for max values up to about 10k)?

There's nothing to be gained by using a HashMap<Integer, Whatever> as opposed to an ArrayList<Whatever> if the indexes you want to use are all numbers between 0 and something, and you'll lose a little performance. HashMaps are better if you need non-integer keys, or if the distribution of the indices is sparse. — millimoose
– millimoose, Commented Feb 3, 2012 at 1:50
Also consider List<List<Integer>> solutions = new ArrayList<List<Integer>>(). — trashgod
– trashgod, Commented Feb 3, 2012 at 1:51
@aclear16 I rejected your edit into my answer because what I was doing was intentional and part of the fix. Please don't change the semantics of other users' answers without a good reason like testing the code change – if you think you've spotted a bug it's probably better to add a comment if you're not sure. — millimoose
– millimoose, Commented Feb 3, 2012 at 2:25

Savvas Dalkitsis · Accepted Answer · 2012-02-03 01:45:51Z

3

You are clearing the localSolutions list.

In Java you copy by value only the reference to an Object not the actual object itself. So when you add the localSolutions list in your solutions list, both the localSolutions reference and the first entry of the solutions list, point to the same object.

Thus, when you clear the localSolutions list, you effectively clear the first entry on your solutions list.

answered Feb 3, 2012 at 1:45

Savvas Dalkitsis

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millimoose · Accepted Answer · 2012-02-03 01:47:24Z

You're doing:

localSolutions.clear();

Adding a list to another list doesn't add a copy of the list, it adds the same list object. What your code is doing in the outerloop is filling the same list with elements, emptying it, and adding it to solutions. solutions contains max references to the same, empty list.

What you want to do is:

ArrayList<ArrayList<Integer>> solutions = new ArrayList<ArrayList<Integer>>();
for(int i = 1; i <= max; i++) {
    ArrayList<Integer> localSolutions = new ArrayList<Integer>();
    for(int j = 1; j < i; j++) {
       //System.out.println(j + "mod" + i + "=" + (j*j)%i);
        if ((j*j)%i == 1) {
            localSolutions.add(j);
        }
    }
    //System.out.println(localSolutions.toString());
    solutions.add(localSolutions);
}

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