How to find multiples of the same integer in an arraylist?

Question

My problem is as follows. I have an arraylist of integers. The arraylist contains 5 ints e.g[5,5,3,3,9] or perhaps [2,2,2,2,7]. Many of the arraylists have duplicate values and i'm unsure how to count how many of each of the values exist.

The problem is how to find the duplicate values in the arraylist and count how many of that particular duplicate there are. In the first example [5,5,3,3,9] there are 2 5's and 2 3's. The second example of [2,2,2,2,7] would be only 4 2's. The resulting information i wish to find is if there are any duplicates how many of them there are and what specific integer has been duplicated.

I'm not too sure how to do this in java.

Any help would be much appreciated. Thanks.

Answer 1

To me, the most straightforward answer, would be using the Collections.frequency method. Something along the lines of this:

// Example ArrayList with Integer values
ArrayList<Integer> intList = new ArrayList<Integer>();
intList.add(2);
intList.add(2);
intList.add(2);
intList.add(2);
intList.add(7);

Set<Integer> noDupes = new HashSet<Integer>();
noDupes.addAll(intList); // Remove duplicates

for (Integer i : noDupes) {
    int occurrences = Collections.frequency(intList, i);
    System.out.println(i + " occurs " + occurrences + " times.");
}

If you want to, you could map each Integer with its number of occurrences:

Map<Integer, Integer> map = new HashMap<Integer, Integer>();
for (Integer i : noDupes) {
    map.put(i, Collections.frequency(intList, i));
}