CODEWITHSUNDEEP
javaarraylistduplicates
bartektartanus
bartektartanus

Reputation: 16080

Remove every duplicated element from ArrayList

Please, before downvote - read the question and check the example - this is not duplicate!

I want to remove every duplicated element from ArrayList in Java, like this:

Original list: [4, 2, 2, 3, 4, 1]
Set result: [1, 2, 3, 4]
Desired result: [1, 3]

The obvious solution for duplicates - set does not work here.

My solution:

        ArrayList<Integer> arrayList = new ArrayList<Integer>();
        arrayList.add(5);
        arrayList.add(1);
        arrayList.add(5);
        arrayList.add(1);
        arrayList.add(5);
        arrayList.add(2);
        arrayList.add(3);

        ArrayList<Integer> temp = new ArrayList<>(arrayList);

        for(Integer number : new HashSet<Integer>(arrayList)){
            temp.remove(number);
        }

        arrayList.removeAll(temp);

Any smarter/more clever solutions?

Upvotes: 2

Views: 1506

Answers (3)

Rob
Rob

Reputation: 6507

There are many ways to do what you are looking to do. Here is one (where T is the type stored by your existing List):

  1. Allocate a Map<T, Integer>. The integer will be used to store the number of times that item appears in your list.
  2. Iterate over your list. Look up each item in the map. If it doesn't exist, put back a count of one. If it does already exist in the map, increment the count and put back the new count.
  3. Iterate over the Entry's in the map to extract your de-duplicated list. You will not include any Entry with a value that is greater than one.

This approach will be fast even for large lists.

Upvotes: 3

Syam S
Syam S

Reputation: 8509

Alternatively you can check for frequency as remove as well. Collections.frequency(arrayList, number)

public static void main(String[] args) {
    ArrayList<Integer> arrayList = new ArrayList<Integer>();
    arrayList.add(5);
    arrayList.add(1);
    arrayList.add(5);
    arrayList.add(1);
    arrayList.add(5);
    arrayList.add(2);
    arrayList.add(3);

    ArrayList<Integer> unique = new ArrayList<>();

    for (Integer number : arrayList) {
        if (Collections.frequency(arrayList, number) == 1) {
            unique.add(number);
        }
    }

    System.out.println(unique);
}

Upvotes: 2

Unihedron
Unihedron

Reputation: 11051

You can use a loop to check for duplicates in an ArrayList, and then use remove() on the ArrayList to remove the elements.

Please see the following:

{
    ArrayList list;
    // ...
    while(true)
        for(final Integer o : list) {
            if(list.indexOf(o) != list.lastIndexOf(o)) {
                while(list.remove(o));
                continue;
            }
            break;
        }
}

Alternatively a better approach would be to use a temporary Set to store the objects to remove:

{
    ArrayList list;
    TreeSet<Integer> set = new TreeSet<>(); // Use your Set!
    // ...
    for(final Integer o : list)
        if(list.indexOf(o) != list.lastIndexOf(o))
            set.add(o);
    for(final Integer o : set)
        while(list.remove(o));
}

Upvotes: 1

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