Remove duplicates from ArrayLists

Question

I have an ArrayList of custom objects. I want to remove duplicate entries.

The objects have three fields: title, subtitle, and id. If a su

Answer 1

If I understand correctly you have an ArrayList<Custom>, let's call it list. Your Custom class has a subtitle field, let's say with a getSubtitle() method that returns String. You want to keep only the first unique subtitle and remove any remaining duplicates. Here's how you can do that:

Set<String> subtitles = new HashSet<String>();
for (Iterator<Custom> it = list.iterator(); it.hasNext(); ) {
    if (!subtitles.add(it.next().getSubtitle())) {
        it.remove();
    }
}

Answer 2

In Java 8, you can also do something like this:

yourList.stream().collect(
     Collectors.toMap(
         obj -> obj.getSubtitle(),
         Function.identity(), 
         (o1,o2) -> o1))
    .values();

The trick is to collect stream to map and provide key collision resolver lambda ((o1,o2) -> o1) which always returns its first parameter. The result is a Collection, not a List but you can easily convert it to a List:

new ArrayList(resultCollection);

Answer 3

Use Collections.sort() to sort and use a simple for cycle to catch doubles, e.g.:

Collections.sort(myList);
A previous = null;
for (A elem: myList) {
    if (elem.compareTo(previous) == 0) continue;
    previous = elem;

    [... process unique element ...]
}

This presumes that you'll implement Comparable in your type A.

Answer 4

private static List<Integer> removeDuplicates(List<Integer> list) {
    ArrayList<Integer> uniqueList = new ArrayList<Integer>();
    for (Integer i : list) {
        if (!inArray(i, uniqueList)) {
            uniqueList.add(i);
        }
    }

    return uniqueList;
}

private static boolean inArray(Integer i, List<Integer> list) {
    for (Integer integer : list) {
        if (integer == i) {
            return true;
        }
    }

    return false;
}

Answer 5

Another method using Java 8 streams you can also do pretty cool:

List<Customer> CustomerLists;
List<Customer> unique = CustomerLists.stream().collect(collectingAndThen(
        toCollection(() -> new TreeSet<>(comparingLong(Customer::getId))),
        ArrayList::new));

Answer 6

Update for Java8:

Using Java8 streams you can also do pretty trivally.

ArrayList<String> deduped;
deduped = yourArrayList.stream()
             .distinct()
             .collect(Collectors.toCollection(ArrayList::new));

This also has the advantage over going ArrayList → Set → ArrayList of maintaining ordering.