How do I remove repeated elements from ArrayList?

Question

I have an ArrayList<String>, and I want to remove repeated strings from it. How can I do this?

Answer 1

If you want to remove duplicates from ArrayList means find the below logic,

public static Object[] removeDuplicate(Object[] inputArray)
{
    long startTime = System.nanoTime();
    int totalSize = inputArray.length;
    Object[] resultArray = new Object[totalSize];
    int newSize = 0;
    for(int i=0; i<totalSize; i++)
    {
        Object value = inputArray[i];
        if(value == null)
        {
            continue;
        }

        for(int j=i+1; j<totalSize; j++)
        {
            if(value.equals(inputArray[j]))
            {
                inputArray[j] = null;
            }
        }
        resultArray[newSize++] = value;
    }

    long endTime = System.nanoTime()-startTime;
    System.out.println("Total Time-B:"+endTime);
    return resultArray;
}

Answer 2

If you don't want duplicates, use a Set instead of a List. To convert a List to a Set you can use the following code:

// list is some List of Strings
Set<String> s = new HashSet<String>(list);

If really necessary you can use the same construction to convert a Set back into a List.

Answer 3

Probably a bit overkill, but I enjoy this kind of isolated problem. :)

This code uses a temporary Set (for the uniqueness check) but removes elements directly inside the original list. Since element removal inside an ArrayList can induce a huge amount of array copying, the remove(int)-method is avoided.

public static <T> void removeDuplicates(ArrayList<T> list) {
    int size = list.size();
    int out = 0;
    {
        final Set<T> encountered = new HashSet<T>();
        for (int in = 0; in < size; in++) {
            final T t = list.get(in);
            final boolean first = encountered.add(t);
            if (first) {
                list.set(out++, t);
            }
        }
    }
    while (out < size) {
        list.remove(--size);
    }
}

While we're at it, here's a version for LinkedList (a lot nicer!):

public static <T> void removeDuplicates(LinkedList<T> list) {
    final Set<T> encountered = new HashSet<T>();
    for (Iterator<T> iter = list.iterator(); iter.hasNext(); ) {
        final T t = iter.next();
        final boolean first = encountered.add(t);
        if (!first) {
            iter.remove();
        }
    }
}

Use the marker interface to present a unified solution for List:

public static <T> void removeDuplicates(List<T> list) {
    if (list instanceof RandomAccess) {
        // use first version here
    } else {
        // use other version here
    }
}

EDIT: I guess the generics-stuff doesn't really add any value here.. Oh well. :)

Answer 4

This is used for your Custom Objects list

   public List<Contact> removeDuplicates(List<Contact> list) {
    // Set set1 = new LinkedHashSet(list);
    Set set = new TreeSet(new Comparator() {

        @Override
        public int compare(Object o1, Object o2) {
            if (((Contact) o1).getId().equalsIgnoreCase(((Contact) o2).getId()) /*&&
                    ((Contact)o1).getName().equalsIgnoreCase(((Contact)o2).getName())*/) {
                return 0;
            }
            return 1;
        }
    });
    set.addAll(list);

    final List newList = new ArrayList(set);
    return newList;
}

Answer 5

If you are using model type List< T>/ArrayList< T> . Hope,it's help you.

---

Here is my code without using any other data structure like set or hashmap

for(int i = 0; i < Models.size(); i++) {
     for(int j = i + 1; j < Models.size(); j++) {                                
       if(Models.get(i).getName().equals(Models.get(j).getName())){    
                                Models.remove(j);

                                j--;
                            }
                        }
                    }

Answer 6

this can solve the problem:

private List<SomeClass> clearListFromDuplicateFirstName(List<SomeClass> list1) {

Map<String, SomeClass> cleanMap = new LinkedHashMap<String, SomeClass>();
for (int i = 0; i < list1.size(); i++) {
     cleanMap.put(list1.get(i).getFirstName(), list1.get(i));
}
List<SomeClass> list = new ArrayList<SomeClass>(cleanMap.values());
return list;
}