Remove duplicate arrays in a ArrayList?

Question

Is there a way to remove duplicates arrays in an ArrayList?
I tried to convert the ArrayList into a HashSet and back to remove the duplicate arrays.
But this won't work:

ArrayList<int[]> mylist = new ArrayList<int[]>(); 
mylist.add(new int[]{1,2});
mylist.add(new int[]{2,2}); // Duplicate
mylist.add(new int[]{2,2}); // Duplicate

Before:

{{1,2},{2,2},{2,2}}

After:

{{1,2},{2,2}}

• Brett Okken: What defines a duplicate? '2' or int[]{2,2}?
• gkrls: Check if your Arraylist contains an Array before adding it
• eldjon: extend ArrayList and override add/addAll methods by checking first if the new item already exists in the list
• user3880174: @xgeorgekx Ok, I gonna try this out using a hashCode based on the value (@laune ´s idea)
• Deepanshu J bedi: javarevisited.blogspot.in/2012/12/…

Answer 1

Different array objects have different hashCode() values even if their contents are equal.

Offhand, I can't think of anything better than to wrap the arrays into a class and to hash all wrapped arrays using a hashCode based on the value and the Arrays.equals as an equals.

This would be O(n) as opposed to searching for duplicates in a nested loop (O(n^2)).

• user3880174: I am gonna check with Arrays.equals a duplicate before adding it. Also thanks to @xgerogekx.
• laune: Preventing the evil is alway best ;-) Although, it'll be costly and costlier with increasing list length.

Answer 2

This cannot be done with an ArrayList, this could be done with a TreeSet and a custom Comparator implementation which is able to accurately compare the int[] objects.

For example:

public final class IntArrayComparator implements Comparator<int[]>
{
 public int compare(int[] a, int[] b)
 {
 if (a == null)
 return b == null ? 0 : 1;
 if (a.length != b.length)
 return a.length - b.length;
 for (int i=0; i<a.length; ++i)
 {
 final int diff = a[i] - b[i];
 if (diff != 0)
 return diff;
 }
 return 0;
 }
}