How do I remove repeated elements from ArrayList?

Question

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

Answer 1

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  void removeDuplicates(ArrayList list) {
    int size = list.size();
    int out = 0;
    {
        final Set encountered = new HashSet();
        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  void removeDuplicates(LinkedList list) {
    final Set encountered = new HashSet();
    for (Iterator 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  void removeDuplicates(List 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. :)