Is there a no-duplicate List implementation out there?

Solution 1:

There's no Java collection in the standard library to do this. LinkedHashSet<E> preserves ordering similarly to a List, though, so if you wrap your set in a List when you want to use it as a List you'll get the semantics you want.

Alternatively, the Commons Collections (or commons-collections4, for the generic version) has a List which does what you want already: SetUniqueList / SetUniqueList<E>.

Solution 2:

Here is what I did and it works.

Assuming I have an ArrayList to work with the first thing I did was created a new LinkedHashMap.

LinkedHashSet<E> hashSet = new LinkedHashSet<E>()

Then I attempt to add my new element to the LinkedHashSet. The add method does not alter the LinkedHasSet and returns false if the new element is a duplicate. So this becomes a condition I can test before adding to the ArrayList.

if (hashSet.add(E)) arrayList.add(E);

This is a simple and elegant way to prevent duplicates from being added to an array list. If you want you can encapsulate it in and override of the add method in a class that extends the ArrayList. Just remember to deal with addAll by looping through the elements and calling the add method.

Solution 3:

So here's what I did eventually. I hope this helps someone else.

class NoDuplicatesList<E> extends LinkedList<E> {
    @Override
    public boolean add(E e) {
        if (this.contains(e)) {
            return false;
        }
        else {
            return super.add(e);
        }
    }

    @Override
    public boolean addAll(Collection<? extends E> collection) {
        Collection<E> copy = new LinkedList<E>(collection);
        copy.removeAll(this);
        return super.addAll(copy);
    }

    @Override
    public boolean addAll(int index, Collection<? extends E> collection) {
        Collection<E> copy = new LinkedList<E>(collection);
        copy.removeAll(this);
        return super.addAll(index, copy);
    }

    @Override
    public void add(int index, E element) {
        if (this.contains(element)) {
            return;
        }
        else {
            super.add(index, element);
        }
    }
}   

Solution 4:

Why not encapsulate a set with a list, sort like:

new ArrayList( new LinkedHashSet() )

This leaves the other implementation for someone who is a real master of Collections ;-)

Solution 5:

You should seriously consider dhiller's answer:

1. Instead of worrying about adding your objects to a duplicate-less List, add them to a Set (any implementation), which will by nature filter out the duplicates.
2. When you need to call the method that requires a List, wrap it in a new ArrayList(set) (or a new LinkedList(set), whatever).

I think that the solution you posted with the NoDuplicatesList has some issues, mostly with the contains() method, plus your class does not handle checking for duplicates in the Collection passed to your addAll() method.