Question

I have a data structure, for which I am currently using an ArrayList. I realised that in this structure I do not want any duplicates to be present. My first thought was to use some form of set, however the order is also important. After a bit of googling and searching the Collections docs I found LinkedHashSet which almost does the job. Unfortunately, one of the primary reasons for preserving order is because I am using the get(int index) method of the ArrayList for random access, and I can’t see any way around this.

More concisely – I need a set that preserves order and allows random access. None of the classes I have so far looked at provide this functionality. Does anyone know of a class that offers this, or will I have to make it myself? If it is the latter case are there any pitfalls when creating such a structure that people are aware of?

(Alternatively, a quick and easy way of checking for and removing duplicates form an ArrayList or similar structure would suffice)

EDIT: for clarity, it is the order that elements are added to the list that is important, not how they compare to one another

Answer 1

I’d just extend ArrayList.

public class SetList<E> extends ArrayList<E> {

    @Override
    public boolean add(E e) {
        return contains(e) ? false : super.add(e);
    }

    @Override
    public void add(int index, E e) {
        if (!contains(e)) {
            super.add(index, e);
        }
    }

    @Override
    public boolean addAll(Collection<? extends E> c) {
        return addAll(size(), c);
    }

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

}

Note that the add() method conforms the contract:

Ensures that this collection contains the specified element (optional operation). Returns true if this collection changed as a result of the call. (Returns false if this collection does not permit duplicates and already contains the specified element.)