Make a unique list of objects Java

Question

I have an ArrayList filled with objects with attributes name and time. I would like to remove duplicates based on the name and keep only records with the latest time. So I have overriden equals and hashcode for name in my object and used code like this.

private List<ChangedRecentlyTO> groupRecords(List<ChangedRecentlyTO> toList) {
    changedRecentlyList.clear(); //static list
    for(ChangedRecentlyTO to : toList) {
        if(!changedRecentlyList.contains(to)) {
            changedRecentlyList.add(to);
        } else {
            if(changedRecentlyList.get(changedRecentlyList.lastIndexOf(to)).getTimeChanged().before(to.getTimeChanged())) {
                changedRecentlyList.remove(to);
                changedRecentlyList.add(to);
            }
        }
    }
    return changedRecentlyList;
}

But I am wondering, is there a better solution?I was thinking about using Set but I am not able to figure out how should I put there the time criterion.

Answer 1

You have to me two ways, one which requires understanding how the set work, and one which is more understandable for people who have littler understanding of Java Collections:

If you want to make it simple, you can simply read in the detail the Javadoc of Set, http://docs.oracle.com/javase/6/docs/api/java/util/Set.html#add(E). It clearly states that if an element is already inside, it won't be added again.

• You implement your equals and hashcode using only the name
• You sort the items by time and then you add them to the Set.

In such a way, the first time you will add the item to Set, you will be adding the elements with the latest times. When you'll add the others, they will be ignored because they are already contained.

---

If someone else who does not know exactly the contract of java.util.Set behaves, you might want to extend Set to make your intention clearer. However, since a Set is not supposed to be accessed to "get back an element after removal", you will need to back your set with an HashMap:

interface TimeChangeable {
   long getTimeChanged();
}
public class TimeChangeableSet<E extends TimeCheangeable> implements Set<E> {

    private final HashMap<Integer,E> hashMap = new HashMap<Integer,E>();

    @Override
    public boolean add(E e) {
        E existingValue = hashMap.remove(e.hashCode());
        if(existingValue==null){
            hashMap.put(e.hashCode(),e);
            return true;
        }
        else{
            E toAdd = e.getTimeChanged() > existingValue.getTimeChanged() ? e : existingValue;
            boolean newAdded = e.getTimeChanged() > existingValue.getTimeChanged() ? true : false;
            hashMap.put(e.hashCode(),e);
            return newAdded;
        }

    }

    @Override
    public int size() {
        return hashMap.size();
    }

    @Override
    public boolean isEmpty() {
        return hashMap.isEmpty();
    }

    @Override
    public boolean contains(Object o) {
        return hashMap.containsKey(o.hashCode());
    }

    @Override
    public Iterator<E> iterator() {
        return hashMap.values().iterator();
    }

    @Override
    public Object[] toArray() {
        return hashMap.values().toArray();
    }

    @Override
    public <T> T[] toArray(T[] a) {
        return hashMap.values().toArray(a);
    }

    @Override
    public boolean remove(Object o) {
        return removeAndGet(o)!=null ? true : false;
    }

    public E removeAndGet (Object o) {
        return hashMap.remove(o.hashCode());
    }

    @Override
    public boolean containsAll(Collection<?> c) {
        boolean containsAll = true;
        for(Object object:c){
            E objectInMap = removeAndGet(object);
            if(objectInMap==null || !objectInMap.equals(object))
                containsAll=false;
        }
        return containsAll;
    }

    @Override
    public boolean addAll(Collection<? extends E> c) {
        boolean  addAll=true;
        for(E e:c){
            if(!add(e)) addAll=false;
        }
        return addAll;

    }

    @Override
    public boolean retainAll(Collection<?> c) {
        boolean setChanged=false;
        for(E e: hashMap.values()){
            if(!c.contains(e)){
                hashMap.remove(e.hashCode());
                setChanged=true;
            }
        }
        return setChanged;
    }

    @Override
    public boolean removeAll(Collection<?> c) {
        throw new UnsupportedOperationException("Please do not use type-unsafe methods in 2012");
    }

    @Override
    public void clear() {
        hashMap.clear();
    }




}