How to avoid Java.util.IllegalStateException in the following code?

I have a List of integer's with duplicate values in it. What I need to do is find the duplicate integers, add their value and then add the result to the list by remov开发者_运维技巧ing the duplicates found. Here is what I am doing:

List<Integer> list1 = new ArrayList<Integer>();
    list1.add(2);
    list1.add(5);
    list1.add(3);
    list1.add(5);
    list1.add(4);

    List<Integer> list2 = new ArrayList<Integer>();
    Iterator<Integer> it = list1.iterator();
    while (it.hasNext()) {
        Integer int1 = it.next();
        if (list2.isEmpty()) {
            list2.add(int1);
            it.remove();
        } else {
            ListIterator<Integer> it2 = list2.listIterator();
            while (it2.hasNext()) {
                Integer int2 = it2.next(); 
                if (int2 != int1) {
                    it2.add(int1);
                    it.remove();// I get exception here

                } else {                        
                    it2.remove();
                    it.remove();
                    Integer newint = int1 + int2;
                    it2.add(newint);
                }                   
            }
        }
    }       
    for(Integer in : list2){
        System.out.println(in);
    }

Output should look like

2

10

3

4

Thanks for your time.

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As the other posters have said, you can't remove while iterating. Even though there are 'tricks', messing with a collection while iterating is a surefire way to get weird runtime bugs.

Anyway, you are working way too hard on the problem.

Here's a quick and dirty solution with a fraction of the code:

private List<Integer> sumAndUniqDuplicates(List<Integer> list) {
    LinkedHashMap<Integer, Integer> lookup = new LinkedHashMap<Integer, Integer>();
    for (Integer value : list) {
        Integer prevValue = lookup.get(value);
        prevValue = (prevValue == null) ? 0 : prevValue;
        lookup.put(value, prevValue + value);
    }
    return new ArrayList<Integer>(lookup.values());
}

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If you are allowed to use a Map you could do something simple like this (incoming pseudocode):

create empty Map m
for each Integer x in list1 do
    if m does not contain key x 
        m.put(x, x)
    else
        m.put(x, m.get(x) + x)
    endif
done

Your result are the values of m (which is a Collection).

Edit: You said you have LatLng instead of Integers - I don't know LatLng but after a quick google I'd take a shot at the following, assuming that you want to "add" up your LatLng points:

create empty Map<LatLng, LatLng> m
for each LatLng x in list1 do
    if not m.containsKey(x) 
        m.put(x, x)
    else
        m.put(x, LatLng.newInstance(m.get(x).getLatitude() + x.getLatitude(),
                                    m.get(x).getLongitude() + x.getLongitude()))
    endif
done

The only problem I can see here is that this m.containsKey(x) depends on the correct implementation of equals, which I'm not sure after reading this

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It is because you remove the same element twice. First time in if(list2.isEmpty()) (because list2 is empty in the beginning and immediately after that in the else body.

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From the documentation for the remove method:

Removes from the underlying collection the last element returned by the iterator (optional operation). This method can be called only once per call to next.

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You cannot remove the current element twice. You need to rethink your logic.