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

Question

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 removing 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.

Just as a side comment, it's recommendable to refer to interfaces instead of concrete implementations (i.e. write List<Integer> list1 = new ArrayList<Integer>();) — Jens Hoffmann
– Jens Hoffmann, Commented Oct 4, 2011 at 22:32

willCode4Beer · Accepted Answer · 2011-10-04 22:32:50Z

3

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());
}

edited Oct 4, 2011 at 22:32

answered Oct 4, 2011 at 22:23

willCode4Beer

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Comments

Jens Hoffmann · Accepted Answer · 2011-10-04 23:24:49Z

2

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

edited Oct 4, 2011 at 23:24

answered Oct 4, 2011 at 22:18

Jens Hoffmann

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2 Comments

Ritesh Mengji Over a year ago

The thing is I cannot use Map, I am actually working with google maps and I have LatLng points instead of Integer, so when I as to get some thing on map with LatLng point it returns null :(

Ritesh Mengji Over a year ago

This is what I tried and it works after overwriting equals method. Thanks for the answer.

AlexR · Accepted Answer · 2011-10-04 21:56:48Z

2

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.

answered Oct 4, 2011 at 21:56

AlexR

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1 Comment

Ritesh Mengji Over a year ago

Any suggestions on how to overcome this issue ? Even if I dont remove first time, I mean in list2.isEmpty block, it still gives me the error.

pablochan · Accepted Answer · 2011-10-04 21:56:19Z

1

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.

answered Oct 4, 2011 at 21:56

pablochan

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Comments

fastcodejava · Accepted Answer · 2011-10-04 22:01:18Z

1

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

answered Oct 4, 2011 at 22:01

fastcodejava

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1 Comment

Ritesh Mengji Over a year ago

Any suggestions on how to achieve it?

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