Understanding ConcurrentModificationException

Question

I have a ArrayList of Metricas. Each Metrica has some properties and another ArrayList lForms. What I want to to is check in the db to see if the form has been filled, and update the lForms. When I do so, I get a ConcurrentModificationException, I don't know how to get rid of it...

public ArrayList<Metrica> addAllForms(ArrayList<Metrica> lMetricas) {
    ArrayList<Metrica> newlMet = lMetricas;
    ArrayList<Formulario> lForms = null;
    ArrayList<Formulario> newlForms;
    Metrica newMetrica;
    Formulario f;
    for (Iterator<Metrica> it = lMetricas.iterator(); it.hasNext();) {
        Metrica metrica = it.next();

        newlForms = new ArrayList<Formulario>();
        lForms = metrica.getlForms();
        for (Iterator<Formulario> it2 = lForms.iterator(); it2.hasNext();) {
            Formulario form = it2.next();

            f = getForm(form);
            if (f == null) {
                addForm(form, metrica.getId());
                Log.e("log", "adding form");
            } else
                form = f;
            Log.e("log", "Getting adding form");

            newlForms.add(form);

        }

        newMetrica = new Metrica();
        newMetrica.setlForms(newlForms);
        newMetrica.setClienteId(metrica.getClienteId());
        newMetrica.setDatoValido(metrica.getDatoValido());
        newMetrica.setDescription(metrica.getDescription());
        newMetrica.setId(metrica.getId());
        newMetrica.setProyectoProductoId(metrica.getProyectoProductoId());
        newMetrica.setTipo(metrica.getTipo());

        newlMet.add(newMetrica);

    }

    return lMetricas;
}

You can't modify the structure of an ArrayList while you're iterating over it... Check out the linked duplicate (adding elements to the list is the same thing as removing them, you're _structurally modifying the list_). — jahroy, May 28 '14 at 18:00
I used Iterator, and still the same problem. Besides, I'm not adding nothing to the ArrayList lMetricas or lForms... I don't understand ! — Juliatzin, May 28 '14 at 18:06
You do add to `IMetricas` since `newlMet = lMetricas;` and `newlMet.add(newMetrica);`. — Sotirios Delimanolis, May 28 '14 at 18:43
Upps, the error was that line : ArrayList newlMet = new ArrayList(); — Juliatzin, May 28 '14 at 18:51

score 0 · Accepted Answer · answered May 28 '14 at 18:36

0

Instead of

for (Metrica metrica : lMetricas)

Use the following:

Iterator<Metrica> it = lMetricas.iterator();
while(it.hasNext())
{
    Metrica metrica = it.next();
    //...
}

Similarly, you need to do the same ting with the lForms.

Edit: If you want to add something, you cannot just add them while iterating. Instead, use another ArrayList to store the elements you want to add. Afterwards, use lMetricas.addAll() to add them all at once.

If you want to add them to specific positions, you need to use a Map<Metrica, Integer> and use the values to find which position to add.

answered May 28 '14 at 18:36

padawan

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What's different between that `for` loop and that `while` loop? – Sotirios Delimanolis May 28 '14 at 18:42
Using an iterator while iterating over a list provides the flexibility of modifying the list. – padawan May 28 '14 at 18:43
I updated my code, I'm using Iterator, and still the same exception – Juliatzin May 28 '14 at 18:45

Understanding ConcurrentModificationException

1 Answers1