Troubles modifying a set with multiple iterators

Question

I'd like to compare elements within a set, in order to merge elements too similar. In order to gain some time, I would like to erase similarities during the iteration. Here how I try to proceed:

Iterator<Personne> i1 = personnes.iterator();
while (i1.hasNext()) {
     Personne personneObservee = i1.next();
     Set<Personne> tmp = new HashSet<Personne>();
     tmp.addAll(personnes);
     Iterator<Personne> i2 = tmp.iterator();
     while (i2.hasNext()) {
        Personne autrePersonne = i2.next();
        if (personneObservee.isSimilarTo(autrePersonne)) {
            personnes.remove(autrePersonne);
        }
    }
    result.add(personneObservee.toString());
}

As you can guess from my presence here, it doesn't work, giving me this nice stacktrace :

java.util.ConcurrentModificationException
        at java.util.HashMap$HashIterator.nextEntry(HashMap.java:926)
        at java.util.HashMap$KeyIterator.next(HashMap.java:960)
        at NameDetectorWithBenefits.mergeSamePersons(NameDetectorWithBenefits.java:41)
        at App.main(App.java:71)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:606)
        at org.codehaus.mojo.exec.ExecJavaMojo$1.run(ExecJavaMojo.java:297)
        at java.lang.Thread.run(Thread.java:744)

At first, I thought i1 and i2 were iterating over the same set and that I would find an answer here. As a consequence, I create a temporary set each time. However, it didn't resolve the problem.

Any ideas where the trouble might come from ?

How does your isSimilarTo(Personne) differ from the code that would be inside equals(Object)? — Josh M
– Josh M, Commented Feb 25, 2014 at 17:56
It has some calculations over the differents attributes but reads onlyinformation. — merours
– merours, Commented Feb 25, 2014 at 18:04

Kakarot · Accepted Answer · 2014-02-25 17:54:31Z

1

While iterating through a set use the iterator to delete elements from the set rather than using set.remove()

// wrong way
personnes.remove(autrePersonne); 

//correct way
it.remove();

answered Feb 25, 2014 at 17:54

Kakarot

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1 Comment

merours Over a year ago

It erased the exception, thanks ! I'll check if it works as expected tomorrow.

Community · Accepted Answer · 2017-05-23 12:10:56Z

1

Use Iterator#remove() rather than Set#remove(Object).

So replace the following line

personnes.remove(autrePersonne);

with

i2.remove();

See here for more details.

edited May 23, 2017 at 12:10

CommunityBot

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answered Feb 25, 2014 at 17:52

asaini007

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3 Comments

Josh M Over a year ago

Although this is correct, how do you apply this to his particular example?

merours Over a year ago

Is there a difference between It#remove and It.remove or is this just convention ?

asaini007 Over a year ago

@fxm Iterator#remove() simply refers to the remove() method of the Iterator class. In your code you'd call that method on your Iterator object (i.e. 12.remove())

Kick · Accepted Answer · 2014-02-25 17:50:48Z

0

You can't update the SET while another thread is iterating it due to which ConcurrentModificationException exception arise.

You are iterating the SET

Iterator<Personne> i1 = personnes.iterator(); and use personnes.remove(autrePersonne); to remove the element.

use Iterator remove method to remove the element

answered Feb 25, 2014 at 17:50

Kick

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6 Comments

Kick Over a year ago

But you cant update parent SET while iterating.Also what is ur desired output ?

merours Over a year ago

A set containing elements which aren't similars.

Kick Over a year ago

you are using set so it will not contain similar entry

merours Over a year ago

Well, similar in my way, not strictly equal (hence the isSimilar function).

Kick Over a year ago

what isSimilarTo method do /

|

user180100 · Accepted Answer · 2014-02-25 17:54:04Z

0

Do it in two steps:

isolate what need removal
remove

.

final Set<Personne> toRemove = new HashSet<>();
for (final Personne personne: personnes) {
    for (final Personne autrePersonne: personnes) {
        if (person.isSimilarTo(autrePersonne)) {
            toRemove.add(autrePersonne);
        }
    }
}
personnes.removeAll(toRemove);

Also, you need to take care of the case where autrePersonne equals personne

answered Feb 25, 2014 at 17:54

user180100

1 Comment

merours Over a year ago

While it probably works, it's more expensive in terms of complexity than what I'm trying to achieve.

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