Concurrent List Modification

Question

Below is some code (removed much of it for clarity) which shows my issue of concurrent list modification:

for (final Week week : this.demand.getWeeks().getAllWeeks()) {
    final List<Week> weeksExcludingThisWeek = this.demand.getWeeks().getAllWeeks();
    weeksExcludingThisWeek.remove(week);
}

As far as I was aware I was removing the reference to the same week object, but in a different list than is used in the for loop.

How do I prevent this from occurring?

I set eclipse to automatically set any non modified variables as final on save. I find it useful to pin point odd errors. — Dr Ken Reid, Jun 27 '16 at 16:04

score 2 · Accepted Answer · answered Jun 27 '16 at 14:39

2

Actually copy it into a new list before removing the element:

List<Week> weeksExcludingThisWeek = new ArrayList<>(this.demand.getWeeks().getAllWeeks());

You are currently removing the week from the same list you are iterating.

answered Jun 27 '16 at 14:39

Andy Turner

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Perfect, that fixed it. Thank you Andy, I'll accept your answer in 8 mins. – Dr Ken Reid Jun 27 '16 at 14:43

score 2 · Answer 2 · edited Jun 27 '16 at 14:49

2

Use iterator to avoid ConcurrentModificationExceptions

Have a look at docs: https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html

You will find:

Iterators allow the caller to remove elements from the underlying collection during the iteration with well-defined semantics

edited Jun 27 '16 at 14:49

Andy Turner

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answered Jun 27 '16 at 14:46

Patrik Mihalčin

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Seems new to me, thanks for info!! – saravanakumar Jun 27 '16 at 14:49

score 1 · Answer 3 · answered Jun 27 '16 at 14:42

Here you are using this.demand.getWeeks().getAllWeeks() For both iteration variable week and inside for loop variable weeksExcludingThisWeek .

Same reference in two object that is the issue. You couldn't modify current iterating object inside the loop.

Create new object inside loop, new ArrayList(this.demand.getWeeks().getAllWeeks())

Concurrent List Modification

3 Answers3