ConcurrentModificationException whilst removing element using nested for-each

Question

CSV Input File.

EmployeeID,FirstName,LastName,Dept
1, John, Smith, Maintenance 
1, John, Smith, Engineering 
1, John, Smith, Transport

From the CSV file I read each line and create an employee object which is then stored into an ArrayList of Employee's.

Before sorting employee's:

employeeID='1', firstName='John', lastName='Smith', department='Maintenance'}
employeeID='1', firstName='John', lastName='Smith', department='Engineering'}
employeeID='1', firstName='John', lastName='Smith', department='Transport'}

Once file processing has finished, I move onto sorting the Employee’s.

Desired output after sort:

employeeID='1', firstName='John', lastName='Smith', department='Maintenance, Engineering, Transport'}

Using the For-Each with a nested For-Each, I compare the EmployeeID’s, if a match is found I append the dept of the duplicate to the original and remove the duplicate. Obviously as I’m modifying the list I’m iterating over I then get a ConcurrentModificationException.

Exception in thread "main" java.util.ConcurrentModificationException
    at java.base/java.util.ArrayList$Itr.checkForComodification(ArrayList.java:1042)
    at java.base/java.util.ArrayList$Itr.next(ArrayList.java:996)
    at com.gillott.csvtomap.CsvToMap.employeeSorter(CsvToMap.java:73)
    at com.gillott.csvtomap.CsvToMap.main(CsvToMap.java:34)

I’ve seen online the advice for carrying out modification on the collection you are iterating over is to use the Iterator, I’ve implemented the Iterator but still get the same results, Is this because I’m using a nested For-Each? See For-Each implementation and For-Each with Iterator implementation.

For-Loop example:

for (Employee emp1: employees) {

    for (Employee emp2: employees){

        if(emp1.getEmployeeID().equals(emp2.getEmployeeID())){

            emp1.setdepartment(emp1.getdepartment() + ", " + emp2.getdepartment());
            System.out.println("Duplicate found for Employee ID: " + emp1.getEmployeeID());
            employees.remove(emp2); //CME Exception Throw here.
        }
    }
}

Iterator For-Loop example:

for (Iterator<Employee> employeeIterator1 = employees.iterator(); employeeIterator1.hasNext();) {
    Employee emp1 = employeeIterator1.next();

    for (Iterator<Employee> employeeIterator2 = employees.iterator(); employeeIterator2.hasNext();){
        Employee emp2 = employeeIterator2.next();

        if(emp1.getEmployeeID().equals(emp2.getEmployeeID())){

            emp1.setdepartment(emp1.getdepartment() + ", " + emp2.getdepartment());
            System.out.println("Duplicate found for Employee ID: " + emp1.getEmployeeID());
            employeeIterator1.remove();
        }
    }
}

My implementation of the Iterator could be incorrect, this is the first time using the Iterator directly.

Andy Turner · Accepted Answer · 2020-05-16 18:59:49Z

2

Group the employees by id, and use a downstream collector to merge the instances:

Map<Integer, Employee> map =
    employees.stream()
        .collect(groupingBy(
            Employee::getId,
            reducing((a, b) -> a.setdepartment(a.getdepartment() + "," + b.getdepartment()));

Then retain the instances in employees which are values in this map:

employees.retainAll(map.values());

You can do this all in a single statement, if you so desire.

answered May 16, 2020 at 18:59

Andy Turner

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2 Comments

Bradley Over a year ago

Hi Andy, when reducing the method takes 2 parameters in your answer (a, b). a and b and not of type Employee so I can't call the a.setdepartment(a.getdepatment() + "," + b.getdepartment())); can you explain how i can make these arguments of type Employee?

Andy Turner Over a year ago

@Bradley you can declare explicit types: (Employee a, Employee b).

0xh3xa · Accepted Answer · 2020-05-16 18:45:39Z

1

Because you loop over the array and remove it, you can use another list that contains duplicates and remove from it or use any Concurrent list like CopyOnWriteArrayList

You can remove the duplicates using Set, and override the equals() and hashCode() in Employee class and determine which attributes to be used in hashCode and equals

Set<Employee> unique = new HashSet<>(employees);

More information: Concurrent collection Oracle doc

answered May 16, 2020 at 18:45

0xh3xa

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2 Comments

Bradley Over a year ago

I have over-ridden the equals and hashCode() in my employee class. How will this work with copying the department from the duplicate employee ID across to the unique version?

Bradley Over a year ago

Not yet, if I create an dept List inside the employee class so when I find a match I can just add the new dept and return the unique Set?

Community · Accepted Answer · 2020-06-20 09:12:55Z

0

Ideally, if implemented right, the iterator way should work. However, the issue with the code you've written in 'iterator for-loop' case is that you are initializing two iterators for the same List (employees) i.e both

Iterator employeeIterator1 = employees.iterator();

Iterator employeeIterator2 = employees.iterator();

both iterators are created for same list. For this reason, ConcurrentModificationException is thrown by the employeeIterator2, which indicates that the list has been changed (except by the Iterator's own remove method, in your case by 'employeeIterator1') after the Iterator employeeIterator2 was created.

For your scenario, you can create a dummy/temporary list which is copy of the original employee list and use that new list while creating the iterator2.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered May 16, 2020 at 19:01

AnonymousFox

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