ConcurrentModificationExample and possible code simplification

Question

I am throwing a ConcurrentModificationExample in the following code. I checked the API and it has to do with me trying to modify an object while another thread is iterating over it. I am clueless on the matter. I have created a comment above the line causing the exception. The Employee class doesn't contain anything other than the three variables for storing information.

I will be including the entire class as I would also like to know if there is a way to simplify my code as it repeats many things such as object creation and adding everything to the lists.

Answer 1

When you call employeesByAge in here with dep.employees:

dep.employeesByAge(dep.employees)

that will pass in dep.employees to employeesByAge such that in:

public class Department{
    LinkedList<Employee> employees = ...;
    public LinkedList<Employee> employeesByAge(LinkedList<Employee> outputList) {
        ...
    }
}

both the employee member field and the outputList parameter refers to the same list, not just two list with the same content, but the same list instance.

Then you do:

for (Employee emp: employees){
    //the list is null. add the first employee

    if (outputList.isEmpty()){
        outputList.add(emp);
    } else
    ...
}

which iterates the employee and modifies outputList, but remember that these two are the same list object. Thus, ConcurrentModificationException.

Answer 2

What you're attempting to do is similar to this...

List list = ...;
for(item: list) {
    list.add(item);
}

That is, you're updating a collection with elements by iterating over the same collection. All

outputList.add(...);

in Department are adding elements to the collection from the same collection 'employees'. In main(), by doing

dep.employeesByAge(dep.employees)

you're attempting to update 'dep.employees' with 'dep.employees.' which results in concurrent modification exception.