使用迭代器时出现 java.util.ConcurrentModificationException 错误？

Question

Why am I getting this problem when trying to print it out? What I'm trying to do is display out the name that was placed in the array.

    ArrayList <Employee> empArray = new ArrayList();
    LinkedList empyrIncm = new LinkedList();
    ListIterator li = empyrIncm.listIterator();
    DecimalFormat df = new DecimalFormat ("#, ##0.00");

for (int i = 0; i<empArray.size(); i++)
    {
        double yrIncm = empArray.get(i).getSalary() * 12;

        if (yrIncm > 80000)
        {
            empyrIncm.add (new String (empArray.get(i).getEmpName()));

            while(li.hasNext())
            {
                System.out.println ("\nName : " + li.next() + "\nTotal yearly income : " + df.format (yrIncm));
            }
        }
        else
        {
            foundyrIncm = false;
        }
    }

From what I know, the iterator I used is to display out the name "one-by-one" without the "comma" when more input are added to the array. Means, without the use of iterator, it will display out by default

Name : Object1, Object 2, Object 3, Object 4 
Total yearly income : 123

I don't want that comma, instead I want the name to be display out one by one with their yearly income. Any help would be appreciated.

Answer 1

You cant do following two line's activity together ieread & write together with regular fail-fast iterator data structures :-

empyrIncm.add (new String (empArray.get(i).getEmpName()));
while(li.hasNext())

If you want to do both read & write together then do either of the following :-

empyrIncm.add (new String (empArray.get(i).getEmpName()));
ListIterator li = empyrIncm.listIterator();
while(li.hasNext())

OR

Use CopyOnWriteArrayList instead of LinkedList/ArrayList.

Now if you try to print any list then default implementation provided by API prints all element in list by comma seperated, hence the output you getting.

Answer 2

You are adding element to the list here:

empyrIncm.add (new String (empArray.get(i).getEmpName()));

while you are iterating the list here:

System.out.println ("\nName : " + li.next() + "\nTotal yearly income : " + df.format (yrIncm));

The iterator in java are Fail-fast. It will not allow you to update the list while iterating over it.

You need to use CopyOnWriteArrayList instead of LinkedList . Normally CopyOnWriteArrayList is very expensive because it involves costly Array copy with every write operation. Use the below statement.

CopyOnWriteArrayList empyrIncm = new CopyOnWriteArrayList();

Also your iterator getting call will be postponed until you need it. Just before the while loop.

ListIterator li = empyrIncm.listIterator();

Answer 3

Try initializing the ListIterator just outside the while loop.

ArrayList <Employee> empArray = new ArrayList();
LinkedList empyrIncm = new LinkedList();
DecimalFormat df = new DecimalFormat ("#, ##0.00");

for (int i = 0; i<empArray.size(); i++)
{
    double yrIncm = empArray.get(i).getSalary() * 12;

    if (yrIncm > 80000)
    {
        empyrIncm.add (new String (empArray.get(i).getEmpName()));

        ListIterator li = empyrIncm.listIterator();
        while(li.hasNext())
        {
            System.out.println ("\nName : " + li.next() + "\nTotal yearly income : " + df.format (yrIncm));
        }
    }
    else
    {
        foundyrIncm = false;
    }
}