Error occurs when running the program

Question about the task

Under discussion

How do I resolve this error when I run my program? I can pinpoint that it has to do with adding a string to the ArrayList in the doubleValues(ArrayList<String list>) method. When I use the loop to print out the list it runs just fine. java.util.ConcurrentModificationException at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:909) at java.util.ArrayList$Itr.next(ArrayList.java:859) at com.codegym.task.task07.task0717.Solution.doubleValues(Solution.java:33) at com.codegym.task.task07.task0717.Solution.main(Solution.java:21) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498)

1. Read 10 words from the keyboard and add them to a list of strings.

2. The doubleValues method should duplicate words like this:
alpha, beta, gamma, -> alpha, alpha, beta, beta, gamma, gamma.

3. Display the result, each value on a new line.

Requirements:

Declare a string list variable and immediately initialize it.
Read 10 lines from the keyboard and add them to the list.
The doubleValues method should duplicate list elements like this: alpha, beta, gamma, -> alpha, alpha, beta, beta, gamma, gamma.
Display the resulting list, each element on a new line.

package com.codegym.task.task07.task0717; import java.io.BufferedReader; import java.io.InputStreamReader; import java.util.ArrayList; /* Duplicating words */ public class Solution { public static void main(String[] args) throws Exception { // Read strings from the console and declare an ArrayList here ArrayList<String> list = new ArrayList<>(); BufferedReader reader = new BufferedReader(new InputStreamReader(System.in)); for (int i = 0; i < 10; i++) { list.add(reader.readLine()); } ArrayList<String> result = doubleValues(list); // Display result /*System.out.println("\nOutput"); for (String str : result) { System.out.println(str); System.out.println(str); }*/ } public static ArrayList<String> doubleValues(ArrayList<String> list) { //write your code here for (String str : list) { list.add(str); /*list.add(str); list.add(str);*/ /*System.out.println(str); System.out.println(str);*/ } return list; } }

Comments (7)

Popular
New
Old

You must be signed in to leave a comment

Rebecca Zee

Level 10 , New York, United States

20 July 2023, 05:25

I was able to complete the task. I realized I was solving it wrong anyway. For some reason, I was iterating through the loop and trying to add an already existing element from the loop. When iterating the loop and and doing this, int i = 0, i < list.size() * 2; i += 2, I would get an IndexOutOfBoundsException. I then did the loop this way: int i = 0, size = list.size() * 2; i < capacity; i += 2. Why do I have to initialize the size to a variable in the loop?

Thomas

Level 41 , Bayreuth, Germany

20 July 2023, 06:56

I guess the learning goal here is to realize that you can't modify the collection in a for-each loop (but you can modify its elements). You have seen that now. Nevertheless you could solve this task with a for-each loop. Or, as already said, with an iterator or an 'ordinary' loop. With a normal loop you can only access existing indices. This is not the same exception as a ConcurrentModificationException (that you have realized as well by now that this is not the same 'error'). If you have a list with the elements {A, B, C} then you access it using the indices 0, 1, 2. If you loop over list.size() * 2 you may access an indices up to 4 which is clearly out of the bounds of the list. That's why you get the exception. Why your second loop header is not throwing that exception I can not tell you as you do not show us what you do with capacity and where you have declared it or if you break out of the loop somewhere inside the loops body. So let's look at the way looping over the collection and modifying that collection. Here it's size may change. That's why you only continue the loop when the current index is less than the collections size.

for (int i = 0; i < source.size(); i++) {

This would look reasonable for me. source: A B C D E The problem however is that you double (insert) elements. If you're at index 2 and the condition is met to insert the current element again, then everything from index 2 on is moved one position to the right. insert step 1 moving to the right: A B _ C D E F insert step 2 inserting reference to C: A B Ccopy C D E F in the next iteration with index 3 you'll again look at the original C and that's why you need to skip one iteration. When removing an element, you need to look at the same index again. Therefore the i++ or i-- (or however you manage to keep track of the index). That way you always will stay inside the bounds of the list because in the header you look at the size

Thomas

Level 41 , Bayreuth, Germany

20 July 2023, 07:03

If you decide to create a new list, the you could use a for-each loop

ArrayList<String> result = new ArrayList<>();
for (String element : liste) {
    if (conditionSkip == true)
        result.add(element);
    else if (conditionDouble == true) {
        result.add(element);
        result.add(element);
    } else if (conditionRemove == true) {
        // do nothing, just for illustration... you'd jusually ust leave that away
    }
}
return result;

with iterator:

ListIterator<String> it = list.listIterator();
while (it.hasNext()) {
    String element = it.next();
    if (conditionSkip == true) {
        // do nothing
    } else if (conditionDouble == true) {
        it.add(element);
    } else { // remove
        it.remove();
    }
}
return list;

Thomas

Level 41 , Bayreuth, Germany

18 July 2023, 15:00

When you use an Iterator you only are allowed to use it's methods to add or remove elements from the collection you iterate over. A for-each loop internally uses an Iterator. So either use a 'normal' loop or an Iterator.

Rebecca Zee

Level 10 , New York, United States

18 July 2023, 15:46

I already changed it to a regular loop and I still get the error, so I do not think that is the reason for why the error is occurring.

Thomas

Level 41 , Bayreuth, Germany

18 July 2023, 16:12

try:

List<Integer> list = IntStream.range(0, 10).boxed().collect(Collectors.toList());

        // throws ConcurrentModificationException
/*
        for (int element : list) {
            if (element == 4) {
                list.remove(4);
            }
        }
*/

        // working alternative 1
        for (int i = 0; i < list.size(); i++) {
            int element = list.get(i);
            if (element == 5) {
                list.remove(i);
            }
        }
        System.out.println(list);

        // working alternative 2
        Iterator<Integer> it = list.iterator(); // ListIterator if you need to add
        while (it.hasNext()) {
            int element = it.next();
            if (element == 6) {
                it.remove();
            }
        }
        System.out.println(list);

the only way to get around the ConcurrentModificationException in a for-each loop is (afaik) to get out of the loop directly after the modification like.

for (int element : list) {
    if (element == 4) {
        list.remove(4);
        break;
    }
}
System.out.println(list);

Guadalupe Gagnon

Level 37 , Tampa, United States

18 July 2023, 16:46

A concurrent modification exception occurs when trying to modify a collection being iterated over as Thomas says. That is the only way for this to occur.