Trying to solve this problem using Iterator and ListIterator - getting "ConcurrentModificationException".

Question about the task

R or L

Java Syntax, Level 7, Lesson 9

Under discussion

1. Create a list of words and populate it yourself.
2. The fix method should:
2.1. remove all words containing the letter "r" from the list
2.2. duplicate all words containing the letter "l".
2.3. if a word contains both "r" and "l", then leave it unchanged.
2.4. don't do anything to other words.

For example:
rose lyre love

Output:
lyre love love

Requirements:

The program must not read data from the keyboard.
The fix method should remove all words containing the letter "r" from the list. There is an exception: words containing both "r" and "l" should be left alone.
The fix method must duplicate words containing the letter "l" (add another instance of this word to the list). There is an exception: words containing both "l" and "r" should not be duplicated.
The fix method should not do anything with words that contain both "l" and "r".
The fix method should not do anything with words that don't contain either "l" or "r".

package com.codegym.task.task07.task0716; import java.util.ArrayList; import java.util.Iterator; import java.util.ListIterator; /* R or L */ public class Solution { public static void main(String[] args) throws Exception { ArrayList<String> list = new ArrayList<String>(); list.add("rose"); // 0 list.add("love"); // 1 list.add("lyre"); // 2 list = fix(list); for (String s : list) { System.out.println(s); } } public static ArrayList<String> fix(ArrayList<String> list) { // write your code here Iterator<String> itr = list.iterator(); ListIterator<String> listItr = list.listIterator(); while(itr.hasNext()){ String nextElement = itr.next(); if (nextElement.contains("r") && nextElement.contains("l")){ } if (nextElement.contains("r") && !nextElement.contains("l")){ itr.remove(); } // Till the point to remove the element, program is working fine. // Below if loop is throwing error if (nextElement.contains("l") && !nextElement.contains("r")){ listItr.add(nextElement); // tried all sort of combination here to add elements used list.add(nextElement) also listItr.add(list.get(listItr.nextIndex())) - nothing worked. } // Tried with separate while loop, didn't work :( /* while (listItr.hasNext()){ if (nextElement.contains("l") && !nextElement.contains("r")){ listItr. } }*/ } return list; } }

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Amit Sharma

Level 11 , Hyderabad, India

22 April 2020, 16:50

Below is the separate while loop code that I used.

// Tried with separate while loop, didn't work :(

               while (listItr.hasNext()){
String nextElement = listItr.next();
                    if (nextElement.contains("l") && !nextElement.contains("r")){
                    listItr.add(nextElement);
                }
                 }

Amit Sharma

Level 11 , Hyderabad, India

22 April 2020, 16:51

But it's not working.

Pavlo Plynko Java Developer at CodeGym

23 April 2020, 11:02

What you mean "not working" ? I'v tried to run this separate while loop code, and don't get ConcurrentModificationException.

Amit Sharma

Level 11 , Hyderabad, India

24 April 2020, 04:11

I am trying to run it with the complete program.

import java.util.ArrayList;
import java.util.Iterator;
import java.util.ListIterator;

/*
R or L
*/
public class Solution {
    public static void main(String[] args) throws Exception {
        ArrayList<String> list = new ArrayList<String>();
        list.add("rose"); // 0
        list.add("love"); // 1
        list.add("lyre"); // 2
        list = fix(list);

        for (String s : list) {
            System.out.println(s);
        }
    }

    public static ArrayList<String> fix(ArrayList<String> list) {
        // write your code here
            Iterator<String> itr = list.iterator();
            ListIterator<String> listItr = list.listIterator();

            while(itr.hasNext()){
                String nextElement = itr.next();

                if (nextElement.contains("r") && nextElement.contains("l")){
                }
                if (nextElement.contains("r") && !nextElement.contains("l")){
                    itr.remove();
                }   // Till the point to remove the element, program is working fine.

                // Below if loop is throwing error
            /*    if (nextElement.contains("l") && !nextElement.contains("r")){
                    listItr.add(nextElement); // tried all sort of combination here to add elements used list.add(nextElement) also listItr.add(list.get(listItr.nextIndex())) - nothing worked.
                } */
            }
                         // Tried with separate while loop, didn't work :(
                         while (listItr.hasNext()){
                                 String nextElement = listItr.next();
                                 if (nextElement.contains("l") && !nextElement.contains("r")){
                                 listItr.add(nextElement);
                         }

Pavlo Plynko Java Developer at CodeGym

24 April 2020, 07:47

You created few iterators for the same list. So when one of them doing remove() or add() operation, other give ConcurrentModificationException. You should try solve the task using one iterator. Try iterate over your list, and add needed elements to new list, which will be returnet by fix() then. So, fix() should not return the same list object it received as argument, and should not change it too.