Getting ConcurrentModification Exception

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Create a Map<String, String> and add ten entries that represent (last name, first name) pairs.
Remove people with the same first name.

Requirements:

The program should not display text on the screen.
The program should not read values from the keyboard.
The createMap() method must create and return a HashMap that has (String, String) elements and contains 10 entries.
The removeFirstNameDuplicates() method must remove from the map all people who have the same first name.
The removeFirstNameDuplicates() method must call the removeItemFromMapByValue() method.

package com.codegym.task.task08.task0817; import java.util.HashMap; import java.util.Map; /* We don't need repeats */ public class Solution { public static HashMap<String, String> createMap() { HashMap<String, String> names = new HashMap<>(); names.put("Majumdar","Joy"); names.put("Ghosh","Shreya"); names.put("Halder","Pritam"); names.put("Mondal","Abhisek"); names.put("Das","Anisha"); names.put("Parveen","Anisha"); names.put("Sran","Abhisek"); names.put("Roy","Aditya"); names.put("Royganguly","Pritam"); names.put("Maity","Shreya"); return names; } public static void removeFirstNameDuplicates(Map<String, String> map) { HashMap<String, String> Copy = new HashMap<>(map); for (Map.Entry<String, String> pair : Copy.entrySet() ) { removeItemFromMapByValue(Copy, pair.getValue()); } } public static void removeItemFromMapByValue(Map<String, String> map, String value) { HashMap<String, String> copy = new HashMap<String, String>(map); for (Map.Entry<String, String> pair : copy.entrySet()) { if (pair.getValue().equals(value)) map.remove(pair.getKey()); } } public static void main(String[] args) throws Exception { HashMap<String, String> Next = createMap(); removeFirstNameDuplicates(Next); for (Map.Entry<String, String> show : Next.entrySet() ) { System.out.println(show); } } }

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Guadalupe Gagnon

Level 37 , Tampa, United States

7 March 2019, 18:19

solution

Your concurrent modification error is happening because you are looping through a list that you are removing items from. This will almost always throw that exception and should be avoided. You need to make a copy (which you already have). Use the copy to loop through while removing from the original (not removing from the copy). Also your code doesn't check if there are duplicate values, it is attempting to remove everything. You need to check if the value is a duplicate first before removing it. I suggest making a new list (either a HashSet or ArrayList). What you do is this: (each loop cycle of the copy map) get the value from the copy map (<key>, <value>) which is the first name check if the value is already in the new list, you can use newList.contains(value); if the value is not contained, then add it into the new list else it is a duplicate and needs to be removed from the original map (end loop cycle)

Joy Majumdar

Level 16 , Kolkata, India

8 March 2019, 08:25

It is first time I am facing a bit difficulty. Means everything is getting confused.

Joy Majumdar

Level 16 , Kolkata, India

11 March 2019, 09:20

uff... atlast Is this thing you are asking to do ? public static void removeFirstNameDuplicates(Map<String, String> map) { ArrayList<String> NewList = new ArrayList<>(); HashMap<String, String> Copy = new HashMap<>(map); for (Map.Entry<String,String> pair : Copy.entrySet() ) { if (!NewList.contains(pair.getValue())) { NewList.add(pair.getValue()); } else { removeItemFromMapByValue(map, pair.getValue()); } } }

Guadalupe Gagnon

Level 37 , Tampa, United States

11 March 2019, 13:17

Yes!! I couldn't have done better myself. Now your code correctly loops through the copy map, checks for duplicate values, and removes duplicates from the original. Well done!

Ifeanyi

Level 22 , Lagos, Nigeria

25 June 2019, 20:19

Thanks, so helpful

Jay

Level 17 , Washington, United States

25 July 2019, 18:02

In that example, isn't the ArrayList completely empty? So how would it be able to compare values? What am I missing?

Dinca

Level 14 , Giurgiu, Romania

26 May 2020, 16:49

It is empty at the begining. First the first value is stored in the list because it doesn't exist yet and while iterating through the loop if it finds the current value from the map in the list it will remove the value from the map if it doesn't find it then the value is stored in the list and is not removed from the map.