java.util.ConcurrentModificationException when removing elements from a hashmap

Question

I am learning about HashMap class and wrote this simple program. this code works good for adding elements to the hashmap and while removing elements from the hashmap , I am encountering java.util.ConcurrentModificationException for example here is a copy of my terminal,

[ravi@doom test]$ java TestHashMap 
.....MENU.....
1. Add
2. remove key
3. remove value
4. display
7. Exit
Your choice :1

 Key : A

 Value : 1
Key/Value : (A,1) added to storage.
.....MENU.....
1. Add
2. remove key
3. remove value
4. display
7. Exit
Your choice :1

 Key : B

 Value : 2
Key/Value : (B,2) added to storage.
.....MENU.....
1. Add
2. remove key
3. remove value
4. display
7. Exit
Your choice :1

 Key : C

 Value : 3
Key/Value : (C,3) added to storage.
.....MENU.....
1. Add
2. remove key
3. remove value
4. display
7. Exit
Your choice :1

 Key : D

 Value : 4
Key/Value : (D,4) added to storage.
.....MENU.....
1. Add
2. remove key
3. remove value
4. display
7. Exit
Your choice :4
( D , 4 );
( A , 1 );
( B , 2 );
( C , 3 );
.....MENU.....
1. Add
2. remove key
3. remove value
4. display
7. Exit
Your choice :2
Key to REMOVE : 
D
Pair (D,4) Removed.
.....MENU.....
1. Add
2. remove key
3. remove value
4. display
7. Exit
Your choice :4
( A , 1 );
( B , 2 );
( C , 3 );
.....MENU.....
1. Add
2. remove key
3. remove value
4. display
7. Exit
Your choice :3
Enter Value to remove : 2
Key : B Removed.
Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.HashMap$HashIterator.nextEntry(HashMap.java:922)
    at java.util.HashMap$EntryIterator.next(HashMap.java:962)
    at java.util.HashMap$EntryIterator.next(HashMap.java:960)
    at TestHashMap.start(TestHashMap.java:60)
    at TestHashMap.main(TestHashMap.java:87)
ABRT problem creation: 'success'

code:

import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

class TestHashMap
{
    private Map<String,Integer> map;
    public TestHashMap()
    {
        map = new HashMap<String,Integer>();
    }
    public void displayMenu()
    {
        System.out.println(".....MENU.....");
        System.out.println("1. Add");
        System.out.println("2. remove key");
        System.out.println("3. remove value");
        System.out.println("4. display");
        System.out.println("7. Exit");
        System.out.print("Your choice :");
    }
    public void start()
    {
        Scanner input = new Scanner(System.in);
        int menuChoice,value;
        String key;
        while(true)
        {
            displayMenu();
            menuChoice = input.nextInt();
            switch(menuChoice)
            {
                case 1:
                    System.out.print("\n Key : ");
                    input.nextLine();
                    key = input.nextLine();
                    System.out.print("\n Value : ");
                    value = input.nextInt();
                    map.put(key,new Integer(value));
                    System.out.println("Key/Value : ("+key+","+value+") added to storage.");
                break;
                case 2:
                    System.out.println("Key to REMOVE : ");
                    input.nextLine();
                    key = input.nextLine();
                    Integer v = map.get(key);
                    if(v == null)
                        System.out.println("No value exists for key "+key);
                    else
                    {
                        map.remove(key);
                        System.out.println("Pair ("+key+","+v.intValue()+") Removed.");
                    }
                    break;
                case 3:
                    System.out.print("Enter Value to remove : ");
                    value = input.nextInt();
                    if(map.containsValue(new Integer(value)))
                    {
                        for(Map.Entry<String,Integer> entry : map.entrySet() )
                        {
                            if(entry.getValue().intValue() == value)
                            {
                                System.out.println("Key : "+entry.getKey()+" Removed.");
                                map.remove(entry.getKey());
                            }
                        }
                    }
                break;
                case 4:
                    for(Map.Entry<String,Integer> entry : map.entrySet() )
                    {
                        System.out.println("( "+entry.getKey()+" , "+entry.getValue()+" );");
                    }
                break;
                case 7:
                    input.close();
                    System.exit(0);
                default:
                    System.out.println("Invalid Choice !");
            }
        }
    }
    public static void main(String args[])
    {
        TestHashMap thm = new TestHashMap();
        thm.start();
    }
}

UPDATE: working code

thanks to both (rgettman,Nathan Hughes) of you.

import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;
import java.util.Iterator;

class TestHashMap
{
    private Map<String,Integer> map;
    public TestHashMap()
    {
        map = new HashMap<String,Integer>();
    }
    public void displayMenu()
    {
        System.out.println(".....MENU.....");
        System.out.println("1. Add");
        System.out.println("2. remove key");
        System.out.println("3. remove value");
        System.out.println("4. display");
        System.out.println("7. Exit");
        System.out.print("Your choice :");
    }
    public void start()
    {
        Scanner input = new Scanner(System.in);
        int menuChoice,value;
        String key;
        while(true)
        {
            displayMenu();
            menuChoice = input.nextInt();
            switch(menuChoice)
            {
                case 1:
                    System.out.print("\n Key : ");
                    input.nextLine();
                    key = input.nextLine();
                    System.out.print("\n Value : ");
                    value = input.nextInt();
                    map.put(key,new Integer(value));
                    System.out.println("Key/Value : ("+key+","+value+") added to storage.");
                break;
                case 2:
                    System.out.println("Key to REMOVE : ");
                    input.nextLine();
                    key = input.nextLine();
                    Integer v = map.get(key);
                    if(v == null)
                        System.out.println("No value exists for key "+key);
                    else
                    {
                        map.remove(key);
                        System.out.println("Pair ("+key+","+v.intValue()+") Removed.");
                    }
                    break;
                case 3:
                    System.out.print("Enter Value to remove : ");
                    value = input.nextInt();
                    if(map.containsValue(new Integer(value)))
                    {
                        for (Iterator<Map.Entry<String, Integer>> it = map.entrySet().iterator();it.hasNext();) 
                        {
                            Map.Entry<String,Integer> x = it.next();
                            if(x.getValue().intValue() == value)
                            {
                                key = x.getKey();
                                it.remove();
                                System.out.println("Key : "+key+" Removed.");
                            }
                        }
                    }
                break;
                case 4:
                    for(Map.Entry<String,Integer> entry : map.entrySet() )
                    {
                        System.out.println("( "+entry.getKey()+" , "+entry.getValue()+" );");
                    }
                break;
                case 7:
                    input.close();
                    System.exit(0);
                default:
                    System.out.println("Invalid Choice !");
            }
        }
    }
    public static void main(String args[])
    {
        TestHashMap thm = new TestHashMap();
        thm.start();
    }
}

The problem is you are mutating the map in the same time that is getting an deleting. Put the system.out.println before the remove. — StackFlowed
– StackFlowed, Commented Oct 21, 2014 at 19:00
possible duplicate of ConcurrentModification Exception with Map and Hashtable — ᴇʟᴇvᴀтᴇ
– ᴇʟᴇvᴀтᴇ, Commented Oct 21, 2014 at 19:16

rgettman · Accepted Answer · 2014-10-21 19:01:34Z

You are calling remove while you're iterating over the Map. This line, the enhanced for loop, runs an Iterator implicitly:

for(Map.Entry<String,Integer> entry : map.entrySet() )

When an Iterator detects that its collection is modified, it throws a ConcurrentModificationException. However, you can call remove() on the Iterator itself without that exception being thrown. Use an Iterator explicitly:

Iterator<Map.Entry<String, Integer>> itr = map.entrySet().iterator();
while(itr.hasNext())
{
   Map.Entry<String, Integer> entry = itr.next();
   if(entry.getValue().intValue() == 2)
   {
      System.out.println("Key : "+entry.getKey()+" Removed.");
      itr.remove();  // Call Iterator's remove method.
   }
}

Nathan Hughes · Accepted Answer · 2014-10-21 19:01:05Z

12

Your for-loop gets the map.entrySet and uses the iterator on it to work through the map entries (this version of the for-loop requires an Iterable, it gets the iterator from the Iterable). When you are using an iterator on a map, but remove things from the map without using that iterator, you get the ConcurrentModificationException. That is the map telling the iterator that it's out of date.

You can write a for loop using the iterator explicitly, like this:

for (Iterator<Map.Entry<String, Integer> it = map.entrySet().iterator();
it.hasNext();) {

and use the iterator's remove method when you need to delete an entry.

answered Oct 21, 2014 at 19:01

Nathan Hughes

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2 Comments

Julien Over a year ago

I prefer the version that uses while(it.hasNext()), as for loops are normally used for loop where the number of iteration are known in advance. Well I've been told so years ago at uni.

Nathan Hughes Over a year ago

@Julien: sure, pretty much the same thing; only difference is the for loop keeps the scope of the iterator variable local to the for-block.

IdusOrtus · Accepted Answer · 2014-10-21 19:27:04Z

You cannot remove an element from a map which you are currently iterating over. You could define an iterator, or you could make a few simple modifications to your code within the block for case 3.

case 3:
    System.out.print("Enter Value to remove : ");
    value = input.nextInt();
    if(map.containsValue(new Integer(value)))
    {
        Map.Entry<String,Integer> foo = null;
        for(Map.Entry<String,Integer> entry : map.entrySet() )
            if(entry.getValue().intValue() == value)
                foo = entry;

        System.out.println("Key : "+foo.getKey()+" Removed.");
        map.remove(foo.getKey());
    }
    break;

Keshav bansal · Accepted Answer · 2015-08-23 20:52:38Z

1

Just Use Iterator class to remove the Iterating Element as

for (Iterator<Map.Entry<String, Integer> it = map.entrySet().iterator();
while(it.hasNext()) {
  Map.Entry<String, Integer> e= itr.next();
  key = e.getKey();
  Value = e.getValue();

  //Your Other Code Here

   it.remove();                    //It removes the current Itertaion from Map


}

answered Aug 23, 2015 at 20:52

Keshav bansal

1301 silver badge2 bronze badges

Comments

SRF · Accepted Answer · 2021-12-07 08:46:05Z

1

It is possible to use removeIf of the Collections class.

map.keySet().removeIf(key -> key.equals("someThing"));

answered Dec 7, 2021 at 8:46

SRF

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