Java.util.ConcurrentModificationException while removing items from hashmap
I am studying a class HashMap
and wrote this simple program. this code works well for adding items to the hashmap and while removing items from the hashmap I come across for java.util.ConcurrentModificationException
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 you (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();
}
}
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Your for-loop gets a map.entrySet and uses an iterator to work through the map entries (this version of the for-loop requires an Iterable, it gets an iterator from an Iterable). When you use an iterator on a map, but remove things from the map without using that iterator, you get a ConcurrentModificationException. It is a map telling the iterator that it is out of date.
You can write a for loop using an iterator explicitly, for example:
for (Iterator<Map.Entry<String, Integer> it = map.entrySet().iterator();
it.hasNext();) {
and use the delete iterator method when you need to delete an entry.
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You call remove
during retry by Map
. This line, amplified for a loop, does Iterator
implicitly:
for(Map.Entry<String,Integer> entry : map.entrySet() )
When an element Iterator
detects that its collection has been modified, it issues ConcurrentModificationException
. However, you can call remove()
on Iterator
yourself if this exception is not thrown. Use 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 remove method.
}
}
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You cannot remove an item from the map you are currently iterating over. You can define an iterator, or you could make some simple changes to your code inside 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;
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Just use the Iterator class to remove the Iterating element like
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
}
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