CODEWITHSUNDEEP
javahashmap
user6121656
user6121656

Reputation:

Java - HashMap Iteration - Exception

I want to iterate a HashMap like:

        for (Map.Entry<Integer, Integer> entry : map.entrySet()) {
            map.replace(entry.getKey(), entry.getValue()-1);
            if (entry.getValue() == 0) {
                map.remove(entry.getKey(), 0);
            }
        }

This ends in an Exception: java.util.ConcurrentModificationException

Any solutions?

Kind regards

Upvotes: 0

Views: 1054

Answers (3)

Louis Wasserman
Louis Wasserman

Reputation: 198033

 Iterator<Map.Entry<Integer, Integer>> entryItr = map.entrySet().iterator();
 while (entryItr.hasNext()) {
   Map.Entry<Integer, Integer> entry = entryItr.next();
   if (entry.getValue() > 1) {
     entry.setValue(entry.getValue() - 1);
   } else {
     entryItr.remove();
   }
 }

Upvotes: 1

Vasu
Vasu

Reputation: 22422

You can't remove elements from an existing Set without using Iterator, otherwise, it will throw ConcurrentModificationException

So, place values to a new Map object if(value-1 !=0) as shown below:

        Map<Integer, Integer> map = new HashMap<>();
        Map<Integer, Integer> newMap = new HashMap<>();
        Iterator<Integer> iterator =map.keySet().iterator();
        while(iterator.hasNext()) {
            int key = iterator.next();
            int value = map.get(key);
            if(value-1 !=0) {
                newMap.put(key, value-1);
            }
        }

Upvotes: 0

clinton3141
clinton3141

Reputation: 4841

You're removing items from the map as you're iterating through it. This is what's causing the exception.

To get a bit of intuition as to why this is: if your map contains three items and on the first iteration you remove the first item, should the next item be the second, or third? i.e. should the iteration continue as though the removed item was still there? And if so, how?

You can get around this by using an iterator on the map which safely goes through each element of the map.

Upvotes: 0

Related Questions