Difference between fail-fast and fail safe in Java

Sr. No.KeyFail-FastFail-Safe
1
Exception
Any changes in the collection, such as adding, removing and updating collection during a thread are iterating collection then Fail fast throw concurrent modification exception. 
The fail-safe collection doesn't throw exception. 
2.
Type of collection
ArrayList and hashmap collection are the examples of fail-fast iterator 
CopyOnWrite and concurrent modification are the examples of a fail-safe iterator 
3.
Performance and Memory
It's work on actual collection instead. So, this iterator doesn't require extra memory and time 
It's working on a clone of the collection instead of actual collection. It is overhead in terms of time and memory
4.
Modifications
 Iterators don't allow modifications of a collection while iterating over it.
Fail-Safe iterators allow modifications of a collection while iterating over it.

Example of FailSafe

public class FailSafeExample{
   public static void main(String[] args){
      ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<String, Integer>();
      //Adding elements to map
      map.put("Dell", 1);
      map.put("IBM", 2);
      //Getting an Iterator from map
      Iterator<String> it = map.keySet().iterator();
      while (it.hasNext()){
      String key = (String) it.next();
         System.out.println(key+" : "+map.get(key));
         map.put("Google", 3);
      }
   }
}

Output

IBM :2
Dell:1

Example of FailSafe

public class FailFastExample{
   public static void main(String[] args){
      List<Integer> list = new ArrayList<Integer>();
      list.add(1);
      list.add(2);
      list.add(3);
      //Getting an Iterator from list
      Iterator<Integer> it = list.iterator();
      while (it.hasNext()){
         Integer integer = (Integer) it.next();
         list.add(4);
      }
   }
}

Output

Exception in thread "main" java.util.ConcurrentModificationException
   at java.util.ArrayList$Itr.checkForComodification(Unknown Source)

Updated on: 18-Nov-2019

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