Effective Java Item 69: Synchronized map vs ConcurrentMap

Synchronized map

 public static void main(String[] args){  
     Map<String, String> syncMap = Collections.synchronizedMap(new HashMap<String, String>());  
     syncMap.put("1","1");  
     syncMap.put("2","2");  
     syncMap.put("3","3");  
     for (String key : syncMap.keySet()){  
       addValue(syncMap);  
     }  
   }  
   
   private static void addValue(Map<String, String> map){  
     map.put("4", "4");  
   }  

Throws ConcurrentModificationException

 Exception in thread "main" java.util.ConcurrentModificationException  
      at java.util.HashMap$HashIterator.nextNode(HashMap.java:1429)  
      at java.util.HashMap$KeyIterator.next(HashMap.java:1453)  

ConcurrentHashMap never throws ConcurrentModificationException

 Map<String, String> syncMap = new ConcurrentHashMap<>();