Java code to Prevent duplicate pairs in HashMap/HashTable

Question

I have a HashMap as below (assuming it has 10,0000 elements)

HashMap hm = new HashMap();
hm.put(\

Answer 1

List keys = new ArrayList(); (1000000)
List values = new ArrayList(); (1000000)
Map map = new HashMap();
int i =0;
for(String key : keys){
  String returnedValue = map.put(key, values.get(i));
  if(returnedValue!=null){
         map.put(key, returnedValue);
         system.out.println("Duplicate key trying to be entered with new value so   reverting the duplicate key ="+key+"new Value"+values.get(i));
  }
}