How to find same keys and values in HashMap

Question

I have an HashMap with names(key) and dates(value) and I need to show if a name that i enter again is a duplicate (I am doing socket programming with different states). Example, I put in the map 3 different names and at the forth time I put a duplicate name.

I tried these codes (they run) but with no success:

HashMap<String, String> map = new HashMap<String, String>();

if(stat.equals("RECEIVE_NAME")){
   name = reader.readLine();
   if(map.containsKey(name)){
      System.out.println("duplicate");
   }
}

or

for(Map.Entry<String, String> entry : map.entrySet()){
   if(name.equals(entry.getKey())){
      sendMessage("duplicate");
   }
}

NB: I have a map.put(name) when is not a duplicate after this sequence of blocks. And i know that an hash map can not have duplicate keys :)

Answer 1

A HashMap cannot contain duplicate keys. If you put a key-value pair in a map which already contains the key, the old value will be replaced by the new value.

Answer 2

map.put() returns the previous value associated with key, or null if there was no mapping for key. So you can store that return value and check if is not null:

String check = map.put(name);
if(check != null){
    System.out.println("dup: " + check);
}

Use map.putIfAbsent(name) to not overwrite the associated value.