How to add value to an existing key

Question

I am using a loop statement to add keys and value in the map from a result set and if the key already exists, update that key and add to its value.

This is my code:

Map<Integer, Integer> item_id = new Hashmap<Integer , Integer>();

for (i=0 ; i<roomtype_id.length(); i++) {
  String query ="SELECT item_id , quantity from amenities_tb where roomtype_id = '"+roomtype_id.get(i)+"'"
  PreparedStatement pst = connection.preparedStatement(query);
  Result Set rs = pst.executeQuery();

  while(rs.next()) {
    if (item_id.containskey(rs.getInt(1))) {
      // update this key and add to its existing value
    }
    else {
      item_id.put(rs.getInt(1),rs.getInt(2));
    }
  }

Answer 1

In java 8 you can make the full if else just a single statement:

item_id.compute (rs.getInt (1), (key, old) -> (old == null ? 0 : old)+rs.getInt (2));