code: public void add(String key, String value)
      {
          if(key == null)
              throw new IllegalArgumentException("Null keys not allowed.");
  
          if(value == null)
              throw new IllegalArgumentException("Null values not allowed.");
  
          List<String> vals = map.get(key);
          if(vals == null)
          {
              vals = new ArrayList<String>();
              map.put(key,vals);
          }
  
          vals.add(value);
      }
docs: Add a value for the supplied key. This will add the value to any existing values stored for this key.
  @param key the key
  @param value The value