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