public void add(String key,Object value){
  if (value == null) {
    throw new IllegalArgumentException("Value cannot be null");
  }
  put(key,Collections.singletonList(value));
}

public void add ( String key , Object value ) { if ( value == null ) { throw new IllegalArgumentException ( "Value cannot be null" ) ; } put ( key , Collections . singletonList ( value ) ) ; }
adds a value to a list with the given key .
