public <U> Leader<U> map(Function<T, U> mapper) {
    return new Leader<>(mapper.apply(id), term, termStartTime);
  }
public < U > Leader < U > map ( Function < T , U > mapper ) { return new Leader < > ( mapper . apply ( id ) , term , termStartTime ) ; }
Converts the leader identifier using the given mapping function.

@param mapper the mapping function with which to convert the identifier
@param <U>    the converted type
@return the converted leader object