Can't store and load an arraylist in an object file

@Override
public String toString() {
  return String.format("%s:%s:%f", nome, estado, montanteMax);
}

public void writeToFile(List<Aluno> objectList) {
   String data = objectList
                   .stream()
                   .map(obj -> obj.toString())
                   .collect(Collectors.joining("\n"));

   Files.write(Paths.get("utilizadores_objetos.txt"),
               data.toByte(StandardCharsets.UTF_8),
               StandardOpenOption.CREATE, 
               StandardOpenOption.TRUNCATE_EXISTING);
}

public List<Aluno> readFile() throws IOException {
   List<Aluno> list = Files.readAllLines(Paths.get("utilizadores_objetos.txt"))
                      .stream()
                      .filter(s -> !s.trim().isEmpty())
                      .map(s -> new Aluno(s))
                      .collect(Collectors.toList());
   return list;
}

public Aluno(String data) {
   String items = data.split(":");
   nome = items[0];
   estado = items[1];
   montanteMax = Float.parseFloat(items[2]);
}