Rest API ResponseEntity add extra attribute to response

document model class

  private String id;
  private String format;
  private List<Attribute> attributes;
  private String type;
  private String accessright;

  public Document() {
  }

  /**
   * Copy constructor.
   */
  public Document(Document source) {
      this.id = source.id;
      this.format = source.format;
      this.type = source.type;
      this.accessright = source.accessright;
      this.attributes = source.attributes;
  }

  public static Document newDocument(String type, String id, String format, String accessright, List<Attribute> attributes) {
      Document document = new Document();
      document.type = type;
      document.id = id;
      document.format = format;
      document.accessright = accessright;
      document.attributes = attributes;

      return document;
  }

  public static Document newCompleteDocument(String id, String type, String format, String accessright, List<Attribute> attributes) {
      Document document = new Document();
      document.id = id;
      document.type = type;
      document.format = format;
      document.accessright = accessright;
      document.attributes = attributes;

      return document;
  }

  //
  // Getters and setters ahead

  public String getId() {
      return id;
  }

  public void setId(String id) {
      this.id = id;
  }

  public String getFormat() {
      return format;
  }

  public void setFormat(String format) {
      this.format = format;
  }

  public String getType() {
      return type;
  }

  public void setType(String type) {
      this.type = type;
  }

  public List<Attribute> getAttributes() {
      return attributes;
  }

  public void setAttributes(List<Attribute> attributes) {
      this.attributes = attributes;
  }

  public String getAccessright() {
      return accessright;
  }

  public void setAccessright(String accessright) {
      this.accessright = accessright;
  }
  
  public Map<String, List<String>> getAttributesMap() {
      Map<String, List<String>> attributesMap = getAttributes().stream().collect(Collectors.toMap(Attribute::getName, Attribute::getValues));
      return attributesMap;
  }

  @Override
  public String toString() {
      final StringBuffer sb = new StringBuffer("Document{");
      sb.append("id='").append(id).append('\'');
      sb.append(", type='").append(type).append('\'');
      sb.append(", format='").append(format).append('\'');
      sb.append(", accessright='").append(accessright).append('\'');
      sb.append(", attributes=[").append(attributes);
      if (attributes != null) {
          for (Attribute attribute:attributes) {
              sb.append(attribute.getName()).append(":").append(attribute.getValues().toArray()).append(",");
          }
      }

      sb.append("]}");
      return sb.toString();
  }

  @Override
  public boolean equals(Object o) {
      if (this == o) {
          return true;
      }
      if (o == null || getClass() != o.getClass()) {
          return false;
      }

      Document document = (Document) o;

      if (id != null ? !id.equals(document.id) : document.id != null) {
          return false;
      }
      if (type != null ? !type.equals(document.type) : document.type != null) {
          return false;
      }
      if (format != null ? !format.equals(document.format) : document.format != null) {
          return false;
      }
      if (accessright != null ? !accessright.equals(document.accessright) : document.accessright != null) {
          return false;
      }
      return attributes != null ? attributes.equals(document.attributes) : document.attributes == null;

  }

  @Override
  public int hashCode() {
      int result = id != null ? id.hashCode() : 0;
      result = 31 * result + (type != null ? type.hashCode() : 0);
      result = 31 * result + (format != null ? format.hashCode() : 0);
      result = 31 * result + (accessright != null ? accessright.hashCode() : 0);
      result = 31 * result + (attributes != null ? attributes.hashCode() : 0);
      return result;
  }
}

but the API reponse JSON as an extra attribute "attributesMap". Sample JSON as below: