class AbstractDocument implements Document {

  private final Map<String, Object> documentProperties;

  protected AbstractDocument(Map<String, Object> properties) {
    Objects.requireNonNull(properties, "properties map is required");
    this.documentProperties = properties;
  }

  @Override
  public Void put(String key, Object value) {
    documentProperties.put(key, value);
    return null;
  }

  @Override
  public Object get(String key) {
    return documentProperties.get(key);
  }

  @Override
  public <T> Stream<T> children(String key, Function<Map<String, Object>, T> childConstructor) {
    return Stream.ofNullable(get(key))
            .filter(Objects::nonNull)
            .map(el -> (List<Map<String, Object>>) el)
            .findAny()
            .stream()
            .flatMap(Collection::stream)
            .map(childConstructor);
  }

  @Override
  public String toString() {
    return buildStringRepresentation();
  }

  private String buildStringRepresentation() {<FILL_FUNCTION_BODY>}

}

    var builder = new StringBuilder();
    builder.append(getClass().getName()).append("[");

    // Explaining variable for document properties map
    Map<String, Object> documentProperties = this.documentProperties;

    // Explaining variable for the size of document properties map
    int numProperties = documentProperties.size();

    // Explaining variable for tracking the current property index
    int currentPropertyIndex = 0;

    // Iterate over document properties map
    for (Map.Entry<String, Object> entry : documentProperties.entrySet()) {
      String key = entry.getKey();
      Object value = entry.getValue();

      // Append key-value pair
      builder.append("[").append(key).append(" : ").append(value).append("]");

      // Add comma if not last property
      if (++currentPropertyIndex < numProperties) {
        builder.append(", ");
      }
    }

    builder.append("]");
    return builder.toString();
  
<no_super_class>