parquet-column/src/main/java/org/apache/parquet/schema/Types.java
public static GroupBuilder<GroupType> optionalGroup() {
    return new GroupBuilder<GroupType>(GroupType.class)
        .repetition(Type.Repetition.OPTIONAL);
  }
public static GroupBuilder<GroupType> optionalGroup() {
    return new GroupBuilder<GroupType>(GroupType.class)
        .repetition(Type.Repetition.OPTIONAL);
  }
Returns a builder to construct an optional {@link GroupType}.

@return a {@link GroupBuilder}