private ColumnConfig newColumnConfig(String column) {
            String[] tuple = column.split(":", 2);
            if (tuple.length != 2) {
                throw new IllegalArgumentException("tuple must be a pair of column name and type");
            }
            String type = tuple[1];
            if (!SUPPORTED_TYPES.contains(type)) {
                throw new IllegalArgumentException(String.format(Locale.ENGLISH,
                            "Unknown column type %s. Supported types are boolean, long, double, string, timestamp and json: %s",
                            tuple[1], column));
            }
            return new ColumnConfig(newConfig()
                    .set("name", tuple[0])
                    .set("type", type));
        }
This method returns PreviewResult.

@return PreviewResult returns the result by PreviewExecutor
@throws IOException in case of failures in file operations
embulk-junit4/src/main/java/org/embulk/test/TestingEmbulk.java