default T one() {
        try (ResultIterator<T> iter = iterator()) {
            if (!iter.hasNext()) {
                throw new IllegalStateException("Expected one element, but found none");
            }

            final T r = iter.next();

            if (iter.hasNext()) {
                throw new IllegalStateException("Expected one element, but found multiple");
            }

            return r;
        }
    }
Returns the only row in the result set. Returns {@code null} if the row itself is
{@code null}.
@throws IllegalStateException if the result set contains zero or multiple rows
@return the only row in the result set.
core/src/main/java/org/jdbi/v3/core/result/ResultIterable.java