@Override
        public int characteristics() {
            // no synchronization here because source.characteristics() is assumed to be thread-safe
            return (source.characteristics() & ~Spliterator.CONCURRENT) | Spliterator.ORDERED
                  | (buffering ? 0 : Spliterator.SIZED);
        }
Returns the estimate size of this Spliterator.

Synchronized to get an accurate sum of <code>buffer.size()</code> and <code>source.estimateSize()</code>.
jOOL-java-8/src/main/java/org/jooq/lambda/SeqBuffer.java