Class for collecting strings and producing a grammatically correct sentence fragment

Grammar.java

public class Grammar<T> {
 
    private T last;
    private int count;
    private final StringJoiner joiner;
 
    public Grammar() {
        this.last = null;
        this.count = 0;
        this.joiner = new StringJoiner(", ");
    }
 
    public int getCount() {
        return count;
    }
 
    public Grammar<T> merge(Grammar<T> other) {
        if (count == 0) {
            return other;
        } else if (other.count == 0) {
            return this;
        }
 
        synchronized (joiner) {
            if (count > 1) {
                joiner.add(Objects.toString(last));
            }
            if (other.count > 1) {
                joiner.merge(other.joiner);
            }
        }
        last = other.last;
        count += other.count;
        return this;
    }
 
    public void add(T other) {
        synchronized (joiner) {
            if (count == 0) {
                joiner.add(Objects.toString(other));
            } else if (count > 1) {
                joiner.add(Objects.toString(last));
            }
        }
        last = other;
        count += 1;
    }
 
    public static <T> Collector<T, ?, Grammar<T>> collector() {
        return Collector.of(Grammar::new,
                            Grammar::add,
                            (singleton, other) -> singleton.merge(other),
                            Collector.Characteristics.UNORDERED,
                            Collector.Characteristics.CONCURRENT);
    }
 
    @Override
    public String toString() {
        StringBuilder builder = new StringBuilder(joiner.toString());
        if (count > 2) {
            builder.append(','); // Oxford comma
        }
        if (count > 1) {
            builder.append(" and ").append(last);
        }
        return builder.toString();
    }
}