java.util.AbstractCollection

1. HeaderMap#getHeaderNames()

public Collection<HttpString> getHeaderNames() {
    if (headerNames != null) {
        return headerNames;
    }
    return headerNames = new AbstractCollection<HttpString>() {

        public boolean contains(final Object o) {
            return o instanceof HttpString && getEntry((HttpString) o) != null;
        }

        public boolean add(final HttpString httpString) {
            getOrCreateEntry(httpString);
            return true;
        }

        public boolean remove(final Object o) {
            if (!(o instanceof HttpString))
                return false;
            HttpString s = (HttpString) o;
            HeaderValues entry = getEntry(s);
            if (entry == null) {
                return false;
            }
            entry.clear();
            return true;
        }

        public void clear() {
            HeaderMap.this.clear();
        }

        public Iterator<HttpString> iterator() {
            final Iterator<HeaderValues> iterator = HeaderMap.this.iterator();
            return new Iterator<HttpString>() {

                public boolean hasNext() {
                    return iterator.hasNext();
                }

                public HttpString next() {
                    return iterator.next().getHeaderName();
                }

                public void remove() {
                    iterator.remove();
                }
            };
        }

        public int size() {
            return HeaderMap.this.size();
        }
    };
}