@Override
        public String toString() {
            final StringBuilder stringBuilder = new StringBuilder();
            stringBuilder.append(articles.length).append(" Articles").append('\n');
            for (final Article article : articles) {
                stringBuilder.append("  ").append(article).append('\n');
            }
            stringBuilder.append(comments.length).append(" Comments").append("\n");
            for (final Comment comment : comments) {
                stringBuilder.append("  ").append(comment).append('\n');
            }
            return stringBuilder.toString();
        }
This test builds on a basic relational data caricature:
a set of articles, each with a set of comments on them.
<p>
The particular relation in this data is that each Comment
has an articleId foreign-key reference to an Article.
<p>
We traverse this relation in both directions to verify
correct operation of Kafka Streams:
- Embed on each Comment a prefix of the text in the Article it references
aka SELECT * FROM Comment JOIN Article ON Comment.ArticleID = Article.ID
- Embed on each Article a count of all the Comments it has
streams/src/test/java/org/apache/kafka/streams/tests/RelationalSmokeTest.java