public Set<KsqlTopic> getSourceTopics() {
    return Collections.unmodifiableSet(sourceTopics);
  }
@Test
  public void shouldExtractJoinTopicsFromJoinSelect() {
    // Given:
    final Statement statement = givenStatement(String.format(
        "SELECT * FROM %s A JOIN %s B ON A.F1 = B.F1;", STREAM_TOPIC_1, STREAM_TOPIC_2
    ));

    // When:
    extractor.process(statement, null);

    // Then:
    assertThat(extractor.getSourceTopics(), contains(TOPIC_2, TOPIC_1));
  }