### Question:
ConfigurationEntry { public boolean isValid() { if (!this.conf.isValid()) { return false; } final Set<PeerId> intersection = listPeers(); intersection.retainAll(listLearners()); if (intersection.isEmpty()) { return true; } LOG.error("Invalid conf entry {}, peers and learners have intersection: {}.", this, intersection); return false; } ConfigurationEntry();  ConfigurationEntry(final LogId id, final Configuration conf, final Configuration oldConf); LogId getId(); void setId(final LogId id); Configuration getConf(); void setConf(final Configuration conf); Configuration getOldConf(); void setOldConf(final Configuration oldConf); boolean isStable(); boolean isEmpty(); Set<PeerId> listPeers(); boolean isValid(); Set<PeerId> listLearners(); boolean containsLearner(final PeerId learner); boolean contains(final PeerId peer); @Override String toString();  }### Answer:
@Test public void testIsValid() { ConfigurationEntry entry = TestUtils.getConfEntry("localhost:8081,localhost:8082,localhost:8083", null); assertTrue(entry.isValid()); entry = TestUtils.getConfEntry("localhost:8081,localhost:8082,localhost:8083", "localhost:8081,localhost:8082,localhost:8084"); assertTrue(entry.isValid()); entry.getConf().addLearner(new PeerId("localhost", 8084)); assertFalse(entry.isValid()); entry.getConf().addLearner(new PeerId("localhost", 8081)); assertFalse(entry.isValid()); }