### Question:
OptimisticEventStoreService implements EventStore { @Override public boolean existsStream(String bucketId, String streamId) { return persistence.existsStream(bucketId, streamId); }  @Override ReadableEventStream openStreamForReading(String bucketId, String streamId); @Override ReadableEventStream openStreamForReading(String bucketId, String streamId, long version); @Override WritableEventStream createStream(String bucketId, String streamId); @Override WritableEventStream openStreamForWriting(
            String bucketId, String streamId, long version); @Override boolean existsStream(String bucketId, String streamId);  }### Answer:
@Test public void testExistsStream() throws Exception { cleanup(); assertEquals(eventStore.existsStream("FOO", "BAR"), false); fill("FOO", "BAR", 1); assertEquals(eventStore.existsStream("FOO", "BAR"), true); }