@Override
    public String toString() {
        return topicId + ":" + topic() + "-" + partition();
    }
@Test
    public void testToString() {
        assertEquals("vDiRhkpVQgmtSLnsAZx7lA:a_topic_name-1", topicIdPartition0.toString());
        assertEquals("vDiRhkpVQgmtSLnsAZx7lA:null-1", topicIdPartitionWithNullTopic0.toString());
    }