### Question:
AWSIotMqttTopic { public void setCallback(AWSIotMqttNewMessageCallback callback) { this.callback = callback; } AWSIotMqttTopic(String topic, AWSIotMqttQos qos, AWSIotMqttNewMessageCallback callback); String getTopic(); void setTopic(String topic); AWSIotMqttQos getQos(); void setQos(AWSIotMqttQos qos); AWSIotMqttNewMessageCallback getCallback(); void setCallback(AWSIotMqttNewMessageCallback callback);  }### Answer:
@Test public void testSetCallback() throws Exception { topicModel.setCallback(callback2); assertEquals(callback2, topicModel.getCallback()); topicModel.setCallback(callback1); }