### Question:
VoiceCallEngine { public void joinChannel(String channelID, EventHandler handler) { mutex.lock(); this.handler = handler; mutex.unlock(); mRtcEngine.joinChannel(null, channelID, "", 0); unmuteMic(); useEarpiece(); } private  VoiceCallEngine(); static VoiceCallEngine getInstance(); RtcEngine getmRtcEngine(); void joinChannel(String channelID, EventHandler handler); void leaveChannel(); void muteMic(); void unmuteMic(); void useSpeaker(); void useEarpiece();  }### Answer:
@Test public void testJoinChannel() { String cid = "channel id"; VoiceCallEngine.getInstance().joinChannel(cid, mock(VoiceCallEngine.EventHandler.class)); verify(VoiceCallEngine.getInstance().getmRtcEngine(), times(1)).joinChannel(null, cid,"",0); }