Notification.Builder builder = new Notification.Builder(this)
        .setAutoCancel(true)
        .setChannelId("test_channel_id")
        .setContentTitle("新消息来了")
        .setContentText("明天是周末...

// 加上这个方法
private NotificationManager getManager(){
    if (manager == null){
        manager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
    }
    return manager;
}