title: How to send message to the topic in Android
body: <p>How to send a topic message via Firebase ?</p>
  
  <p>I only found the way to send by using Firebase Console ,
  as well as some ways about Post HTTP requests , 
  but I don't understand how to do it in Android.</p>
  
  <p><strong>How can I write some codes in Android
  to specify a topic which to target then send a message to the topic?</strong> </p>
  
  <p>Thanks.</p>