title: How should be the content in a file which can be easily used to serialize further using protocol buffers
body: <p>I'm new to protocol buffers and I'm trying to understand and learn about it. So that, I can implement it in my project.</p>
  <p>As a first step, I followed the <a href="https://developers.google.com/protocol-buffers/docs/javatutorial" rel="nofollow noreferrer">protocol buffer java tutorial</a>. In this tutorial, under a <strong>Writing a Message</strong> section I can see they are trying to read addressbook from the file.</p>
  <p><a href="https://i.stack.imgur.com/fONVs.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/fONVs.png" alt="code which is reading the file" /></a></p>
  <p>Now, Is the file which is trying to read as addressbook is the one which is already written using <strong>writeTo()</strong>?? if not written with the help of writeTo(),how the data in that file content would be like?? (i.e., Can we have our own file in certain format which can be serialized into other file? if yes, what should be the format??).</p>