title: Retrieving data from firebase database in android
body: <p>I'm recently migrated to Firebase. I have gone through Firebase Android api docs for retrieving data from database through JSON. there is method <code>onDataChange(DataSnapshot snapshot)</code> to retrieve data whenever there is change in data in database but I could find how to retrieve data even if there is no change in database.</p>
  
  <p>For example:- when a user logs into my app, I want that data which is stored under the unique id node should be retrieved. How can I retrieve data anytime from Firebase database if i want?</p>