code: public void setReadPermissions(String... permissions) {
          properties.setReadPermissions(Arrays.asList(permissions), sessionTracker.getSession());
      }
docs: Set the permissions to use when the session is opened. The permissions here
  can only be read permissions. If any publish permissions are included, the login
  attempt by the user will fail. The LoginButton can only be associated with either
  read permissions or publish permissions, but not both. Calling both
  setReadPermissions and setPublishPermissions on the same instance of LoginButton
  will result in an exception being thrown unless clearPermissions is called in between.
  <p/>
  This method is only meaningful if called before the session is open. If this is called
  after the session is opened, and the list of permissions passed in is not a subset
  of the permissions granted during the authorization, it will log an error.
  <p/>
  Since the session can be automatically opened when the LoginButton is constructed,
  it's important to always pass in a consistent set of permissions to this method, or
  manage the setting of permissions outside of the LoginButton class altogether
  (by managing the session explicitly).
  
  @param permissions the read permissions to use
  
  @throws UnsupportedOperationException if setPublishPermissions has been called