Heroku Postgres is one of many services you can provision from addons.heroku.com. Just like other addons, you can add Heroku Postgress via the Heroku Dashboard or via the Heroku Toolbelt.
There is no need to add Heroku Postgres to a Java app, this is done automatically by the Java buildpack.
PG Backups is a service that captures and restores backups from your Heroku Postgres databases for an application. PG Backups imports data from an existing database backup, and exports backups for off-site storage
Just like other addons you can provision PG Backups via the dashboard or toolbelt.
Add the PG Backups service to your application
Hint See Importing & Exporting Heroku databases with PG Backups article for further details.