No, they don't necessarily. A resource bundly does not have to be
file-based. The common implementation, however is. And that common
implementation (PropertyResourceBundle) indeed uses properties files and
thr Properties class.

However, resource bundles also come with a particular lookup-schema to
select a particular bundle according to the Locale. Something which you
don't get with Properties, but which is important when you do some
actual localization.

A resource bundle (properties file based or otherwise) when in need for
the lookup of locale-specific data. A naked properties file when in need
for keeping some small (FSWO) set of data persistent (e.g. application
configuration data), which is not supposed to be translated.

You didn't mention Preferences. I still preferen Properties over
Preferences because of the easy-to-work-with text file format, but they
can be used for the same purpose as Properties, without the need of
having to deal with files.

Both map keys to values. The biggest difference is how they get that
mapping. ResourceBundles follow a build-in lookup schema taking a locale
into account. Properties need to be provided with some InputStream,
which the programmer has to handle explicitely. Properties have an easy
way to be written to an OutputStream, while ResourceBundles are ment to
be read-only.

/Thomas