There are two different Java distributions for end users: one from Apple, and one from Oracle. They don't overlap in function.
The Apple Java runtime (version 6) is needed to run bare jar files and older standalone Java applications. Newer Java applications have the Java runtime built in, and don't depend on Apple's Java.
The Oracle Java runtime (version 8 or later) is a web plugin only. It's used to run Java applets on web pages and Java WebStart applications. You can determine whether it's installed and up to date by looking for a preference pane named Java in System Preferences. If it's present, open it. It will launch the "Java Control Panel." Select the Update tab.