They started with Java 1.0 and 1.1 - then to "Java 2" at version 1.2, so you have Java 2 version 1.2 ... they then stuck with that through the version after 1.4 - but rather than 1.5, that was special as it was Java 2 version 5.0 ... thereafter they're just numbered with the plain number.
Oh, that's the JRE software version - that's completely different from the language version.
For the JRE, I want to say that they stuck with the 1.version.minor numbering (although the last version with a minor other zero was I think the Very Popular And Probably Still In Prod 1.4.2), with updates suffixed with "u" + number through the Java 8 series, but then with 9 they switched to version.minor.update instead.
It's completely incomprehensible; not even Oracle can keep it straight.
They started with Java 1.0 and 1.1 - then to "Java 2" at version 1.2, so you have Java 2 version 1.2 ... they then stuck with that through the version after 1.4 - but rather than 1.5, that was special as it was Java 2 version 5.0 ... thereafter they're just numbered with the plain number.