Java versioning and terminology, 1.6 vs 6.0 OpenJDK vs Sun

The versioning is simply a mess:

• Java 1.0 and 1.1 were normal
• Then came Java 1.2, but you were supposed to call it "Java 2, JDK 1.2"
• This continued until 1.4 (There were also minor releases like 1.4.2)
• The next version was then supposedly "Java 5.0", but there was still "1.5" all over the place in the file names and URLs.
• Starting with Java 6, they've dropped the minor version and mostly (but not completely, see output of java -version) eliminated the traces of the old versioning scheme, but people have gotten used to it and continue to use it colloquially.
• Starting with Java 9 or 10, the 1.X notation also disappeared from the output of java -version (which caused some code that depended on parsing it to break), and people have pretty much stopped using it. We now have Java 15, Java 16, Java 17, etc.

Note also that when this question was asked, Sun JDK and OpenJDK were separate codebases (whith a large overlap), and Sun JDK was the official reference implementation.

In the more than 10 years since then, Java was sold to Oracle, OpenJDK became the official Java reference implementation, and Oracle stopped maintaining the Oracle JDK as a separate codebase. Instead, they just provide OpenJDK builds and provide commercial long term support for them with bugfixes and security patches. But you can also get builds for free from AdoptOpenJDK (which recently rebranded as "Adoptium"), they just aren't supported as long.

This is explained (cough) in Java SE Naming and Versions:

The current release is Java Platform, Standard Edition 6 (Java SE 6). The previous release was Java 2 Platform, Standard Edition 5.0 (J2SE 5.0).

Sun Microsystems simplified the platform name in 2006 to better reflect the level of maturity, stability, scalability, and security built into the Java platform. Sun dropped the "2" from the name and deleted the "dot number" (the number following the period). Any updates to Java platforms will simply be noted as updates rather than adding a "dot number" at the end of the platform name.

Due to significant popularity within the Java developer community, the development kit has reverted back to the name "JDK" from "Java 2 SDK" (or "J2SDK"). The runtime environment has reverted back to "JRE" from "J2RE." (Note that "JDK" stands for "J2SE Development Kit" in version 5.0.)

For more information on platform names and version numbers, see the following pages:

• Java SE 6 Platform Name and Version Numbers
• J2SE Version 1.5.0 or 5.0?
• J2SE SDK/JRE Version String Naming Convention

And quoting Java SE 6 Platform Name and Version Numbers:

At this release, the platform name has changed from J2SE^TM to Java^TM SE. The official name is Java^TM Platform, Standard Edition 6.

Both version numbers (1.6.0 and 6) are used to identify this release of the Java Platform. Version 6 is the product version, while 1.6.0 is the developer version. The number 6 is used to reflect the evolving level of maturity, stability, scalability and security of Java SE.

And J2SE Version 1.5.0 or 5.0?:

Both version numbers "1.5.0" and "5.0" are used to identify this release of the Java 2 Platform Standard Edition. Version "5.0" is the product version, while "1.5.0" is the developer version. The number "5.0" is used to better reflect the level of maturity, stability, scalability and security of the J2SE.

The number "5.0" was arrived at by dropping the leading "1." from "1.5.0". Where you might have expected to see 1.5.0, it is now 5.0 (and where it was 1.5, it is now 5).

So, to summarize, the whole naming is a mess but this is finally over and the current platform name is Java^TM Platform, Standard Edition 6 (abbrev. Java^TM SE 6). Two Products are delivered under the platform: the JDK^TM 6 and the JRE^TM 6.

OpenJDK 6 is an open-source implementation of the Java SE 6 Platform, it's different from Sun's JDK.

• Java 1.5.x is the same as Java 5.x
• Java 1.6.x is the same as Java 6.x

Basically throw away the 1. part of the version number.

Both 1.6 and 6 are used to refer to the same version (and similarly for 1.5 and 5).

The only difference is the single digit numbers are usually used in conjunction with Java SE 5 and Java EE 6.

The 1.5 and 1.6 versions are what the Java compiler says using java -version and sometimes 'the minor release' is used as a version string, like SunOS 5.8 was called Solaris 8 as a product name.