The Java Platform name has changed a few times over the years.
Java was first released in January 1996 and was named Java Development Kit, abbreviated JDK.
Version 1.2 was a large change in the platform and was therefore rebranded as Java 2. Full name: Java 2 Software Development Kit, abbreviated to Java 2 SDK or J2SDK.
Version 1.5 was released in 2004 as J2SDK 5.0 –dropping the “1.” from the official name and was further renamed in 2006. Sun simplified the platform name to better reflect the level of maturity, stability, scalability, and security built into the Java platform. Sun dropped the "2" from the name. The development kit reverted back to the name "JDK" from "Java 2 SDK". The runtime environment has reverted back to "JRE" from "J2RE."
JDK 6 and above no longer use the “dot number” at the end of the platform version.
For more information on platform names and version numbers, see the following pages: