Java technology was introduced 9 years ago and it's been over 5 years since the introduction of J2SE, the 2nd generation Java platform.
Given its rich history, changing the version of the next release from 1.5 to 5.0 better reflects the level of maturity, stability, scalability and security built into J2SE. Here are the official naming and versioning conventions for the upcoming release (also known as "Tiger"):
Java(TM) 2 Platform Standard Edition 5.0 or J2SE(TM) 5.0
Products delivered under the platform*:
Due to significant popularity within the Java developer community, the development kit has reverted back to the naming convention "JDK" from "SDK."
*Note: Full names for JDK(TM) and the JRE:
J2SE Development Kit 5.0
J2SE Runtime Environment 5.0
For more detailed information on version string convention and more: link