Java Versions and Terminology Demystified

Though Sun Microsystems and the other Java Community Process participants have built a marvelous platform for software development, they have not done such a fabulous job managing their buzzwords. There are not only many overlapping terms to understand -- Java, J2SE, J2EE, SDK, JDK, JRE, etc. -- there are multiple versions and, in fact, multiple version numbering systems in play.

This document aims first to clarify the terms and the version numbers, and then offers some tips on which Java version(s) are most likely to be appropriate in training situations.

• The frequently asked question
• What do the terms mean?
• Which version is which?
• Which version do I want?

The frequently asked question

The most common version question that occurs when considering Java training is simply, "Do I want Java 1.4 or Java 5?" The shortest answer we can manage to that question is here, though you may want to dig into other sections for details.

Java 1.4 is the dominant form of Java today. Java 5.0 introduces a number of powerful new features to the Java language and runtime, especially improving ease of use and type safety. Java 5.0 is clearly a superior tool, but because classes compiled with Java 5.0 won't work in older runtimes, there is a significant barrier to migration, and many companies will hold off on moving to Java 5.0 for months or even years.

This means there is no simple, single answer to the question! The very best thing to do is to ask the client or the trainees themselves what version of J2SE they are or will soon be using, and train for that version. For a brief decision framework, see which version do I want?.

What do the terms mean?

First a quick explanation of terminology, in glossary form:

• Java -- 1. The programming language itself. 2. The complete platform of tools making it possible to use and to develop software using the Java language: this includes the JRE for users and the SDK or JDK for developers.
• JRE -- The Java Runtime Environment: this is the package of software that must be installed on a machine in order to run Java applications. It does not include development tools.
• SDK and JDK --The Software Developers Kit and Java Developers Kit: these include a JRE and also development tools such as compilers and debuggers. Sun deftly moved from the term JDK to SDK a few years ago, and now with the latest release of Java they are just as deftly returning to JDK.
• J2SE --The Java 2 Platform, Standard Edition: this is one of three editions of the Java platform; it is indeed the "standard" one, and it is the one most people mean when they talk about plain old Java software.
• J2EE --The Java 2 Platform, Enterprise Edition: another of the Java platforms, J2EE includes the J2SE and is dedicated to enterprise software development. J2EE systems typically include Web servers, application servers, messaging software, and increasingly Web-services engines. J2EE is an umbrella specification that includes many popular technologies: Servlets, JSP, EJB, JMS, and the Java Web-services APIs (SAAJ and JAX-RPC) are all part of J2EE.

Which version is which?

Within the J2SE -- again, this is the platform most people are referring to when they talk about Java software, unless they specifically mention J2EE -- there are a few things to know about version numbers. First, the Java language and the J2SE platform share a version number, so to say "Java 1.3" is to say "J2SE 1.3." Technically, one is the language and one is the platform, but they move in synchronization.

Of course the "2" in "J2SE" looks like a version number of sorts, but it mostly confuses the issue of what "version of Java" one is working with or wants. The primary version numbering has historically run from Java 1.0 (released in 1996) up through 1.1, 1.2, and 1.3, to the currently dominant Java 1.4; there have been minor upgrades to each platform along the way and so, for instance, many companies are currently using Java 1.4.2. As of Java 1.2, Sun defined the three-platform architecture and developed the "Java 2" platform as a brand of sorts. Thus -- sadly -- "Java 2" overlaps "Java 1.2," All versions since Java 1.2 have been versions of the "Java 2 Platform."

Most recently, Java 1.5 was released -- "Tiger" by its code name. At this point the version-numbering scheme was also revised, with the aim of making the first digit in the number more meaningful: we'll start talking about version X, not version 1.X. So Java 1.5 is also Java 5.0! Fantastic. Either number is valid, but Sun generally favors 5.0 for the current version (and so does Object Innovations). Future revisions will just be labeled 6.0, 7.0, etc., with minor upgrades appearing as 5.1, 6.2, and so on.

So these three overlapping schemes lay out like this:

Which version do I want?

J2SE releases are backward-compatible, for the most part. However there is less forward compatibility. By "backward-compatible," we mean that old code will run on a new version of the platform; and by "forward-compatible," that code written on a new version of the platform will run on an old version of the platform. Especially with the new release of Java 5.0, there is no such forward-compatibility: this version uses a new format for its binary files (class files), which cannot be understood by an old (1.4 or earlier) runtime.

Object Innovations offers multiple versions of many of its Java courses, but which version should you choose for training? It can be critical to get this right, as the language changed significantly between 1.4 and 5.0. If you are not sure, the first, best option is to contact someone in your company or at your client's site who knows. Typically there is either a Java infrastructure in place, or one is forming, or there is a specific product with specific requirements including a Java version.

If you have a mixed classroom (perhaps a public offering) or simply don't know what the participants will want, here are a few guidelines and tips:

• 1.4 is right now the dominant version.
• Early adopters are jumping on 5.0 with some gusto, because it introduces a number of new features that strengthen the language and make it easier to write code.
• Remember that we're talking about Java or J2SE versions! Many companies have investments in J2EE, and right now there are no J2EE products that fully support the 5.0 J2SE runtime. This is probably the major brake on adoption of J2SE 5.0: no one wants to be stuck writing one version of the language for some applications and an older version for J2EE components. J2EE 5.0 is due out later this year, and will synch up with J2SE 5.0 for a seamless architecture.

So there's a tradeoff in training situations. 1.4 coding techniques are valid for both 1.4 and 5.0 environments, and 1.4 is the most popular version of the platform today. 5.0 introduces some exciting new features that may give trainees a more valuable learning experience; but they may also learn skills that they can't apply if they are or will be working in a 1.4 context.