Java versions are even more confusing than you think they are, so I wanted to clear up at least a little of it here.
Java 1.4.2 is also called Java 2 Platform Version 1.4.
Java 1.5 is also called Java 5
Java 1.6 is also called Java 6
Java 1.5 (aka Java 5) introduced a LARGE number of features that aren't available in version 1.4.2. And from what I've found, it's pretty difficult to have both Java 1.5 (or 1.6) and 1.4.2 installed on the same system. If you NEED to run some apps that require Java 1.4.2 (like the TEP desktop client, for example) and some apps that require Java 1.5, I would truly create a separate Windows virtual machine (using the virtual machine software of your choosing, though I would recommend VMWare) to run version 1.4.2.
After 1.4.2, however, it really appears (so far) that you can install the different versions on the same machine AND have them play well together. My next post will have some information about this.
If some piece of software states that it requires Java 1.5, that *should* mean that the JDK or JRE with a version number of 1.5.0_xx, where xx is the Update Number, will work; it's not guaranteed, but in my experience, it does NORMALLY work. Similar is true for Java 1.6 - version 1.6.0_xx *should* work.
What's the difference between the JDK and JRE
The JRE (Java Runtime Engine) is what most people need. It contains the java executable and all of the other executables that are needed to run Java clients, but it does NOT contain any of the executables needed to WRITE and package Java applications (i.e. javac and jar, among others).
The JDK (Java Developer Kit) contains all of the JRE, plus all of the developer tools needed to write and package Java applications.
What about SE and EE variants?
SE (Standard Edition) and EE (Enterprise Edition) are used to distinguish between types of developer environments.
SE is for creating client applications that will run inside a standalone JVM (Java Virtual Machine) on a user's local machine. Applets and Java Web Start applications, though they are accessed over the web, are examples of applications created using the SE developer kit.
EE contains all of the SE, PLUS it allows a developer to create applications that will run inside an application server, such as WebSphere, WebLogic, JBoss, Oracle App Server, and many others. These applications actually run on a JVM on a server. A user connects to that server to access the application, but the Java application itself is using CPU cycles on the server.
What's Java Web Start
Java Web Start (JavaWS) is a technology that allows application developers to create a web-launchable Java application directly from the browser. A Java Web Start application is defined in a .JNLP file (which is just a text file, so you can open one up to look at it), and is launched using the javaws executable. This is different than an applet in a few ways:
1. An applet generally runs inside the browser itself, or it can open a new window in which it runs *seemingly* outside of the browser. I say "seemingly" because if you close the browser, the applet will still die. Any windows opened by the applet are child windows of the browser and will automatically be closed when the browser is closed. A JavaWS application's control file (the JNLP file) is downloaded by the browser, but is then launched by the javaws executable. So if you close the browser, the application won't close.
2. A JavaWS application is a completely standalone application, which has full control over menus, windowing elements, look-and-feel, etc. of itself. An applet is constrained by the browser in several ways. (This difference is mainly interesting to developers, but I think it's useful for users to be aware of).
3. A JavaWS application can be launched outside of a browser; you can just double-click the javaws executable (or a .JNLP file, for that matter) to launch a JavaWS application. An applet MUST be launched in a browser.
Where to get Java
ANY VERSION older than the current version: http://java.sun.com/products/archive/index.html
Current Update of 1.5: http://java.sun.com/javase/downloads/index_jdk5.jsp
Current Update of 1.6: http://java.sun.com/javase/downloads/index.jsp
Links to different platforms and versions: http://www.ibm.com/developerworks/java/jdk/