Ubuntu – How to “install” multiple Java versions

Yes. you can. Check this article. http://lj4newbies.blogspot.com/2007/04/2-jvm-on-one-linux-box.html

Hope this helps.

Are they all different versions of JDK? If not, then what?

They are different implementations of Java/development kits for Java (JDK). Java is a standard platform, and all of these implement that platform in their own way. Of these:

• OpenJDK and GNU Compiler are the only ones that are open source. I usually prefer them -- the ease of installation (apt-get or Software Center) is a big factor.
  • OpenJDK in particular is derived mostly from the "official" Java source code that Oracle/Sun open sourced; it is backed by Oracle.
• Oracle Java 6 and 7 are "the" Java, if you will, from the company that invented Java (that was Sun, but Oracle bought them a while back). That's why they are called reference implementations. You may want to use them if you are using Oracle JDK on another platform too, or if OpenJDK is giving you problems.
  • The 6 and 7 refer to versions of the language/platform. 7 is newer and has more features, but has been tested less; 6 is extremely mature and stable. I'd prefer 6 if you are just starting out.
• Contrary to the wiki, IBM makes JDKs/JREs for all platforms, although theirs might be the only one for the PowerPC.

If you are starting out in programming, almost any of these will be a good fit. Go for OpenJDK just because its easy to install.

The only experience I have in differentiating these is compiling in a low-resource environment (128-192MB RAM - a VPS); I found Oracle Java 6 JDK worked best for one program (i.e succeeded) and IBM Java 6 JDK for another...