Apple has a pretty lousy history with Java support, but this still surprised me. If you are having trouble getting programs to compile with older Java versions (e.g. for backward compatibility), you might want to take a look in this directory: /System/Library/Frameworks/JavaVM.framework/Versions
Here's what I found on my OS 10.6 system:
lrwxrwxr-x 1 root wheel 5 Jun 29 13:37 1.3 -> 1.3.1 drwxr-xr-x 3 root wheel 102 Feb 11 2010 1.3.1 lrwxrwxr-x 1 root wheel 10 Jun 29 13:37 1.4 -> CurrentJDK lrwxrwxr-x 1 root wheel 10 Jun 29 13:37 1.4.2 -> CurrentJDK lrwxrwxr-x 1 root wheel 10 Jun 29 13:37 1.5 -> CurrentJDK lrwxrwxr-x 1 root wheel 10 Jun 29 13:37 1.5.0 -> CurrentJDK lrwxrwxr-x 1 root wheel 10 Jun 29 13:37 1.6 -> CurrentJDK lrwxrwxr-x 1 root wheel 10 Jun 29 13:37 1.6.0 -> CurrentJDK lrwxrwxr-x 1 root wheel 59 Jun 29 13:37 CurrentJDK -> /System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents
I kept trying to get Eclipse to compile with 1.5 and kept getting incompatible binaries. Thanks Apple! YMMV, but I found this post a good guide to getting a real Java 1.5 installation.
