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In JBuilder it searches your system and finds all installed SDKs. You can subsequently search again or add a new one to the list. There's an easy to option where you pick one of these from the list and you build, compile, debug etc. all using that SDK. This makes, for example, switching between 1.3 and 1.4 a snap. In Netbeans 3.4.1 (which is a huge improvement over previous versions), it's still a nightmare to accomplish this. You can compile in 1.4 but then the debugger craps out if you haven't configured that. But getting the debugger to work requires you to specify some jars and directories in the Boot Class Path. You then have to do this all over again if you want to go back to the other SDK.
I believe this is already planned for NetBeans 4.0's new Projects infrastructure.
Yes, this is the new 'platforms support'.
this is discussed in java spec: http://projects.netbeans.org/uispecs/java/JavaProjectsUISpec.html
Reorganization of java component