I need to do this too, & the docs aren't helping much... this FAQ
http://developers.sun.com/prodtech/javatools/jscreator/reference/faqs/configura tion/java5limits.html
seems to indicate that apps will be compiled & run w/a target of 1.4 by default, but this must be old as the Project Properties > Libraries > Java Platform popup only shows 1.5 (Default).
It must have something to do with the "Manage Platforms..." button, but i can't get the "next" button to light up in the "Choose Platform Folder" dialog, nor do i know what a "Platform Folder" is - and the docs & website are no help at all...
Anyone?
OK, my problem was i just had the JRE installed, not the full JDK - damn client laptop... ;-)
Anyway, this appears to work for me:
1) Right-click your project & choose "Properties"
2) Select the "Libraries" category
3) At the top of the dialog box, click the "Manage Platforms..." button
4) Click the "Add Platform" button at the lower left
5) Point at a full SDK (*not* just a JRE) folder & click "Next"
6) Verify that it found the "Platform Sources" (if not, browse for them) and click "Finish"
7) You can then select your new platform from the "Java Platform" popup at the top of the Libraries category of Project Properties
Hope this is what you were looking for, best of luck,
alex.