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A new editor into netbeans, as a UML modeller would be a very nice feature. this could offer classes design capabilities from a diagram editor, and retro-engineering.
Changing to enhancement to follow the issuezilla type use.
There exist third-party modules for UML modeling: http://www.netbeans.org/catalogue/13-stable.html Check, if some of them satisfies your requirements.
No known plans to add a UML modeller to NetBeans.