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Ask and answer user questions on the users@netbeansthe users@netbeans.incubator.apache.org mailing org mailing list (subscribe via users-subscribe@netbeans.incubator.apache.org). Ask and answer questions on StackOverflow.
Join the development mailing list at dev@netbeans at dev@netbeans.incubator.apache.org org (subscribe via dev-subscribe@netbeans.incubator.apache.org).
Join NetBeans on Slack. Go to https://netbeans.signup.team, enter your e-mail address, and join in discussions going on there about/around NetBeans. Note that these are informal discussions and that important discussions and decisions around Apache NetBeans must happen on the dev@netbeans.incubator.apache.org mailing list. Slack and other non-official channels are fine for coffee machine style discussions, but nothing more.
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