Java documentation is not included neither in NetBeans installation, nor in “standard” Java SDK installation.

However, it’s possible to download it manually and add to the NetBeans IDE.

1. Get the Java Documentation (currently “Java SE 6 Documentation”) from Sun site.
2. Start NetBeans
3. Select menu item “Tools->Java platforms”, then click “Javadoc” tab
4. Click “Add Zip/Folder” button and select the path to the downloaded Zip-file