You are wondering why Netbeans give you code completion but no javadoc, rather saying “javadoc not found” ? It is not part of the Netbeans Installer or the regular JDK package. You need to download it separately (link), copy the zip file to a folder of your choice and configure the javadoc in Java platforms.
Netbeans Java Platform
Netbeans Java Platform
The JavaDude Weblog 
Replicating your every move and with the same file, the javadoc still does not appear in the editor.
Update. Okay yes it works in Java mode. Still can’t make it work in exs. Scala mode (for neither scala or java). But but… thats another issue.
It works when i use in J2SE environment, but it doesn’t work when i use in J2ME environmet… the error message still ‘Javadoc not found’, do you know why ?
And how to make it work ? Do i have to use *.jar file or something else ?
Please provide suggestion this issue.I am trying a Spring related Javadocs,I am failing..Please help…
Excellent info. Most recent javadocs are available at https://cds.sun.com/is-bin/INTERSHOP.enfinity/WFS/CDS-CDS_Developer-Site/en_US/-/USD/ViewFilteredProducts-SingleVariationTypeFilter
For those you can’t make it work even with this tutorial, remove all docs which are already present in Java platforms and add the “jdk-xxx-docs.zip. It worked for me.