When I am coding at Netbeans it autocompletes the code and show Javadocs. However when I am using Intellij IDEA if I click Ctrl+Space
File -> Settings -> Editor -> Code completion.
Check Autopopup documentation in (ms) and choose delay.
Settings | Editor | General |Code Completion | Autopopup documentation in (ms).
UPDATE: latest versions have an option to show JavaDoc on mouse move.
Show Javadoc in IntelliJ IDEA 14:
When you write code:
File -> Settings -> Editor -> General -> Code Completion -> Autopopup documentation in (ms)
It's only available when you press CTRL+SPACE It's more usable if you select the pin icon when it popups. The documentation window will be added to the tabs on the right. Later on I recommend to resize the window and to uncheck the Pinned property.
On mouse hover:
File -> Settings -> Editor -> General -> Show quick documentation on mouse move