You can view the javadoc in Netbeans by clicking on the method and press Ctrl + Shift + Space
(Command + Shift + Space on Mac) You can also see the various shortkeys in the help menu>keyboard shortcuts card.
UPDATE:
You can instead open a “javadoc” window from the menu Window>IDE Tools>Javadoc Documentation. This will show the javadoc in a window near the output window for that method when you click on the method or object