Download JDK documentation from http://java.sun.com/j2se/1.5.0/download.jsp
Unzip the zip file to the JDK directory
In NetBeans IDE
1. Choose Tools > Java Platform Manager from the main window.
2. Select the platform to which you want to add Javadoc in the left panel of the dialog box.
3. In the Javadoc tab, click Add ZIP/Folder and specify the location of the Javadoc files.
4. Click Close.