APIs relevant to your installation are automatically installed or updated during corresponding product installs and patches. Reference documentation for APIs is in the Javadoc files on the product installation CD. Javadoc files are also viewable locally after installation in <install>/xapidocs, and they are updated during product installs and patches.
If you want to view the Javadoc files online in a development environment, you can build the doc EAR file with your application.