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0. Use the Export Wizard to generate Javadoc for your classes. Check "Save the settings as an Ant script" on the last page.
1. Open javadoc.xml.
2. For each external API, add the following line as child of <javadoc> element:
<link href="url to the API directory on the web" />
Some of the most common URLs are:
JDK => http://java.sun.com/j2se/1.5.0/docs/api/
Eclipse => http://help.eclipse.org/help31/nftopic/org.eclipse.platform.doc.isv/reference/api/
3. Right click javadoc.xml => "Run As" => "Ant Build".
4. Don't ever use the Export Wizard to generate Javadoc again.
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