Bug 19886 setProjectJavadocLocation() only in internal package [api] [javadoc]
If You want to set the project javadoc location by API so You have to use an internal class, there is no public interface. see class org.eclipse.jdt.internal.corext.javadoc.JavaDocLocations (Otherwise the method setLibraryJavadocLocation() is public at org.eclipse.jdt.ui.JavaUI - thats ok.)
org.eclipse.jdt.ui/ui/org/eclipse/jdt/ui/JavaUI.java
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 * Copyright (c) 2000, 2002 International Business Machines Corp. and others.
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 * Contributors:
 *     IBM Corporation - initial API and implementation
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package org.eclipse.jdt.ui;