Bug 4240 Can't edit class path entry (1GKEPIF)
There is no way to edit an existing entry for a library on the Java Build Path project properties page (other than editing the .classpath file directly). NOTES: EG (9/22/2001 11:42:11 AM) this is particularly annoying for variables. we should have an Edit button when a variable is selected.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/wizards/buildpaths/SourceContainerWorkbookPage.java
private CPListElement newCPSourceElement(IResource res) {
		Assert.isNotNull(res);
		return new CPListElement(fCurrJProject, IClasspathEntry.CPE_SOURCE, res.getFullPath(), res);
	}
	
	/*
	 * @see BuildPathBasePage#getSelection
	 */
	public List getSelection() {
		if (fIsProjSelected) {
			ArrayList list= new ArrayList(1);
			list.add(fProjectCPEntry);
			return list;
		} else {
			return fFoldersList.getSelectedElements();
		}
	}
	/*
	 * @see BuildPathBasePage#setSelection
	 */	
	public void setSelection(List selElements) {
		if (!fIsProjSelected) {
			filterSelection(selElements, IClasspathEntry.CPE_SOURCE);
			fFoldersList.selectElements(new StructuredSelection(selElements));
		}
	}	
	
		
}