Bug 28510 java build path inconsistency [build path]
20021216 select rt.jar in the package explorer open properties it allows you to set the javadoc location but for source attachment you are sent somewhere else it's inconsistent - on the java build path they both look sort of the same (they sit under the same node in the tree)
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/wizards/buildpaths/CPListElement.java
public static CPListElement createFromExisting(IClasspathEntry curr, IJavaProject project) {
		IPath path= curr.getPath();
		IWorkspaceRoot root= project.getProject().getWorkspace().getRoot();
		
		IResource res= null;
		boolean isMissing= false;
		URL javaDocLocation= null;
		switch (curr.getEntryKind()) {
			case IClasspathEntry.CPE_CONTAINER:
				res= null;
				try {
					isMissing= (JavaCore.getClasspathContainer(path, project) == null);
				} catch (JavaModelException e) {
					isMissing= true;
				}
				break;
			case IClasspathEntry.CPE_VARIABLE:
				IPath resolvedPath= JavaCore.getResolvedVariablePath(path);
				res= null;
				isMissing=  root.findMember(resolvedPath) == null && !resolvedPath.toFile().isFile();
				javaDocLocation= JavaUI.getLibraryJavadocLocation(resolvedPath);
				break;
			case IClasspathEntry.CPE_LIBRARY:
				res= root.findMember(path);
				if (res == null) {
					if (!ArchiveFileFilter.isArchivePath(path)) {
						if (root.getWorkspace().validatePath(path.toString(), IResource.FOLDER).isOK()) {
							res= root.getFolder(path);
						}
					}