code: public String getPrefix(final String namespaceURI) {
          if (namespaceURI == null) {
              // Be compliant with the JAXB contract for NamespaceResolver.
              throw new IllegalArgumentException("Cannot acquire prefix for null namespaceURI.");
          }
  
          return uri2Prefix.get(namespaceURI);
      }
docs: {@inheritDoc}