code: @Override
      public int compareTo(Identifier o) {
          if (o == null) {
              throw new IllegalArgumentException("Unable to compare a null identifier");
          }
          return new CompareToBuilder()
                  .append(this.value, o.toString())
                  .append(this.url, o.getUrl())
                  .append(this.confidence, o.getConfidence())
                  .toComparison();
      }
docs: Implementation of the comparator interface.
  
  @param o the object being compared
  @return an integer indicating the ordering