code: static String toStringImpl(Map<?, ?> map) {
      StringBuilder sb = Collections2.newStringBuilderForCollection(map.size()).append('{');
      boolean first = true;
      for (Entry<?, ?> entry : map.entrySet()) {
        if (!first) {
          sb.append(", ");
        }
        first = false;
        sb.append(entry.getKey()).append('=').append(entry.getValue());
      }
      return sb.append('}').toString();
    }
docs: An implementation of {@link Map#toString}.