public Builder addJoints(
          int index, org.box2d.proto.Box2D.PbJoint.Builder builderForValue) {
        if (jointsBuilder_ == null) {
          ensureJointsIsMutable();
          joints_.add(index, builderForValue.build());
          onChanged();
        } else {
          jointsBuilder_.addMessage(index, builderForValue.build());
        }
        return this;
      }