private com.google.protobuf.SingleFieldBuilder<
          org.box2d.proto.Box2D.PbVec2, org.box2d.proto.Box2D.PbVec2.Builder, org.box2d.proto.Box2D.PbVec2OrBuilder> 
          getCenterFieldBuilder() {
        if (centerBuilder_ == null) {
          centerBuilder_ = new com.google.protobuf.SingleFieldBuilder<
              org.box2d.proto.Box2D.PbVec2, org.box2d.proto.Box2D.PbVec2.Builder, org.box2d.proto.Box2D.PbVec2OrBuilder>(
                  center_,
                  getParentForChildren(),
                  isClean());
          center_ = null;
        }
        return centerBuilder_;
      }