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