private com.google.protobuf.SingleFieldBuilder<
          org.box2d.proto.Box2D.PbFilter, org.box2d.proto.Box2D.PbFilter.Builder, org.box2d.proto.Box2D.PbFilterOrBuilder> 
          getFilterFieldBuilder() {
        if (filterBuilder_ == null) {
          filterBuilder_ = new com.google.protobuf.SingleFieldBuilder<
              org.box2d.proto.Box2D.PbFilter, org.box2d.proto.Box2D.PbFilter.Builder, org.box2d.proto.Box2D.PbFilterOrBuilder>(
                  filter_,
                  getParentForChildren(),
                  isClean());
          filter_ = null;
        }
        return filterBuilder_;
      }