private void initFields() {
      tag_ = 0L;
      restitution_ = 0F;
      friction_ = 0F;
      density_ = 0F;
      sensor_ = false;
      filter_ = org.box2d.proto.Box2D.PbFilter.getDefaultInstance();
      shape_ = org.box2d.proto.Box2D.PbShape.getDefaultInstance();
    }