private void initFields() {
      tag_ = 0L;
      gravity_ = org.box2d.proto.Box2D.PbVec2.getDefaultInstance();
      allowSleep_ = false;
      autoClearForces_ = false;
      warmStarting_ = false;
      continuousPhysics_ = false;
      subStepping_ = false;
      bodies_ = java.util.Collections.emptyList();
      joints_ = java.util.Collections.emptyList();
    }