public Builder setPosition(org.box2d.proto.Box2D.PbVec2 value) {
        if (positionBuilder_ == null) {
          if (value == null) {
            throw new NullPointerException();
          }
          position_ = value;
          onChanged();
        } else {
          positionBuilder_.setMessage(value);
        }
        bitField0_ |= 0x00000004;
        return this;
      }