我正在嘗試發送請求以取回對象數組-Coupon當我提交請求時，我得到了答案-直接自引用導致循環（通過引用鏈： java.util.HashSet[0]=> model.Coupon["emptyCoupon"]=> model.Coupon["emptyCoupon"]）model.Coupon可能會解決問題。如果請求的優惠券不存在，則打算退回空優惠券。public static final int NO_ID = -1;private static final Coupon EMPTY_COUPON = new Coupon(NO_ID, null, null, null, NO_ID, NO_ID, null, NO_ID, null);private long id = NO_ID;private String title;private LocalDate startDate;private LocalDate endDate;private int amount;private int category;private String message;private double price;private String image;public Coupon() {}private Coupon(long id, String title, LocalDate start_date, LocalDate end_date, int amount, int category,        String message, double price, String image) {    this.id = id;    this.title = title;    this.startDate = start_date;    this.endDate = end_date;    this.amount = amount;    this.category = category;    this.message = message;    this.price = price;    this.image = image;}public Coupon getEmptyCoupon() {    return EMPTY_COUPON;}在我添加之前，我EMPTY_COUPON對請求沒有任何問題。我想要代碼中的emptyCoupon，我很樂意提供幫助