从 HashSet 集合中打印对象 [重复]

Question

我试图打印 hashSet 集合中的对象。控制台仅显示最后一个对象（一个对象）。当我用同样的方法使用 ArrayList 时，我可以打印所有的对象。我使用了迭代器方法来打印集合集，请参阅附件测试。

public Set<Coupon> getAllCoupouns() throws Exception {

    Coupon coupon = new Coupon();
    Set<Coupon> coupons = new HashSet<Coupon>();

    // Open a connection
    conn = DriverManager.getConnection(Utils.getDBUrl());
    // Define the Execute query
    java.sql.Statement stmt = null;

    try {
        stmt = conn.createStatement();
        // build The SQL query
        String sql = "SELECT * FROM COUPON";
        // Set the results from the database
        ResultSet resultSet = stmt.executeQuery(sql);
        // constructor the object, retrieve the attributes from the results
        while (resultSet.next()) {

            coupon.setId(resultSet.getLong(1));
            coupon.setTitle(resultSet.getString(2));
            coupon.setStartDate((Date) resultSet.getDate(3));
            coupon.setEndDate((Date) resultSet.getDate(4));
            coupon.setAmount(resultSet.getInt(5));
            CouponType type = CouponType.valueOf(resultSet.getString(6)); // Convert String to Enum
            coupon.setType(type);
            coupon.setMessage(resultSet.getString(7));
            coupon.setPrice(resultSet.getDouble(8));
            coupon.setImage(resultSet.getString(9));

            coupons.add(coupon);

        }

    } catch (SQLException e) {
        throw new Exception("Retriev all the coupons failed");
    } finally {
        // finally block used to close resources
        try {
            if (stmt != null)
                conn.close();
        } catch (SQLException se) {
            // do nothing
        }
        try {
            if (conn != null)
                conn.close();
        } catch (SQLException se) {
            se.printStackTrace();
        }

    }
    return coupons;
}

Answer 1

当您从 while 循环外部初始化 Coupon 时，它每次都会不断添加相同的对象，因此，覆盖结果只会显示最后一个结果。

您需要做的是从while 循环中实例化Coupon，例如：

public Set<Coupon> getAllCoupouns() throws Exception {
    Set<Coupon> coupons = new HashSet<Coupon>();

    // Open a connection
    conn = DriverManager.getConnection(Utils.getDBUrl());
    // Define the Execute query
    java.sql.Statement stmt = null;

    try {
        stmt = conn.createStatement();
        // build The SQL query
        String sql = "SELECT * FROM COUPON";
        // Set the results from the database
        ResultSet resultSet = stmt.executeQuery(sql);
        // constructor the object, retrieve the attributes from the results
        while (resultSet.next()) {
            Coupon coupon = new Coupon();
            coupon.setId(resultSet.getLong(1));
            coupon.setTitle(resultSet.getString(2));
            coupon.setStartDate((Date) resultSet.getDate(3));
            coupon.setEndDate((Date) resultSet.getDate(4));
            coupon.setAmount(resultSet.getInt(5));
            CouponType type = CouponType.valueOf(resultSet.getString(6)); // Convert String to Enum
            coupon.setType(type);
            coupon.setMessage(resultSet.getString(7));
            coupon.setPrice(resultSet.getDouble(8));
            coupon.setImage(resultSet.getString(9));

            coupons.add(coupon);

        }

    } catch (SQLException e) {
        throw new Exception("Retriev all the coupons failed");
    } finally {
        // finally block used to close resources
        try {
            if (stmt != null)
                conn.close();
        } catch (SQLException se) {
            // do nothing
        }
        try {
            if (conn != null)
                conn.close();
        } catch (SQLException se) {
            se.printStackTrace();
        }

    }
    return coupons;
}

Answer 2

cupon 始终是同一个对象。您只制作了一个 Cupon 类的对象，因此该集合仅包含一个对象（您始终添加相同的对象）。您必须在 while 循环的每次迭代中创建一个新对象。