在编写 hashcode 和 equals 方法方面需要帮助吗？

发布于 2024-12-03 06:00:03 字数 244 浏览 2 评论 0原文

我有一个提供有关付款详细信息的类。属性在

    accountNo, transactionAmount, dateOfTransaction.

这里我想编写哈希函数，以便当我将此类对象存储在哈希集中时它会高效。

主要限制是付款详细信息应该是唯一的（假设某个人不应在一个月内支付两次费用）。

任何人都可以帮助我为这种情况编写 hashCode 以及 equals 方法吗？

原文

I have a class that gives details about payment.The attributes are

    accountNo, transactionAmount, dateOfTransaction.

Here I want to write hash function such that it will efficient when I store this class objects in a hashSet.

The main constraint is payment details should be unique(suppose a particular person should not pay fee two times in a month).

Can any one help me in hashCode to be written for this scenario and also equals method?

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梦年海沫深 2024-12-10 06:00:03

您需要准确地确定平等的含义。特别是，您谈到一个月内不支付两次 - 这是否意味着一笔交易应该与另一笔交易在同一个月内相等，即使是在不同的一天？这听起来很奇怪——而且是特定于用法而不是特定于类型的——平等的想法。另请注意，交易只有一个帐号 - 当然它应该有一个“发件人”帐户和一个“收件人”帐户，因为可能有多个人向同一帐户付款，并且可能有在同一个月内从一个帐户向多个帐户付款。

所以，就我个人而言，我不想以这种方式覆盖平等，但如果你真的必须这样做，这并不太难......一旦你决定了什么构成平等，我将实现equals - 那时 hashCode 通常相当简单。

我强烈建议您阅读 Josh Bloch 在中关于平等的部分《Effective Java》（第二版）了解更多详细信息，但 equals 通常看起来像这样：

@Override public boolean equals(Object other)
{
    if (other == null || other.getClass() != this.getClass())
    {
        return false;
    }
    BankTransaction otherTransaction = (BankTransaction) other;

    return accountNo == otherTransaction.accountNo 
        && transactionAmount == otherTransaction.transactionAmount
        && // etc;
}

请注意，对于任何属于引用类型，您需要确定要在那里应用哪种相等性 - 通常您需要调用 equals 而不是仅使用 ==。

You need to decide exactly what you mean by equality. In particular, you talk about not paying twice in a month - does that mean one transaction should be equal to another if it's in the same month even if it's on a different day? That sounds like quite an odd - and very usage-specific rather than type-specific - idea of equality. Also note that the transaction only has one account number - surely it should have both a "from" and a "to" account, as there could be payments from multiple people to the same account, and there could be payments from one account to multiple accounts in the same month.

So, personally I wouldn't want to override equality in this way, but if you really do have to, it's not too hard... Once you've decided on what consitutes equality, I would implement equals - at that point hashCode is usually fairly easy.

I would strongly recommend that you read Josh Bloch's section on equality in Effective Java (second edition) for more details, but equals would typically look something like this:

@Override public boolean equals(Object other)
{
    if (other == null || other.getClass() != this.getClass())
    {
        return false;
    }
    BankTransaction otherTransaction = (BankTransaction) other;

    return accountNo == otherTransaction.accountNo 
        && transactionAmount == otherTransaction.transactionAmount
        && // etc;
}

Note that for any field which is a reference type, you need to determine what sort of equality you want to apply there - often you'll want to call equals instead of just using the reference comparison provided by ==.

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