Need help in writing hashcode and equals method?

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开发者_如何学C 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?

---

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 ==.

---

I suggest you use the hashcodebuilder of the apache commons package:

http://commons.apache.org/lang/api-2.6/org/apache/commons/lang/builder/HashCodeBuilder

There is also an EqualsBuilder:

http://commons.apache.org/lang/api-2.6/org/apache/commons/lang/builder/EqualsBuilder

If you implememt both you should not worry about storing your objects in a hashset

---

The definitive answer to this question is in Effective Java (second edition).