equals合同的重要字段是什么（有效的java项目8）

Question

In Effective Java in Item 8 the recommendation is that

For each significant field in the class to check the corresponding field of the this object.

I understand that we can have secondary fields that are calculated by primary fields but what exactly is the meaning of "for each significant field"? Is the equals contract implemented properly only when all fields of an object is compared?

If I have eg a class Employee which has a multitude of fields like id, first and last name, dob, position, location etc all these seem significant but to me it seems that just using the id would suffice for a proper and performant equals implementation.

Am I wrong on this? Or the id I mention is exactly what Bloch means by "significant" fields?

Answer 1

class Employee {

    private UUID id;
    private String firstName;
    private String lastName;

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (!(obj instanceof Employee))
            return false;
        return id.equals(((Employee)obj).id);
    }

    @Override
    public int hashCode() {
        return Objects.hash(id);
    }
}

In case of Employee is stored into DB, ie has unique id , then no need to check other fields like firstName and lastName in equals ; according to data object comparison, only id filed is significant .

Answer 2

A significant field is merely one that, if omitted, would result in an incorrect implementation of equals (according to the notion of equality you have defined for instances of your class).

I appreciate that is a bit of a self-referential definition, but that's what it means.

The canonical example of a non-significant field is String.hashCode : as you observe, this is calculated from other fields (and lazily), so it would not be appropriate to include in the equality because there is no guarantee that it has been calculated for either of the strings being compared; and, if it has been calculated for both, it tells you nothing more than you already know.

In your case, yes, it sounds like comparing instances using only the id would suffice: this is a significant field, the name (etc) is not significant: there should only be one person (little p, as in an actual real human) with a particular id.

It does raise a question of how you would deal with "same id, different name" instances, but this is getting into the realm of Falsehoods Programmers Believe About Names :

1. People have exactly one canonical full name.
2. People have exactly one full name which they go by.
3. People have, at this point in time, exactly one canonical full name.
4. People have, at this point in time, one full name which they go by.
5. People have exactly N names, for any value of N.
6. (People's names fit within a certain defined amount of space.)
7. People's names do not change.
8. ...

Taking these into consideration, if you want to say "this Person is the same as that Person" (and you want to do that using equals ), id seems like the only reasonable thing to use.