在 Java 中覆盖 equals 和 hashCode 时应该考虑哪些问题?

2022-01-30 00:00:00 equals overriding java hashcode

在覆盖 equals 和 hashCode 时必须考虑哪些问题/陷阱?

What issues / pitfalls must be considered when overriding equals and hashCode?

推荐答案

理论(语言律师和数学爱好者):

equals() (javadoc) 必须定义一个等价关系(它必须是reflexive、symmetric和transitive).此外，它必须是一致的(如果对象没有被修改，那么它必须保持返回相同的值).此外，o.equals(null) 必须始终返回 false.

The theory (for the language lawyers and the mathematically inclined):

equals() (javadoc) must define an equivalence relation (it must be reflexive, symmetric, and transitive). In addition, it must be consistent (if the objects are not modified, then it must keep returning the same value). Furthermore, o.equals(null) must always return false.

hashCode() (javadoc) 也必须是 consistent (如果对象没有根据 equals() 进行修改，它必须保持返回相同的值).

hashCode() (javadoc) must also be consistent (if the object is not modified in terms of equals(), it must keep returning the same value).

这两种方法之间的关系是:

只要 a.equals(b)，则 a.hashCode() 必须与 b.hashCode() 相同.

Whenever a.equals(b), then a.hashCode() must be same as b.hashCode().

在实践中:

如果你覆盖了一个，那么你应该覆盖另一个.

In practice:

If you override one, then you should override the other.

使用与计算 equals() 相同的字段集来计算 hashCode().

Use the same set of fields that you use to compute equals() to compute hashCode().

使用优秀的辅助类 EqualsBuilder 和 Apache Commons Lang 库中的 HashCodeBuilder.一个例子:

Use the excellent helper classes EqualsBuilder and HashCodeBuilder from the Apache Commons Lang library. An example:

public class Person {
    private String name;
    private int age;
    // ...

    @Override
    public int hashCode() {
        return new HashCodeBuilder(17, 31). // two randomly chosen prime numbers
            // if deriving: appendSuper(super.hashCode()).
            append(name).
            append(age).
            toHashCode();
    }

    @Override
    public boolean equals(Object obj) {
       if (!(obj instanceof Person))
            return false;
        if (obj == this)
            return true;

        Person rhs = (Person) obj;
        return new EqualsBuilder().
            // if deriving: appendSuper(super.equals(obj)).
            append(name, rhs.name).
            append(age, rhs.age).
            isEquals();
    }
}

还要记住:

使用基于哈希的Collection 或 地图 例如HashSet, LinkedHashSet, HashMap, 哈希表，或 WeakHashMap，确保你放入集合中的关键对象的 hashCode() 在对象存在时永远不会改变集合.确保这一点的防弹方法是使您的密钥不可变，这还有其他好处.

Also remember:

When using a hash-based Collection or Map such as HashSet, LinkedHashSet, HashMap, Hashtable, or WeakHashMap, make sure that the hashCode() of the key objects that you put into the collection never changes while the object is in the collection. The bulletproof way to ensure this is to make your keys immutable, which has also other benefits.

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