为什么 lombok 添加 canEqual 方法

Question

使用lombok时@Data (which adds EqualsAndHashCode)

添加canEqual方法

protected boolean canEqual(Object other) {
  return other instanceof Exercise;
}

只调用一次：

if (!other.canEqual((Object)this)) return false;

我搜索并找到 discussions 关于访问级别

If you implement equals and hashCode in a non-final class the safest thing we can do is add the can equal the way we do. Since we don't add any field the costs, especially if the method is protected, are slim.

但是为什么我们需要这个生成的方法呢？不能内联吗？

Answer 1

canEqual 方法在一篇名为 How to Write an Equality Method in Java 的论文中定义。此方法旨在允许在 class 层次结构 的多个级别上重新定义相等性，同时保持其合同：

The idea is that as soon as a class redefines equals (and hashCode), it should also explicitly state that objects of this class are never equal to objects of some superclass that implement a different equality method. This is achieved by adding a method canEqual to every class that redefines equals.

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它似乎是在 Lombok 0.10 中引入的，如 @EqualsAndHashCode 文档中所述：

NEW in Lombok 0.10: Unless your class is final and extends java.lang.Object, lombok generates a canEqual method which means JPA proxies can still be equal to their base class, but subclasses that add new state don't break the equals contract.

文档更进一步，参考了上面引用的论文：

The complicated reasons for why such a method is necessary are explained in this paper: How to Write an Equality Method in Java. If all classes in a hierarchy are a mix of scala case classes and classes with lombok-generated equals methods, all equality will 'just work'. If you need to write your own equals methods, you should always override canEqual if you change equals and hashCode.