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写一个“完美”的Java的equals方法

原文

 摘自Core Java(Volume I--Fundamentals 9th Edition)：

1. Name the explicit parameter otherObject—later, you will need to cast it to another variable that you should call other.
2. Test whether this happens to be identical to otherObject:
   if (this == otherObject) return true;
   This statement is just an optimization. In practice, this is a common case. It is
   much cheaper to check for identity than to compare the fields.
3. Test whether otherObject is null and return false if it is. This test is
   required.
   if (otherObject == null) return false;
4. Compare the classes of this and otherObject. If the semantics of equals
   can change in subclasses, use the getClass test:
   if (getClass() != otherObject.getClass()) return false;
   If the same semantics holds for all subclasses, you can use an instanceof test:
   if (!(otherObject instanceof ClassName)) return false;
5. Cast otherObject to a variable of your class type:
   ClassName other = (ClassName) otherObject
6. Now compare the fields, as required by your notion of equality. Use == for
   primitive type fields, Objects.equals for object fields. Return true if all fields
   match, false otherwise.
   return field1 == other.field1 && Objects.equals(field2, other.field2) && . . .;
   If you redefine equals in a subclass, include a call to super.equals(other).

笔记

1. 直接比较是否是同一个引用；
2. 判断需要去比较的对象是否为null；
3. 比较：
   equals的语义在子类有所变化，使用getClass；
   子类和父类的equals语义一致，使用instanceof；
4. 类型转换；
5. 根据要求进一步比较域。

关于getClass和instanceof的理解：
getClass的规则是：采用==来进行检查是否相等的，是严格的判断，不会存在继承方面的考虑，也就是说只判断该继承层次上是否equals，进一步说子类的equals语义变化了。
instanceof的规则是：你属于该类或者该类的子类吗？考虑了继承，继承层次上equals语义一致；

参考：http://blog.csdn.net/hzw19920329/article/details/51095413

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