Why should the hashcode () method be rewritten when rewriting the equals () method

First of all, we need to know one thing , For two different objects , They use hashcode（） The value calculated by the method may be the same , We can't use hashcode（​ ） To determine if two objects are equal , Must pass equals（） Method .

     Two objects equals() equal ,hashcode() It must be equal
​     Two objects hashcode() Unequal ,equals() It must be different
​     Two objects hashcode() equal , Objects are not necessarily equal , Need to pass through equals() Further judgment .）

  Why should we rewrite equals（） Methods? ？

 

Sometimes, when we judge objects in our business system, we sometimes need not a strict equality , It's a business where objects are equal . under these circumstances , Native equals Methods can not meet our needs .

What we know JavaBean Superclass of ( Parent class ) yes Object class ,JavaBean Medium equals The method is inherited from Object The method in .Object Class equals() Method is used to compare whether the memory addresses of the objects pointed to by two references are consistent . It is not to compare whether the attribute values of two objects are consistent , So at this point we need to rewrite equals() Method .

Let me put it in a simple way , There's a student class id,name, then A.id=1;A.name=“ Zhang San ”, There's one at this time B.id Also equal to 1,B.name=“ Zhang San ”, Use at this time equals Method to calculate, we get the result is false, But our business needs just want to compare literally , What you want is true, At this time, we have to rewrite it equals（） Method , When we rewrite equals（） Why rewrite after method hashcode（） Methods? ？

We stipulated equals In the case of equality ,hashCode（） Must be equal , Another official explanation is that we are writing a class to rewrite equals Method is not overridden hashcode When the method is used , There will be a situation ：equals When the comparison results are equal ,hashcode It's not equal , Although there is no problem in this class , But if we put it in a hash set , In the hash set, a equals Equal and hashcode Different situations , This causes the hash set to equals Wrong judgment （ Because it passes first hashcode Comparative ,hashcode Different people will think it is different ）, So in order to match the comparison in the hash set , We need to rewrite equals Method when overridden hashcode Method .