Why must hashcode() be overridden when overriding the equals() method? | HashMap

Yard farmland 2020-11-11 09:01:29
hashcode overridden overriding equals method


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First of all, we have a hypothesis : Any two object Of hashCode It's all different .

So under this condition , There are two object They are equal. , So if you don't rewrite hashCode(), The calculated hash values are all different , It's going to be different buckets 了 , Lost in the sea of people , No more recognition , Just like equals() The conditions are contradictory , Certificate completion .

And the flower ~~

Now let's explore these two methods :

Actually hashCode() and equals() The methods are all in Object class That's what our ancestors defined ,Object It's all Java Medium class The originator of , By default , I can't get rid of it .

Since it's for nothing , Let's see what's in the big bag first , Google Object Of Oracle file :

So these methods can be used directly ~

go back to hashCode() and equals(), So if this new class No rewriting in (override) These two methods , Is the default inheritance Object class The definition in .

Let's go in and have a look equals() How to define it :

Note taking :

equals() The way is to compare the two references Whether it points to the same object.

Um. ??? Are you making fun of me? ?? Isn't it with == The same ??

Add :
One of the symbols we use to compare sizes ==
If it is primitive type, that == Is to compare the values ;
If it is reference type, So the comparison is between the two reference Whether it points to the same object.

Replenish :
Java There are two types of data :
Primitive type Yes and no 8 Kind of :byte, short, int, long, float, double, char, boolean.
Others are Reference type.
So although Java claim “Everything is object”, But there are still problems object Existence of data type .

I don't believe it , I will go to the source code to see how it is implemented .

Ha , It's true , I've been around for so long ,equals() Just use == To achieve !

Then why do you come up with such a method ?

answer : For you override~

For example, generally speaking, we want to compare the contents of these two strings , that :

str1 = “tianxiaoqi”;
str2 =  new String(“tianxiaoqi”);

str1 == str2; // return false
str1.equals(str2); // return true 

because String It's rewritten equals() Methodical :

Your ancestors left you for your own use , If you don't have to , The family also provided a default method , That's enough .

Okay , Let's see it again hashCode() Introduction to :

As for hashCode() What is it that returns , Not very relevant to this article , Interested students can see the reference This article [1], The conclusion is that :

What is returned is not necessarily the object's ( fictitious ) Memory address , Depending on the runtime library and JVM The concrete realization of .

But no matter how , You need to follow the conventions on the document , That is to say, for different object Returns the Unique hash value .

So ,

hashCode() To determine the key The number in this bucket , That's in the array index;

equals() To compare two object Same or not .

If you like this article , Remember to leave me a like message ~ Your support and recognition , It's the biggest driving force of my creation , See you in the next article !

This is Qi , New York program , Lifelong learners , Every night 9 spot , Let's meet you in the cloud study room !

See my... For more dry articles Github: https://github.com/xiaoqi6666/NYCSDE

Reference material

[1]

hashCode() Reference article : https://blog.csdn.net/xusiwei1236/article/details/45152201

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