When do you override equals and hashcode

So if you need compare its state as well then you can override that as it is done in String class. I've found the EqualsVerifier library to be very useful and comprehensive.

It is also very easy to use. Also, building equals and hashCode methods from scratch involves a lot of boilerplate code. These classes greatly simplify implementing equals and hashCode methods for complex classes.

As an aside, it's worth considering overriding the toString method to aid debugging. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Collectives on Stack Overflow.

Learn more. Why do I need to override the equals and hashCode methods in Java? Ask Question. Asked 11 years, 9 months ago. Active 28 days ago. Viewed k times. How can I take the decision to implement these methods efficiently? Improve this question. Exploring 2, 6 6 gold badges 38 38 silver badges 63 63 bronze badges. Shashi Shashi The link appears to be dead. Can I obtain the IBM's developer works document?

Add a comment. Active Oldest Votes. Override only hashCode If you only override hashCode then when you call myMap. Hope it was clear.

Improve this answer. SebastianWilke 1 1 silver badge 12 12 bronze badges. Lombo Lombo I don't like this answer because it suggests that you can't override hashCode without overriding equals , which is simply not true.

You say your example code the "override only hashCode" part won't work because you define your two objects as equal, but - sorry - this definition is only in your head.

In your first example you have two un-equal objects with the same hashCode, and that is perfectly legal. So the reason you need to override equals is not because you have already overridden hashCode , but because you want to move your "equals" definition from your head to the code.

You need to override hashCode if your class overrides equals but reverse is not true. I think it's totally ok to override only hashCode without overriding equals as well. It's also whats written in Effective Java : books. PhantomReference, note that only overriding equals would violate the contract spelled out in the javadoc of Object : "If two objects are equal according to the equals Object method, then calling the hashCode method on each of the two objects must produce the same integer result.

Show 15 more comments. Hashing retrieval is a two-step process: Find the right bucket using hashCode Search the bucket for the right element using equals Here is a small example on why we should overrride equals and hashcode. Consider an Employee class which has two fields: age and name. In any case, hashmap replaces the value if object's hashcode is equal. VikasVerma equals object will have equal hashcode doesn't mean unequal object will have unequal hashcode.

What if objects are actually different, but their hashcode is same? Even if we comment the equals method and uncomment the hashcode method, then also it will be false ,as even though the right bucket is found using the hashcode buth the correct element is not found. Can we use any random numbers?

JavaYouth Yes, you can — rajeev pani.. Show 1 more comment. JuanZe JuanZe 7, 42 42 silver badges 58 58 bronze badges. This the correct answer. To corollary being, of course, that if you never use the class in a hash-based collection, then it doesn't matter that you haven't implemented hashCode. In a more complex cases, you never know if the collections you use are using hashes, so stay away from "it doesn't matter that you haven't implemented hashCode " — Victor Sergienko. Can I override hashCode without overriding equals?

Johnny certainly you can override the hascode without override the equals. But what would be the use case? Gi1ber7 check my answer a little under from here to understand analytically what is happening with HashMap and HashTable for equals and hashCode — Panagiotis Bougioukos. Identity is not equality. First we have to understand the use of equals method. In order to identity differences between two objects we need to override equals method.

Assume we have override equals method of Customer as above, customer1. Premraj Premraj This is what I was looking for since last 1 hour.

Awesome mate y — Adnan. Shashi Why we override hashCode method Some Data Structures in java like HashSet, HashMap store their elements based on a hash function which is applied on those elements.

The hashing function is the hashCode If we have a choice of overriding. Let's get back to those hash data structures. There is a rule for those data structures. HashSet can not contain duplicate values and HashMap can not contain duplicate keys HashSet is implemented with a HashMap behind the scenes where each value of a HashSet is stored as a key in a HashMap.

So we have to understand how a HashMap works. Our map might end with those persons in different linkedLists. Now we are aligned with the rule of Hash Map that says no multiple equal keys are allowed! Panagiotis Bougioukos Panagiotis Bougioukos 6, 2 2 gold badges 7 7 silver badges 20 20 bronze badges. Natasha Kurian 3 3 silver badges 11 11 bronze badges.

Rinkal Gupta Rinkal Gupta 1 1 silver badge 2 2 bronze badges. That's interesting point, about override only hashCode. It's totally fine, right? Or can there be problematic cases as well? This is a misleading and wrong answer. But won't be useful as none of them will be equal to each other.

Let me explain the concept in very simple words. Now why is a hashmap used? Hashing EG: we have array 1,2,3,4,5,6,7,8,9,10,11 and we apply a hash function mod 10 so 1,11 will be grouped in together. That datastructure used to store all the above information can be thought of as a 2d array for simplicity Now apart from the above hashmap also tells that it wont add any Duplicates in it. And this is the main reason why we have to override the equals and hashcode So when its said that explain the internal working of hashmap , we need to find what methods the hashmap has and how does it follow the above rules which i explained above so the hashmap has method called as put K,V , and according to hashmap it should follow the above rules of efficiently distributing the array and not adding any duplicates so what put does is that it will first generate the hashcode for the given key to decide which index the value should go in.

You could also refer to Detail working import java. Lii Chetan Chetan 3, 6 6 gold badges 42 42 silver badges 52 52 bronze badges. I have one confusion, why do we need to override equals method when we override hashCode method in case of HashMap? VikasVerma hashmap doesn't replace any kind of value if the objects' hashcode is equal, it only decides the index where the newly added object to the hashmap has to be placed.

Now there can be objects at the index, so to avoid duplicated we override the equals method and we write the logic for defining when the two objects in comparison are to be treated as equal. If not overridden then though objects having same values will be stored because the reference of both the objects will be different — Chetan.

Java puts a rule that "If two objects are equal using Object class equals method, then the hashcode method should give the same value for these two objects. Rany Albeg Wein 2, 2 2 gold badges 14 14 silver badges 26 26 bronze badges. Ritesh Kaushik Ritesh Kaushik 1 1 gold badge 12 12 silver badges 22 22 bronze badges. Because if you do not override them you will be use the default implentation in Object. Prashanth Prashanth 10 10 silver badges 7 7 bronze badges.

Adding to Lombo 's answer When will you need to override equals? Override only equals Addition to Lombo 's answer myMap. But returns false!!! Then you are missing the point of Hash based Collections.

The following are the keys stored in the form of buckets. Bucket 1 : 1,10,19, Bucket 3 : 3,21,30, HashCode Equal Contract Two keys equal according to equal method should generate same hashCode Two Keys generating same hashCode need not be equal In above example all even numbers generate same hash Code. Problem caused by hashCode The problem is caused by the un-overridden method hashCode.

The contract between equals and hashCode is: If two objects are equal, then they must have the same hash code. If two objects have the same hash code, they may or may not be equal. Suraj Rao Neeraj Gahlawat Neeraj Gahlawat 1, 14 14 silver badges 9 9 bronze badges. The following is an excerpt from the Portland Pattern Repository : Examples of value objects are things like numbers, dates, monies and strings.

Stan k 18 18 gold badges silver badges bronze badges. Dewfy Dewfy Your Job is to color those balls as follows and use it for appropriate game, For Tennis - Yellow, Red. Coloring the balls - Hashing. Choosing the ball for game - Equals. Aakash Goplani 1 1 gold badge 8 8 silver badges 18 18 bronze badges.

Narinder Narinder 41 1 1 bronze badge. Aarti Aarti 39 1 1 bronze badge. If the equals method returns true, there's no need to check the hashcode. If two objects have different hashcodes, however, one should be able to regard them as different without having to call equals.

Further, knowledge that none of the things on a list have a particular hash code implies that none of the things on the list can match nay object with that hash code. As a simple example, if one has a list of objects whose hash codes are even numbers, and a list of objects where they are odd numbers, no object whose hash code is an even number will be in the second list.

If one had two objects X and Y whose "equals" methods indicated they matched, but X's hash code was an even number and Y's hash code was an odd number, a collection as described above which noted that object Y's hash code was odd and stored it on the second list would not be able to find a match for object X.

It would observe that X's hash code was even, and since the second list doesn't have any objects with even-numbered hash codes, it wouldn't bother to search there for something that matches X, even though Y would match X.

When calling students. Now any time an element with the same hash code is inserted into the set, it will just replace alex1. However, since alex2 has a different hash code, it will be stored in a separate bucket and will be considered a totally different object. Now when HashSet searches for an element inside it, it first generates the element's hash code and looks for a bucket which corresponds to this hash code.

Here comes the importance of overriding hashcode , so let's override it in Student and set it to be equal to the ID so that students who have the same ID are stored in the same bucket:. See the magic of hashcode! The two elements are now considered as equal and stored in the same memory bucket, so any time you call contains and pass a student object holding the same hash code, the set will be able to find the element.

The same is applied for HashMap, HashTable , or any data structure that uses a hashing mechanism for storing elements. In order to achieve a fully working custom equality mechanism, it is mandatory to override hashcode each time you override equals.

Follow the tips below and you'll never have leaks in your custom equality mechanism:. See the original article here. Thanks for visiting DZone today,. Edit Profile. Sign Out View Profile. Over 2 million developers have joined DZone. Working With hashcode and equals. Need to implement your own custom equality-checking mechanism? Here are some tips for when you need to override hashcode and equals.

Like Join the DZone community and get the full member experience. Post a Comment. A couple of questions, which are often asked to me was why do we need to override equals and hashcode method, Why should I implement toString , What will happen if I don't override them or in a different way, I have never overridden equals and hashcode and not faced any problem, why should I override them now.

You guessed it correctly, almost all of these questions come from beginners, who have either taken some Java programming classes or started learning Java by their own. Though the concept of equality is something you cannot ignore, correct understanding of equals , hashcode , toString , and some other method from java.

Object class, often goes unnoticed, at least until they self-realize it or Interviewer forces them to explore that part. Why you should override equals or hashcode From the face, you can guess that equals are used to check if two objects are equal or not.

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