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.
What you should say Given two objects whose hash codes are unknown, it is often faster to compare them directly than compute their hash codes, so there's no guarantee that things which report unequal hash codes but return true for equals will not be regarded as matching.
On the other hand, if collections happen notice that things cannot have the same hash code, they're likely not to notice that they're equal. Equals and Hashcode methods in Java They are methods of java. Implementation: public boolean equals Object obj public int hashCode public boolean equals Object obj This method simply checks if two object references x and y refer to the same object. It is reflexive: for any reference value x, x. For any non-null reference value x, x. The general contract of hashCode is: Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified.
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HashSet; import java. But if I have not overridden hashCode , it will cause nightmare as objects of Rishav with same member content will no longer be treated as unique as the hashCode will be different, as generated by default behavior, here's the would be output :- true false 2. If you override one, then you should override the other. Paramesh Korrakuti Paramesh Korrakuti 1, 3 3 gold badges 23 23 silver badges 34 34 bronze badges.
That is impossible. You've contradicted this yourself in the second sentence of the fourth paragraph. EJP, most of the times hascode will return the unique interger for two different objects. But there is will be chances of collide hascode for two different object, this concept is called as Hashcode Collision. Please refer : tech. That's it! Arun Raaj Arun Raaj 1, 1 1 gold badge 17 17 silver badges 20 20 bronze badges. Cleonjoys Cleonjoys 4 4 silver badges 10 10 bronze badges. To help you check for duplicate Objects, we need a custom equals and hashCode.
Tavash Tavash 1 1 silver badge 7 7 bronze badges. Ambrish Rajput Ambrish Rajput 2 2 silver badges 7 7 bronze badges. Please dont add an answer as two incomplete ones — Suraj Rao. GuruKulki GuruKulki I am happy to discuss if something results into un-expected behavior. I believe, since you are overriding hashcode which returns different numbers for the same object it still depends on which bucket these object go to.
Lets say, if hashcodes are 11, 12 and 13 but the hashset api applies another hashing function on the hashcode which determines the bucket. The result is you will have only one bucket and that will have the last object entered. With the same argument, SIZE can even be 2.
How it is working in default implementation of equals method. For some reason I was in a situation where I needed to customize equals and hashCode methods. The situation was that based on the primary key, say the employee ID of an employee, I needed to insert elements into an HashSet.
However to my discomfort, I am not getting the desired behavior out of the HashSet collection. I am quoting the code below with my comments:. Could you explain the behavior please? It is because you are not overriding the equals method correctly.
Correct way is to pass Object as method argument. Best way to detect is add an override annotation. It will tell you that method is overridden or local. Basically, this enforces the contract that equals must return true if the hashCode of two objects are the same… does it not?
Can u explain situation like master detail when id is unknown for more than one child objects ids will be generated on persist. When initial id is 0 if primitive for more then one object, but I want put it in HashSet like childs, and then persist the Master with childs.
Thank you for helpful information. For whatever reason, is it ok to override hashcode and do not override equals? All blogs talk only about if you override equals , then you have to override hashcode. If you might want to implement logical comparison instead of default comparison mechanism in java, you can live happy with only overriding equals method only.
If there is none of above scenario, then there is no need to override any of above. But if still you want to override, do at your will. It will not make much difference. Thanks Lokesh for clearing out my doubts …I went through many blogs but this one was the best, giving a clear picture.
I have read that the default hashcode is computed by using the class name, method names and member variables. Is it right or not? This getIdentityHashCode method is native method. Please note that identity hashcode takes no account of the content of the object, just where it is located. Object is another method similar to VMMemoryManager. A blog about Java and its related technologies, the best practices, algorithms, interview questions, scripting languages, and Python.
About Me. Contact Us. Privacy policy. Guest Posts. Secure Hash Algorithms. Best Way to Learn Java. How to Start New Blog. Skip to content. Table of Contents: 1. Uses of hashCode and equals Methods 2. Override the default behavior 3.
EqualsBuilder and HashCodeBuilder 4. Generate hashCode and equals using Eclipse 5. Important things to remember 6. Uses of hashCode and equals Methods equals Object otherObject — verifies the equality of two objects.
Its default implementation simply checks the object references of two objects to verify their equality. By default, two objects are equal if and only if they are refer to the same memory location.
Most Java classes override this method to provide their own comparison logic. Contract between hashCode and equals Overriding the the hashCode is generally necessary whenever equals is overridden to maintain the general contract for the hashCode method, which states that equal objects must have equal hash codes. Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode must consistently return the same integer , provided no information used in equals comparisons on the object is modified.
This integer need not remain consistent between the two executions of the same application or program. If two objects are equal according to the equals method, then calling the hashCode on each of the two objects must produce the same integer result. It is not required that if two objects are unequal according to the equals , then calling the hashCode on each of the both objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hash tables.
Overriding the Default Behavior Everything works fine until we do not override any of both methods in our classes. Let us understand why we need to override equals and hashcode methods. Should we override only equals method? HashSet; import java. Overriding hashCode is necessary We are missing the second important method hashCode.
We can use these classes in the following manner. EqualsBuilder; import org. Was this post helpful? As you can see in above diagram that every object is placed in Hash bucket depending on the hashcode they have. It is not necessary that every different object must have different hashcode. When we try to insert any key in HashMap first it checks whether any other object present with same hashcode and if yes then it checks for the equals method. If two objects are same then HashMap will not add that key instead it will replace the old value by new one.
Ans : If the object does not implement hashcode method and used as key then we will not get the object back as shown in below code. Ans : This is the common question asked in interview. In JAVA 5 : we can use advance for loop as shown in above code, use map. This will return the Set As Keys must be unique. In JAVA 4 : use map. But my emphasys is on need of hashcode method and that is correct.
You can refer java api and try to run program also. Now to get value, we just need to create an object and that will act as key, so need to insert in map again. You must note that object m2,m3 and m4 will have same hashcode as implemented in example.
Hi shailendra, statement is perfect. He already specified that to be object same equal method is required. But what if somebody does not override hashcode? Comparing two object will return true if they are same and hashcode is not implemented.
Try to run the program, you will get. It is said above that when v try ti insert key into hashmap…what is key…please may i know this…. I did not understand ur code, as u have not added m4 object in map like map. If it is not added u r not able to retrieve and getting output as object not found. Hi Dhara, Please check line 70 in first code and line 81 in second code snippet, i have added object m4.
Thanks, Jitendra Zaa. Can you please correct it. That is the reason i have used code like this. Yes got your explanation now.. Pjoshi : Jitendra Zaa is absolutly correct, although the m4 object not been added in the map, but m4 is the exact copy of m2 and m2 is getting used here as a KEY, so it will return m2 object value. Please correct me if i am wrong.
Thanks K.
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