Since the default hashCode implementation in the Object class return distinct integers for distinct objects, if only equals method is overridden, e1 will be placed in some bucket and e2 will be placed in some other bucket as e1.
If we only override the hashCode method, both e1 and e2 will hash to the same bucket as they produce the same hash code. But since the equals method is not overridden, when the set hashes e2 and iterates through the bucket looking if there is an Employee e such that e2.
Please note that even though equal objects must have equal hash codes, the reverse is not true. It is perfectly valid to override hashCode without overriding equals as objects with equal hash codes need not be equal. Reference: StackOverflow. Average rating 4.
Vote count: No votes so far! Be the first to rate this post. Primary Primary. This is for instance important when putting your objects into a container that utilizes equals and hashcode, like HashMap and Set.
Let's say we have a class like:. Well maybe, if this is what you want. But let's say we want objects with the same id to be the same object, regardless if it's two different instances. We override the equals and hashcode :.
As for implementing equals and hashcode I can recommend using Guava's helper methods. They are behaving a certain way. It checks if 2 references that we compare point to the same instance in memory. So let's say that in your program you have built 2 Person objects on different places and you wish to compare them. Those 2 objects from business perspective look the same right? For JVM they are not the same. Since they are both created with new keyword those instances are located in different segments in memory.
There comes the. You can override equals to check if some objects have same values for specific fields to be considered equal.
You can select which fields you want to be compared. If we say that 2 Person objects will be the same if and only if they have the same age and same name, then the IDE will create something like the following for automatic generation of equals. Keep in mind however, if we don't provide our custom version of. Default equals method which is inherited from Object will check whether both compared instances are the same in memory! 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. There is a reason for that. Default implementation of hashCode which is inherited from Object considers all objects in memory unique! In a simple way a HashMap is a native array that has some buckets.
Each bucket has a linkedList. In that linkedList our keys are stored. HashMap locates the correct linkedList for each key by applying hashCode method and after that it iterates through all elements of that linkedList and applies equals method on each of these elements to check if that element is already contained there. No duplicate keys are allowed. When we put something inside a HashMap, the key is stored in one of those linkedLists.
In which linkedList that key will be stored is shown by the result of hashCode method on that key. So if key1. By default hashCode method returns a different result for each different instance. But in our previous example we said we want Person instances to be considered equal if their ages and names match. In Person class we have not overridden the hashCode method but we have overridden equals method.
Since the default hashCode provides different results for different java instances person1. But ours now has and the reason is that the default hashCode which was inherited from Object Class was not enough. Not after we have overridden the equals method on Person Class. That is the reason why we must override hashCode method after we have overridden equals method.
Now let's fix that. Let's override our hashCode method to consider the same fields that equals considers, namely age, name. Let's say it is 0. HashMap will go to bucket 0 and in that LinkedList will save the person1 as key with the value "1". For the second put HashMap is intelligent enough and when it goes again to bucket 0 to save person2 key with value "2" it will see that another equal key already exists there. So it will overwrite the previous key. So in the end only person2 key will exist in our HashMap.
If you only override the hash-code method nothing happens, because it always returns a new hashCode for each object as an Object class. If you only override the equals method, if a. Note : hashCode method of Object class always returns a new hashCode for each object. So when you need to use your object in the hashing based collection, you must override both equals and hashCode. Firstly from a broader perspective we have collections, and hashmap is one of the datastructure in the collections.
To understand why we have to override the both equals and hashcode method, if need to first understand what is hashmap and what is does.
A hashmap is a datastructure which stores key value pairs of data in array fashion. Lets say a[], where each element in 'a' is a key value pair. Also each index in the above array can be linked list thereby having more than one values at one index.
If we have to search among a large array then searching through each if them will not be efficient, so what hash technique tells us that lets pre process the array with some logic and group the elements based on that logic i. 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.
So if we had to search for 11 in previous array then we would have to iterate the complete array but when we group it we limit our scope of iteration thereby improving speed.
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. As every object derived from the object class, the default implementation for comparing two objects is that it compares the reference and not values inside the object. So in the above case both though semantically equal will fail the equality test, and possibility that two objects which same hashcode and same values will exists thereby creating duplicates.
If we override then we could avoid adding duplicates. You could also refer to Detail working. So, if in our class we override equals we should override hashcode method also to follow this rule. Both methods, equals and hashcode , are used in Hashtable , for example, to store values as key-value pairs. If we override one and not the other, there is a possibility that the Hashtable may not work as we want, if we use such object as a key.
Given that instance equality and hascode values generally require knowledge of what makes up an object they generally will need to be redefined in your class to have any tangible meaning. In order to use our own class objects as keys in collections like HashMap, Hashtable etc.. Otherwise, it leads to wrong results which we are not expected. But you might want to consider two objects the same if they have the same value for one or more of their properties Refer the example given in Lombo 's answer.
So you will override equals in these situations and you would give your own conditions for equality. I have successfully implemented equals and it is working great. So why are they asking to override hashCode as well? As long as you don't use "Hash" based Collections on your user-defined class,it is fine. But some time in the future you might want to use HashMap or HashSet and if you don't override and "correctly implement" hashCode , these Hash based collection won't work as intended.
First of all,HashMap checks if the hashCode of second is the same as first. Only if the values are the same,it will proceed to check the equality in the same bucket. But here the hashCode is different for these 2 objects because they have different memory address-from default implementation. Hence it will not even care to check for equality. If you have a breakpoint inside your overridden equals method,it wouldn't step in if they have different hashCodes.
Why can't we make the HashMap check for equality in all the buckets? So there is no necessity for me to override hashCode!! Say,you want to know if the map contains the key Would you want to search all the buckets? Based on the hashCode,you would identify that if 10 is present,it must be present in Bucket 1.
So only Bucket 1 will be searched!! The problem is caused by the un-overridden method hashCode. The contract between equals and hashCode is:. It is useful when using Value Objects. The following is an excerpt from the Portland Pattern Repository :. Examples of value objects are things like numbers, dates, monies and strings.
Usually, they are small objects which are used quite widely. Their identity is based on their state rather than on their object identity. This way, you can have multiple copies of the same conceptual value object. So I can have multiple copies of an object that represents the date 16 Jan Any of these copies will be equal to each other.
For a small object such as this, it is often easier to create new ones and move them around rather than rely on a single object to represent the date. A value object should always override. Remember to override. Assume you have class A that aggregates two other B C , and you need to store instances of A inside hashtable. Default implementation only allows distinguishing of instances, but not by B and C.
So two instances of A could be equal, but default wouldn't allow you to compare them in correct way.
Consider collection of balls in a bucket all in black color. Your Job is to color those balls as follows and use it for appropriate game,. Now bucket has balls in three colors Yellow, Red and White. And that now you did the coloring Only you know which color is for which game. If you did the coloring and some one chooses the ball for either cricket or tennis they wont mind the color!!!
I was looking into the explanation " If you only override hashCode then when you call myMap. I think 2nd time when we are adding in myMap then it should be the 'second' object like myMap. The methods equals and hashcode are defined in the object class. By default if the equals method returns true, then the system will go further and check the value of the hash code.
If the hash code of the 2 objects is also same only then the objects will be considered as same. So if you override only equals method, then even though the overridden equals method indicates 2 objects to be equal , the system defined hashcode may not indicate that the 2 objects are equal. So we need to override hash code as well. They are methods of java. Object class which is the super class of all the classes custom classes as well and others defined in java API. This method simply checks if two object references x and y refer to the same object.
It is symmetric: for any reference values x and y, x. It is transitive: for any reference values x, y, and z, if x. It is consistent: for any reference values x and y, multiple invocations of x. This method returns the hash code value for the object on which this method is invoked. Here hash code for both the string are same but they are not equal. Can you please help me out from this. As you know that two different objects can have the same hashcode.
For String Class. So when you compare hashcode of 2 strings with same content as in your case you will get same hashcode. If you compare str and str1 with. Hi, your blog really helps a lot for all, I appreciate for sharing your knowledge with us.
I am having a small problem, i would be glad if you help me out. Nice Article. It helped me refreshing my knowledge on Hashcode and Equals. Please keep up the good work. Two objects may be located in separate location on heap, but they can still be equal. But the Equals-Hashcode contract says, if two objects are equal then they must have the same hashcode. A complete method will look like this:.
I find it tedious to implement equals and hashCode by hand. I also see too many mistakes in code in this area. The generated output can be automatically updated when you change your interfaces — no need to maintain the generated code. The generated output can be automatically updated when you change your interfaces so no need to maintain the generated code.
Compared to alternatives it is much more customisable so no problem if you want to add your own value based custom methods, have the generated code subclass a base class with a non-default constructor, change hashCode to cache results etc. Everything can be customised. Why implementations of both are required? What does it convey? What if I override only hashCode?
You must override hashCode in every class that overrides equals. Failure to do so will result in a violation of the general contract for Object. In fact, there is very are chance that you will be first modifying your hashcode method. In most of the cases, you will need to compare two instances logically rather than default compare which compare the object references in memory. But for application, they actually represent same user entity and they should be treated as same instance.
If one session in application says, user with id 1 is logged in, then in another part of application some code should be able to say same thing about user 1 using different instance. To do this, you must override equals method first of all before even overriding hashcode. If rule is broken, you must override to make rule happy. You see, java has not made it mandatory.
No compilation errors. For more discussion, please read comments below. Some good questions already have been answered there. If both employee objects have been equal, in a Set which stores only unique objects, there must be only one instance inside HashSet, after all both objects refer to same employee. This would help add more detail about the working of the Hashing mechanism in Java. HI Nagesh, Thanks for asking a good question. I would suggest anybody who wants to know why it printed two objects : Please drill down the sourcecode of HashSet.
Now how hashmap works , is another topic you may be interested in. Am I wrong? Excellent Explanation. Finally i understood why we override hashcode and eqals methods.
Thank you Sir. If class has overridden the hashCode method then it is possible. Integer class. Then why you have written this line? Is it O. K to use getId which returns an Integer and which can be null?
And is there any reason that you compared the id Integer with a double-equal-sign? Hello Sir, Tutorials are very good, only the red mark on the image is misplace, i think it should on hashCode and equals , but its at generate getters and setters… You may rectify this…. I am sorry. I am behind some firewall which is preventing image loading from wordpress.
Please ignore my comment. Also, I have written my equals in such that it always return true and hashcode always returns a diff number. Will the above set contain 3 entries of the duplicate obj? What is the order in which equals and hashcode are called? Because ,if hashcode is diff then there is no need to even look at equals method 3. How many times equals and hashcode will be called in above case? Please elaborate a little on this.
Have you tried above anything yourself. I will appreciate if you try first. 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.
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