Why override equals and hashcode

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What is Factory method Design Pattern in Java with A very popular usage of equals is defining an array list of Student and searching for a particular student inside it. So we modified our testing class in order the achieve this.

Okay, so we override equals and we get the expected behavior — even though the hash code of the two objects are different. So, what's the purpose of overriding hashcode? Let's consider a new test scenario. We want to store all the students in a HashSet , so we update HashcodeEquals as the following:.

We already override equals and verify that alex1 and alex2 are equal, and we all know that HashSet stores unique objects, so why did it consider them as different objects? HashSet stores its elements in memory buckets. Each bucket is linked to a particular hash code. 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. As you can see clearly in image, both values are getting stored into different bucket locations. Like that every insert into map will get different bucket location whether we are using same key objects or different i. And when we call map. So it should replace first with second as per rule. Reason is, when it iterate through that bucket and seeks to find k such that k.

But it fails to find because equals Object method has not been overridden. It is violation of rule of hashing. It it total violation of rule as key are unique in map. Reference : StackOverflow This article is contributed by Nitsdheerendra. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.

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