What is the difference between equals and hashcode

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. Thanks for visiting DZone today,. Edit Profile. Sign Out View Profile.

Collectives on Stack Overflow. Learn more. Asked 7 years, 4 months ago. Active 3 years, 1 month ago. Viewed 51k times. Improve this question. DavidPostill 7, 9 9 gold badges 36 36 silver badges 53 53 bronze badges. Dhivakar Dhivakar 1, 5 5 gold badges 12 12 silver badges 17 17 bronze badges. I suggest you read the definition for hashCode again. There is no such thing as a reference number. Add a comment. Active Oldest Votes. So if you pass a null as an argument to your object o1, then it should return false.

Hash code value: o1. This is very important. If you define a equals method then you must define a hashCode method as well. Before overriding this method, you need to keep in mind Whenever hashCode method is invoked on the same object more than once during an execution of a Java program, this method must consistently return the same result. The integer result need not remain consistent from one execution of the program to the next execution of the same program. Improve this answer.

As i understand that the hashcode and memory address are the two different things, how the hash codes can be same for different objects and those are not equal.

Martin Dinov Martin Dinov 8, 2 2 gold badges 27 27 silver badges 40 40 bronze badges. For Strings, it is a checksum of all characters inside. You can check the details in the Javadoc for String. Pablo Lozano Pablo Lozano 9, 2 2 gold badges 35 35 silver badges 58 58 bronze badges. An object is passed to the method, and it is type casted to Student. Then, the id values are checked. If the id values are similar, it will return true. If not, it will return false. The ids of s1 and s2 are similar.

So, it will print true. The ids of s1 and s3 are also similar, so it will print true. The hashCode is used in hashing to decide to which group an object should be categorized into. A group of objects can share the same hashCode.

A correct hashing function can evenly distribute objects into different groups. A correct hashCode can have properties as follows. Assume that there are two objects as obj1 and obj2. If obj1. The two unequal object might also have the same hashCode. The Student class contains the equals and hashCode methods. The equals method in the Student class will receive an object. If the object is null, it will return false.

If the classes of the objects are not the same, it will return false.