Why always override hashcode() if overriding equals()? This method is supported for the benefit of hashtables such as those provided by. The hashCode() method is supposed to return an int that should uniquely identify different objects. When storing objects in a hash, Java uses the hashCode() method which is a method that returns a hash code value for the object. The nature of hash based collections is to store keys and values. The int value returned from hashCode() is of particular use with the hash based Collection classes e.g. ![]() Returns a hash code value for the object. Let’s now discuss second part of this article and learn how to override hashCode() method. The argument to the equals() method must be an Object. Public boolean equals(Book book) will not override it. To make method overriding to occur, the signatures must be identical (with same visibility) Mistake while overriding equals() methodĪ silly mistake is to overload equals() method instead of overriding it. So, getClass() is preferred over instanceof(). To strictly follow equals() contract, symmetry must be ensured, and thus the class needs to be the same. Give Animal a and Dog d (with other fields the same). By using instanceof(), it's possible to not be symmetric.Ĭonsider the example: Dog extends Animal. Symmetric: For any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true. The reason to use getClass() is to ensure the symmetric property of the equals() contract. Reason to prefer getClass() over instanceof() ("book4 equals book2 : " + book4.equals(book2)) īook1 equals book2 : true because book and author name is sameīook3 equals book1 : true because book3 and book1 are same references.īook4 equals book2 : false because author name is not same. ("book3 equals book1 : " + book3.equals(book1)) ("book1 equals book2 : " + book1.equals(book2)) Source code of Object class equals() methodĬode: public boolean equals(Object obj) else if (!nameOfBook.equals(book.nameOfBook))īook book1 = new Book("Alchemist", "Paule Calhoe") īook book2 = new Book("Alchemist", "Paule Calhoe") īook book4 = new Book("Alchemist", "Salman Rushdi") The equals() method provided by Object class tests referential equality of objects and returns true if object references are equal. ![]() Default implementation of equals() in Object class We will discuss why it is necessary to override these methods and how we can override them. In this article we will discuss in detail the concepts of equals() and hashCode() method. Object class has the following five non-final methods.Įvery Java class has Object as a superclass so by default all above methods are provided in every java class. So now I am gonna tell you how to override the equals() and hashcode() method. I have more than 6 years of experience in coding. I were one of the developers, who made app VPN for windows and macOS version. In 2015 I have got a Master's Degree from the Massachusetts Institute of Technology (MIT). I work as Java Developer at VeePN VPN company.
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