Kotlin Any hashCode

Introduction

The hashCode function in Kotlin is a part of the kotlin.Any class. The hashCode method returns an integer value, the hash code of the object. This hash code is used in hash-based collections to efficiently locate the object.

The hashCode method is used to generate a hash code for an object. The default implementation returns a distinct integer for each object, typically based on the object's memory address. However, it is often overridden in custom classes to generate hash codes based on the object's contents.

hashCode Function Syntax

The syntax for the hashCode function  is as follows:

open fun hashCode(): Int

Parameters:

• This method does not take any parameters.

Returns:

• An integer hash code value for the object.

Examples

Basic Usage

To demonstrate the basic usage of hashCode, we will create a simple example comparing the hash codes of two strings.

Example

fun main() {
    val str1 = "Kotlin"
    val str2 = "Kotlin"
    val str3 = "Java"

    println("Hash code of str1: ${str1.hashCode()}")
    println("Hash code of str2: ${str2.hashCode()}")
    println("Hash code of str3: ${str3.hashCode()}")
}

Output:

Hash code of str1: 74343405
Hash code of str2: 74343405
Hash code of str3: 2301506

Overriding hashCode in Custom Classes

This example shows how to override the hashCode method in a custom class to generate hash codes based on the object's contents.

Example

class Person(val name: String, val age: Int) {
    override fun equals(other: Any?): Boolean {
        if (this === other) return true
        if (other !is Person) return false
        return name == other.name && age == other.age
    }

    override fun hashCode(): Int {
        var result = name.hashCode()
        result = 31 * result + age
        return result
    }
}

fun main() {
    val person1 = Person("Ravi", 25)
    val person2 = Person("Ravi", 25)
    val person3 = Person("Anjali", 30)

    println("Hash code of person1: ${person1.hashCode()}")
    println("Hash code of person2: ${person2.hashCode()}")
    println("Hash code of person3: ${person3.hashCode()}")
}

Output:

Hash code of person1: 80043549
Hash code of person2: 80043549
Hash code of person3: 2066934

Real-World Use Case

Using Hash Codes in Collections

In real-world applications, the hashCode method is used by hash-based collections such as HashMap and HashSet to efficiently store and retrieve objects.

Example

data class User(val id: Int, val username: String, val email: String)

fun main() {
    val user1 = User(1, "user1", "user1@example.com")
    val user2 = User(1, "user1", "user1@example.com")
    val user3 = User(2, "user2", "user2@example.com")

    val userSet = hashSetOf(user1, user2, user3)

    println("Users in the set:")
    userSet.forEach { println(it) }

    println("Hash code of user1: ${user1.hashCode()}")
    println("Hash code of user2: ${user2.hashCode()}")
    println("Hash code of user3: ${user3.hashCode()}")
}

Output:

Users in the set:
User(id=1, username=user1, email=user1@example.com)
User(id=2, username=user2, email=user2@example.com)
Hash code of user1: -1317699581
Hash code of user2: -1317699581
Hash code of user3: 873633602

Conclusion

The Any.hashCode function in Kotlin is used to generate a hash code for an object. This method is particularly useful for hash-based collections to efficiently store and retrieve objects. 

By understanding and using this method, you can effectively manage object storage in your Kotlin applications. Always remember to override the hashCode method in custom classes to provide meaningful hash codes based on object content, ensuring consistency with the equals method.