HashCode() Method in Java - Naukri Code 360

Introduction

The hashCode() method in Java returns an integer hash code representing an object's value. Java hashcode used as follows: int hash = targetString.hashCode();. This method is vital for efficient data retrieval, particularly with Java collections such as HashMap and HashSet.

In this blog, we will explore the HashCode method in Java and its practical applications.

What is the HashCode method in Java?

The HashCode() method is a built-in method in Java that returns a unique integer value, known as a hash code, for an object. The hash code is generated based on the object's content and memory address.

The hashcode method in Java is defined in the Object class, which is the root of the Java class hierarchy. Therefore, all Java objects inherit this method by default. However, the HashCode() method can be overridden in a subclass to provide a more meaningful hash code for that particular object.

The HashCode() method is commonly used in hashing algorithms and data structures, such as hash tables, to quickly search for and retrieve objects.

Example

Java

Java

public class Student {
    private int id;
    private String name;

    // Constructor
    public Student(int id, String name) {
        this.id = id;
        this.name = name;
    }

    // Getters and setters
    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    // Override the hashCode() method
    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + id;
        result = prime * result + ((name == null) ? 0 : name.hashCode());
        return result;
    }

    public static void main(String[] args) {
        Student student1 = new Student(1, "Alice");
        Student student2 = new Student(2, "Bob");

        // Displaying hash codes
        System.out.println("Hash code for student1: " + student1.hashCode());
        System.out.println("Hash code for student2: " + student2.hashCode());
    }
}

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Output

Hash code for student1: 63351360
Hash code for student2: 67988

Explanation

In this example, we have a Student class with two attributes: id and name. We have overridden the hashCode() method to generate a hash code based on both id and name fields. The formula used here for generating the hash code is a common approach. We used a prime number (31 in this case) to avoid collisions. We calculated the hash code based on the fields of the object. If a field is an object itself (like name), we calculate its hash code as well.
We combined the hash codes using multiplication and addition.

Types of HashCode Method in Java

In Java, there are mainly two types of hashcode based on parameters:-

1. Default hashcode() Method

The default implementation of the hashCode method is provided by the Object class, which returns a unique integer value for each object based on its memory address.

The syntax of the default hashCode Method in Java:

public int hashCode()

This method returns the hashcode of the calling Integer object.

2. Parameterized hashCode(int value) Method

A custom implementation of the hashCode method can be provided by the programmer to calculate a hash code based on the contents of the object's fields. This method, which has a parameter, finds the hashcode of the value of that parameter.

The syntax of parametrized hashCode Method in Java:

public static int hashCode(int value)

This method returns the hashcode of the value, an initial primitive int argument.

Importance of hashCode() method

Some importance of the hashCode() method are:

Object identification: The hashCode() method generates a unique identifier, called a hash code, for an object. This hash code helps to differentiate one object from another.
Efficient data organization: The hashCode() method is used by data structures like hash maps or hash tables to store and organize objects. It enables object insertion, deletion, and retrieval based on their hash codes.
Fast searching: The hashCode() method plays a vital role in searching for objects within a large collection by reducing the search space. It enables the data structure to efficiently locate potential matches based on the object's hash code. It leads to significant time and resource savings.
Performance optimization: A well-implemented hashCode() method can enhance the performance of your code. It facilitates efficient data retrieval and improves the speed of operations involving hashing and object comparison. It leads to overall performance improvements.
Equality checking: The hashCode() method is generally used with the equals() method to determine the equality of two objects. If two objects produce the same hash code, it suggests a potential equality, prompting a more thorough comparison using the equals() method.

When to override hashCode() method?

There are different scenarios when we need to override the hashcode() method. Some of them are below:

Whenever you override the equals() method in a class, it is required to override the hashCode() method. This will ensure that the objects considered equal will have the same hash code, as required, between hashCode() and equals().
If you plan to use the custom class as a key in hash-based data structures like HashMap or HashSet, you should override hashCode(). These data structures work on the hash code to efficiently store and get objects.
If you override the equals() method for a class but do not override the hashCode(), it may lead to unexpected behavior. Therefore, you need to override the hashcode() method in this situation.
If you have a class where objects that are considered equal should have the same hash code, you need to override hashCode(). This is important when working with collections that depend on hash codes, like HashMap or HashSet.

Properties of HashCode() in Java

The hashCode method in Java has the following properties:

Consistency: If an object is not modified, multiple invocations of the hashCode method should return the same hash code value.
Equal objects must have equal hash codes: If two objects are equal, they must have the same hash code value.
Unequal objects should have different hash codes: If two objects are not equal, they should have different hash code values. However, two different objects can have the same hash code value (known as a hash code collision).
Hash code values must be deterministic: The hashCode method in Java should always return the same hash code value for a given object, regardless of the state of the JVM or the order in which methods are called on the object.
Efficient computation: The hashCode method should be computationally inexpensive and fast to execute since it may be called frequently (e.g., when objects are added to or retrieved from a hash table).

Example of HashCode Method in Java

We will understand the working of hashCode() in Java with examples.

Example 1:

Java

Java

import java.util.*;

public class Coding_Ninjas {

    public static void main(String args[]) {
        String ans1 = "120";
        String ans2 = "120";

        // Displaying the hashcodes of ans1 and ans2
        System.out.println(ans1 + ": " + ans1.hashCode());
        System.out.println(ans2 + ": " + ans2.hashCode());

        // Declaring a different variable
        String ans3 = "5000";

        // Printing the hashcode of ans3
        System.out.println(ans3 + ": " + ans3.hashCode());

    }
}

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Output:

120: 48687
120: 48687
5000: 1626587

In this example, the hashCode() of ans1 and ans2 are the same since both are assigned the same value, whereas the hashCode() of ans3 are different.

Example 2:

Java

Java

import java.util.*;

class Coding_Ninjas {
    public static void main(String args[]) {
        ArrayList < Float > lis = new ArrayList < > ();
        System.out.println(lis.hashCode());

        // hashCode() with String
        String s = new String();
        System.out.println(s.hashCode());

    }
}

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Output:

1
0

In the above example, the hashCode of ArrayList is one, while the hashCode of a string is zero.

Also see, Java Ioexception

Uses of HashCode Method in Java

The hashCode() method in Java has several important uses:

Hash Tables: The primary use of hashCode() is to support hash tables like HashMap, HashSet, etc. These data structures use the hash code to store and retrieve objects efficiently. An object's hashCode() method is used as the key to computing the object's index in the table.
Object Identity: The hashCode() method can be used to determine the identity of an object. Two objects with the same hash code are not necessarily equal, but two objects with different hash codes are not equal. This is because the hash code of an object is typically based on its memory address, which is unique for each object.
Caching: The hashCode() method can be used to cache the hash code of an object. This can be useful if the object is expensive to compute and the hash code is needed frequently. By caching the hash code, the application's performance can be improved.
Equality Testing: The hashCode() method is used in conjunction with the equals() method to test for the equality of objects. The general contract is that if two objects are equal according to the equals() method, they must have the same hash code. This is used to optimize equality testing in some cases.
Custom Classes: The hashCode() method can be overridden in custom classes to provide a custom implementation. This is useful if the default implementation in the Object class is inappropriate. The custom implementation should follow the general contract of hashCode(), which specifies that equal objects must have the same hash code.

Best Practices for implementing hashCode() method

Here are the best practices for implementing the hashCode() method:

Consistency with equals(): Maintaining consistency between the hashCode() and equals() methods is important. When two objects are considered equal according to the equals() method, their hash codes should be identical.
Use relevant fields: To ensure that the hash code accurately represents the object's properties. It includes the same fields used in the equals() method when calculating the hash code.
Efficiency: Implementing the hashCode() method aim for an efficient calculation to minimize unnecessary overhead. However, it is important to maintain a good balance and avoid sacrificing too much accuracy or distribution in performance optimizations.
Immutable fields: It is generally suggested to use only immutable fields when calculating the hash code for an object with mutable fields. This ensures the hash code remains consistent throughout the object's lifetime.
Follow Java conventions: It is important to follow the conventions and guidelines provided by Java when implementing the hashCode() method. This includes utilizing the objects.hash() method to combine hash codes and ensure a valid implementation within the hashCode() method.
Avoid unnecessary complexity: To ensure an efficient hashCode() implementation, it is important to keep the calculation simple and avoid unnecessary complexity. Complex calculations can lead to a chance of higher collisions and degrade overall performance.

Exploring Alternative HashCode Generators

In Java, the default hashCode() method inherited from the Object class often works well for many scenarios. However, in some cases, you may need to customize the hashCode() method to improve performance or to better suit the specific characteristics of your class. Here are some considerations and techniques for generating alternative hash codes in Java:

1. Combining Field Hash Codes:

One common approach to generating a hash code for a class is to combine the hash codes of its fields.
You can use the hashCode() method of each field and combine them using addition, multiplication, or bitwise operations to generate the final hash code.
It's essential to choose a combination method that distributes the hash codes effectively to minimize collisions.

2. Prime Numbers:

When combining hash codes, using prime numbers for multiplication and addition can help distribute hash codes more evenly and reduce the likelihood of collisions.
Prime numbers reduce the chance of common factors between different hash codes, which can lead to more distributed values across the hash space.

3. Ignoring Certain Fields:

In some cases, you may have fields in your class that should be ignored when computing the hash code.
If certain fields do not contribute to the object's identity or equality, you can exclude them from the hash code calculation to improve performance and reduce the chance of collisions.

4. Apache Commons Lang HashCodeBuilder:

Apache Commons Lang provides a utility class called HashCodeBuilder, which simplifies the process of generating hash codes for classes with multiple fields.
HashCodeBuilder allows you to chain method calls to include fields and provides options for specifying prime numbers and initial values for hash code generation.

5. Guava's Objects.hashCode():

Google's Guava library provides a utility method called Objects.hashCode() which generates hash codes for objects.
This method takes any number of objects as arguments and combines their hash codes to produce a final hash code.
It simplifies the process of generating hash codes for classes without having to implement custom hash code logic.

HashCode’s Role in Java Data Structures

In Java, the hashCode() method plays a critical role in hash-based data structures such as HashMap, HashSet, and Hashtable. Its primary function is to provide a unique identifier (hash code) for objects, facilitating efficient storage, retrieval, and management within these data structures. Here's a breakdown of the key roles hashCode() plays:

Indexing and Storage: Hash-based data structures use the hash code returned by hashCode() to determine the storage location (index) for objects. When an object is inserted into the data structure, its hash code is computed, and the data structure uses this code to calculate the appropriate index for storage. This indexing mechanism allows for rapid insertion and retrieval of objects, as it enables direct access to the desired storage location based on the hash code.
Efficient Retrieval: One of the primary advantages of hash-based data structures is their ability to facilitate fast retrieval of objects. Upon retrieval, the data structure computes the hash code of the target object and uses it to locate the corresponding storage location. By leveraging the hash code, these data structures achieve near-constant time retrieval complexity on average, even for large datasets.
Ensuring Uniqueness: The hash code generated by hashCode() is intended to be unique for each distinct object instance. This uniqueness ensures that objects with different properties or states produce distinct hash codes, preventing unintended collisions and maintaining the integrity of the data structure.
Handling Collisions: Despite efforts to generate unique hash codes, collisions can occur when multiple objects produce the same hash code. Hash-based data structures employ collision resolution strategies to manage these scenarios effectively. Common collision resolution techniques include chaining (using linked lists or other data structures to store colliding objects) and open addressing (finding alternative storage locations for colliding objects within the same array).

Frequently Asked Questions

What is the hashCode () and equals () used for?

Java hashing and equality checks are performed using the methods hashCode() and equals(). While equals() checks for equality between two objects, hashCode() produces an integer value that represents an item.

How to calculate hashCode in Java?

To create a distinct integer representation of an object, create a custom class that overrides the hashCode() method supplied by the Object class.

What is the difference between hashCode and hash in Java?

In Java, a method called hashCode is used to obtain an object's integer representation, but the term "hash" usually refers to data structures like HashMap that employ hash codes to facilitate quick data retrieval and storage.

What is the purpose of the hashCode?

The purpose of hashCode() is to facilitate efficient storage and retrieval in hash-based data structures by providing a unique identifier for objects.

What is the return type of the hashCode () method?

hashCode() method returns an integer value (primitive type int).

What is the formula for the hashCode?

Common formula involves combining object fields using multiplication and addition, often incorporating prime numbers to distribute hash codes evenly across the hash space.

Conclusion

In this article, we have discussed HashCode() Method in Java. We have also discussed the properties and uses of hashcode() in Java. The hashCode() method in Java serves as a pivotal component, enabling efficient organization and retrieval of objects within hash-based data structures. Its role extends beyond mere computation, influencing integrity, performance, and reliability across diverse Java applications.

We hope this blog has helped you enhance your knowledge of the HashCode Method in Java. Do not stop learning! We recommend you read some of our Java articles:

1. Introduction to Hashing

2. String Hashing

3. Hash Functions in Data Structure

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