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Use of hashCode method in java

HashCode():

The hashCode method in Java is a fundamental method inherited by all classes from the Object class. 

It is used to generate an integer value, known as the hash code, that represents the memory address of the object or a value derived from the object's fields. 

The hashCode method is primarily used in hash-based collections like HashMap, HashSet, and HashTable.

Purpose:

The hash code is used to optimize the performance of hash-based collections by distributing objects into buckets based on their hash codes.

Method Signature:

java code:

public native int hashCode();

The default implementation of hashCode in the Object class returns a value derived from the object's memory address, typically unique for each object instance.

Relationship Between equals and hashCode:

If two objects are equal according to the equals method, their hashCode values must also be the same.

If two objects are not equal according to the equals method, their hashCode values can be the same (but ideally should be different to minimize collisions).

When overriding equals, you must also override hashCode to maintain this contract.

Example Override:

Here’s an example of how to override hashCode in a custom class:

java code:

import java.util.Objects;

public class Person {
    private String name;
    private int age;

    // Constructor
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    // Override equals
    @Override
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        }
        if (!(obj instanceof Person)) {
            return false;
        }
        Person other = (Person) obj;
        return this.age == other.age && this.name.equals(other.name);
    }

    // Override hashCode
    @Override
    public int hashCode() {
        return Objects.hash(name, age);
    }

    // Main method for demonstration
    public static void main(String[] args) {
        Person p1 = new Person("Alice", 30);
        Person p2 = new Person("Alice", 30);
        Person p3 = new Person("Bob", 25);

        System.out.println("p1.equals(p2): " + p1.equals(p2)); // true
        System.out.println("p1.hashCode() == p2.hashCode(): " + (p1.hashCode() == p2.hashCode())); // true
        System.out.println("p1.equals(p3): " + p1.equals(p3)); // false
        System.out.println("p1.hashCode() == p3.hashCode(): " + (p1.hashCode() == p3.hashCode())); // false
    }
}

Explanation of the Code:

    Equals Method:

        Compares two Person objects for equality based on their name and age fields.

    HashCode Method:

        Uses Objects.hash() to generate a hash code based on the name and age fields.

        This ensures that hashCode is consistent with equals.

    Demonstration:

        Two Person objects with the same name and age have the same hashCode value and are considered         equal.

        Different Person objects produce different hash codes.

Key Points:

    Consistency:

        If the state of an object does not change, the hashCode method should consistently return the same         value.

    Performance:

        A good hashCode implementation minimizes collisions, improving the performance of hash-based         collections.

Best Practices:

    Use the Objects.hash() utility method for simplicity and reliability when calculating hash codes.

    Include the same fields in hashCode as in equals.

This ensures that your custom class works effectively in collections like HashMap or HashSet


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