Hashcode() in Java

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Introduction

In computer science, hashing is a concept that involves mapping entities or objects to integer values. The values of those entities are called hashcodes or hash values.

In Java, to compute hash values of objects, the hashCode() method is used. In Java, the Integer class has two methods:

  • hashCode(): It computes the hash values of the Integer.
  • hashCode(int value): It computes the hash values of primitive int values.

Properties of Hashing

  • When the output of the equals() method is the same as the object then they have the same hash value. That means they need to be mapped to the same Integer value.
  • When two objects are not equal, they may or may not have the same hash values. That is why different objects do not need to have different hash values.
  • When hash values of objects are computed multiple times, their hash values should remain consistent.
  • When invoked more than once during the execution, the output of hashing algorithm must be the same for the same objects.
  • But, from one execution to another execution the value does not need to stay consistent.

Java hashCode() Method

To compute the hash values of input objects, there is the hashCode() method in java. The value of that integer which represents the input object’s hash value, that integer is returned.
To produce the hash values of objects, the hashCode() method is used. These objects are then stored in Java collections such as HashMap, HashSet and HashTable by using these hash values.

In this example, we will see the properties of hashing by testing the following things:

Two objects having the same values also have the same hashcodes.
Objects that do not have the same values also don’t have the same hashcodes.
When the hashcodes of the same object are computed again, it gives the same hashcode or the hashcode is consistent.

Code

import java.io.*;

public class hash {

  public static void main(String args[]) {
    String a1 = "100";         // declaring two variables
    String a2 = "100";        // having same value
     
    // Printing the hashcodes of a1 and a2
    System.out.println("HashCode of a1 = " + a1 + " " + "is: " + a1.hashCode());
    System.out.println("HashCode of a2 = " + a2 + " " + "is: " + a2.hashCode());
   
    // Declaring a third variable with a different value
    String a3 = "500";

    // Printing the hashcode of a3
    System.out.println("HashCode of a3 = " + a3 + " " + "is: " + a3.hashCode());
   
    // Computing hashcode of variable a2 again
    System.out.println("HashCode of a2 = " + a2 + " " + "is: " + a2.hashCode());
  }
}

Output:

HashCode of a1 = 100 is: 48625
HashCode of a2 = 100 is: 48625
HashCode of a3 = 500 is: 52469
HashCode of a2 = 100 is: 48625

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Integer class hashCode(int value) Method

As can be seen clearly, this method computes the hash value of its parameter and return its hashcode.

Code

// Example of hashCode(int value)
public class Main {

  public static void main(String[] args) {
    int x = 144;

    System.out.println(
      "HashCode of x = " + x + " "+ "is: " + Integer.hashCode(x)
    );
  }
}

Output:

HashCode of value = 144 is : 144

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