What is Java Integer hashCode() Method?

In Java, the hashCode() method is a method of the Object class that returns an integer value representing the hash code of an object. It is widely used in hash-based collections like HashMap, HashSet, and Hashtable. Every object in Java inherits the hashCode() method from the Object class, but it is often overridden for custom classes to improve performance in hash-based collections.

hashCode() in the Integer class

The Integer class, which is a part of java.lang, overrides the hashCode() method from the Object class to return the int value itself. This makes sense because the hash code for integers can simply be the integer value, as it’s already a primitive type and doesn’t require additional processing.

Method Signature:

public int hashCode()

• The method returns an int value, which is the hash code of the integer object.

Usage in the Integer class:

For Integer, the hashCode() method simply returns the integer value stored in the object, as Integer is a wrapper class around the primitive int.

Example:

public class IntegerHashCodeExample {
    public static void main(String[] args) {
        Integer num1 = new Integer(42);
        Integer num2 = new Integer(42);
        Integer num3 = new Integer(99);
        
        // Printing hash codes of Integer objects
        System.out.println("HashCode of num1: " + num1.hashCode());  // Output: 42
        System.out.println("HashCode of num2: " + num2.hashCode());  // Output: 42
        System.out.println("HashCode of num3: " + num3.hashCode());  // Output: 99
        
        // Checking if two Integer objects with the same value have the same hash code
        System.out.println("Hash codes of num1 and num2 are equal: " + (num1.hashCode() == num2.hashCode()));  // Output: true
    }
}

Explanation:

• In the example above, the hashCode() method of the Integer class returns the integer value itself (42 for num1 and num2, and 99 for num3).
• Since both num1 and num2 store the value 42, they return the same hash code, demonstrating that two Integer objects with the same value have the same hash code.

Key Points About hashCode() in Integer:

1. hashCode() Returns the Integer Value:
   • For Integer, the hashCode() method returns the int value directly, which is stored inside the Integer object.
   • For example, Integer x = 42; would return 42 when calling x.hashCode().
2. Consistency:
   • The hashCode() method for the Integer class always returns the same value as long as the value of the integer object doesn’t change. This ensures consistency in collections like HashMap or HashSet.
3. Equality and Hash Code:
   • The hashCode() method must fulfill the contract of equals() and hashCode(). If two objects are considered equal (i.e., x.equals(y) returns true), they must have the same hash code. However, it is not necessary for two objects with the same hash code to be equal, though it is a desirable property for performance reasons.

import java.util.HashMap;

public class HashMapExample {
    public static void main(String[] args) {
        HashMap<Integer, String> map = new HashMap<>();
        
        map.put(42, "Answer to the Ultimate Question");
        map.put(99, "Highway Number");
        
        // Accessing the values using keys
        System.out.println("Value for key 42: " + map.get(42));
        System.out.println("Value for key 99: " + map.get(99));
    }
}