How to Implement a Custom Hash function for Keys in a HashMap in Java?

In Java, HashMap is the data structure that implements the Map interface. This is used to save the data in the form of key-value pairs. In this article, we will learn how to implement a Custom Hash function for keys in a HashMap in Java.

In Java, implementing the custom hash function for keys in a HashMap requires overriding the hashcode() method in the class of the keys to correctly identify and store the keys in the HashMap. The hashcode() method generates a hash code for an object, which is used by HashMap.

Program to Implement a Custom Hash Function For Keys in a HashMap

Below is the Program to Implement a Custom Hash Function For Keys in a HashMap:

Java

// Java Program to Implement a Custom
// Hash function for keys in a HashMap
import java.util.HashMap;

// Driver Class
public class GfGCustomHashCode
{
      // Main Method
    public static void main(String[] args) {
        // Create a HashMap with CustomKey as keys
        HashMap<CustomKey, String> map = new HashMap<>();

        // Create CustomKey objects
        CustomKey key1 = new CustomKey("key1");
        CustomKey key2 = new CustomKey("key2");
        CustomKey key3 = new CustomKey("key3");

        // Put key-value pairs into the map
        map.put(key1, "Java");
        map.put(key2, "JavaScript");
        map.put(key3, "Java");

        System.out.println(map);

        // retrieve values using keys
        System.out.println(map.get(key1));
        System.out.println(map.get(key2));
        System.out.println(map.get(key3));
    }
}

class CustomKey
{
    private String keyData;

    public CustomKey(String keyData)
    {
        this.keyData = keyData;
    }

    // Override hashCode() method
    @Override
    public int hashCode() {
        return keyData.hashCode();
    }

    // Override equals() method
    @Override
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        }
        if (obj == null || getClass() != obj.getClass()) {
            return false;
        }
        CustomKey other = (CustomKey) obj;
        return keyData.equals(other.keyData);
    }

    // Override toString() method
    @Override
    public String toString() {
        return keyData;
    }
}

Output

{key1=Java, key2=JavaScript, key3=Java}
Java
JavaScript
Java

Explanation of the Program:

• We have defined a HashMap that uses custom keys and stores string values.
• Each CustomKey is associated with a string, and the HashMap is populated with three key-value pairs.
• Then the CustomKey class overrides the hashCode(), equals(), and toString() methods.
• This identified that keys are correctly identified and stored in the HashMap.
• After that, it retrieves the values associated with each key from the HashMap and prints them.