HashMap in Java

A HashMap is a part of Java’s Collection Framework and implements the Map interface. It stores elements in key-value pairs, where, Keys are unique. and Values can be duplicated.

• Internally uses Hashing, hence allows efficient key-based retrieval, insertion, and removal with an average of O(1) time.
• HashMap is not thread-safe, to make it synchronized, use Collections.synchronizedMap().
• Insertion order is not preserved in HashMap. To preserve the insertion order, LinkedHashMap is used and to maintain sorted order, TreeMap is used.

HashMap Declaration;

public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable

It takes two parameters, namely as follows:

• K -> Type of keys maintained by this map
• V -> Type of mapped values

import java.util.HashMap;
import java.util.Map;

public class GFG{
    
    public static void main(String[] args){

        // Create a HashMap
        HashMap<String, Integer> hashMap = new HashMap<>();

        // Add elements to the HashMap
        hashMap.put("John", 25);
        hashMap.put("Jane", 30);
        hashMap.put("Jim", 35);

        // Iterate through the HashMap
        for (Map.Entry<String, Integer> entry : hashMap.entrySet()) {
            System.out.println(entry.getKey() + " -> " + entry.getValue());
        }
    }
}

John -> 25
Jane -> 30
Jim -> 35

Hierarchy of HashMap in Java

It extends AbstractMap and implements the Map Interface.