Java HashMap class implements the map interface by using a hash table. It inherits the AbstractMap class and implements the Map interface.

The important points about Java HashMap class:

A HashMap contains values based on the key.
It contains only unique elements.
It may have one null key and multiple null values.
It maintains no order.
Java HashMap is not thread-safe. You must explicitly synchronize concurrent modifications to the HashMap.

Create a HashMap in Java Example

The following example shows how to create a HashMap, and add new key-value pairs to it.

package com.javaguides.collections.hashmapexamples;

import java.util.HashMap;

import java.util.Map;

public class CreateHashMapExample {
    public static void main(String[] args) {
        // Creating a HashMap
     Map<String, Integer> numberMapping = new HashMap<>();

        // Adding key-value pairs to a HashMap
        numberMapping.put("One", 1);
        numberMapping.put("Two", 2);
        numberMapping.put("Three", 3);

        // Add a new key-value pair only if the key does not exist in the HashMap, or is mapped to `null`
        numberMapping.putIfAbsent("Four", 4);

        System.out.println(numberMapping);
        
    }
}