In Hashmap one null key is allowed and multiple null values are allowed. Since hashcode of null is 0 so null key actually stored at index 0 in hashmap.
Hashmap in java actsually created as Array of linkedlist where index in array is find out by hashing on key.
package com.w3schools; import java.util.HashMap; import java.util.Map; public class Test { public static void main(String args[]){ MaphashMap = new HashMap<>(); Site site = new Site("w3schools", 1); //Insert element with null key hashMap.put(null, site); } } class Site{ private String name; private int id; public Site(String name, int id) { super(); this.name = name; this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } public int getId() { return id; } public void setId(int id) { this.id = id; } @Override public String toString() { return this.getId() + "-" + this.getName(); } @Override public int hashCode() { //For simplicity we are returning id as hashCode value return this.id; } @Override public boolean equals(final Object obj) { if (this == obj) return true; if (obj == null) return false; if (getClass() != obj.getClass()) return false; final Site siteObject = (Site) obj; if(this.getName() == siteObject.getName() && this.getId() == siteObject.getId()) return true; else return false; } }