How HashMap works in Java

https://www.javainterviewpoint.com/hashmap-works-internally-java/

How a HashMap Works internally has become a popular question in almost all the interview. As almost everybody knows how to use a HashMap or the difference between HashMap and Hashtable. But many fails when the question is how does a hashmap internally works.

So the answer to the question how does a hashmap works is that is it works based on the hashing principle but it is not as simple as it sounds. Hashing is the mechanism of assigning the unique code to a variable or attribute using some algorithm to enable easy retrieval. A true hashing mechanism should always return the same hashCode() when it is applied to the same object.

Then comes the question how does hashing help in storing and retrieving the value in HashMap. Many answers the value will be stored in the bucket and retrieved using the key if you think that is how it works then you are absolutely wrong.  To prove let’s take a look at the hashmap class.

     /**
     * The table, resized as necessary. Length MUST always be a power of two.
     */
     transient Entry[] table;

So what is the use of Entry[] in a hashmap? Because the HashMap stores the Objects as Entry instances, not as key and value

What is Entry Class?

The HashMap has an inner class called as Entry Class which hold the key, value stuff. And there is something called as next, hash which you will get to know a bit later.

 static class Entry<K,V> implements Map.Entry<K,V>
 {
     final K key;
     V value;
     Entry<K,V> next;
     final int hash;
     ........
 }

As of now, you know that the hashmap stores the Entry instances in an array and not as a key,value pairs. In order to store a value you will use the put() method of the hashmap, so now let’s dig into that and see how it works.

how put() method works internally ?

The Code implementation of the put method will be like below

public V put(K key, V value)
{
    if (key == null)
       return putForNullKey(value);
    int hash = hash(key.hashCode());
    int i = indexFor(hash, table.length);
    for (Entry<K,V> e = table[i]; e != null; e = e.next)
    {
        Object k;
        if (e.hash == hash && ((k = e.key) == key || key.equals(k)))
         {
             V oldValue = e.value;
             e.value = value;
             e.recordAccess(this);
             return oldValue;
          }
     }
     modCount++;
     addEntry(hash, key, value, i);
     return null;
 }

• First, it checks for the if the key given is null or not, if the given key is null it will be stored in the ‘0’th position as the hashcode of null will be zero.
• Then it applies the hashcode to the key.hashCode() by calling the hashcode method. In order to get the value within the limits of an array the hash(key.hashCode())is called which does some shifting operations to the hashcode.
• The indexFor() method is used to get the exact location to store the Entry object.
• Then comes the most important part what happens if two different object has the same hashcode( eg : Aa,BB will have the same hashcode) and will it be stored in the same bucket. To handle this let’s think of the LinkedList in data structure it will have a next attribute which will always point to the next object . The same way the next attribute in the Entry class points to the next object. Using this different objects with the same hashcode will be placed next to each other.
• In the case of the Collision , the HashMap checks for the value of the next attribute if it is null it inserts the Entry object in that location , if next attribute is not null then it keeps the loop running till next attribute is null then stores the Entry object there.

How duplicate key is prevented in hashmap ?

As we all know hashmap doesn’t allow duplicates in the key even though we insert the same key with different values the latest value only is returned.

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

public class HashMapEg
{
	public static void main(String[] args)
	{
		Map map = new HashMap();
		map.put(1,"sam");
		map.put(1,"Ian");
		map.put(1,"Scott");
		map.put(null,"asdf");

		System.out.println(map);  

	}

}

For the above code, you will get the output as {null=asdf, 1=Scott}  as the values sam,Ian will be replaced by Scott , So how does this happen.

All the Entry Objects in the LinkedList will have the same hashcode but hashmap uses  equals () method checks the equality if key.equals(k) is true then it will replace the value object inside the Entry class and not the key. So this way it prevents the duplicate key being inserted.

how get() method works internally ?

It is almost the same logic as applied in put will be used to retrieve the value.

public V get(Object key)
{
    if (key == null)
       return getForNullKey();
     int hash = hash(key.hashCode());
     for (Entry<K,V> e = table[indexFor(hash, table.length)];e != null;e = e.next)
     {
         Object k;
         if (e.hash == hash && ((k = e.key) == key || key.equals(k)))
             return e.value;
     }
         return null;
 }

• First, it gets the hashcode of the key object which is passed and finds the bucket location.
• If the correct bucket is found it returns the value (e.value)
• If no match is found it returns null.

What happens if two keys have the same hashcode?

The same collision resolution mechanism will be used here  key.equals(k)  will check until it is true, if true it returns the value of it.

Hope this article clarifies the troublesome HashMap internal mechanism. Happy Learning !!