How does a Java HashMap handle different objects with the same hash code?

Question

As per my understanding I think:

1. It is perfectly legal for two objects to have the same hashcode.
2. If two objects are equal (using the equals() method)

Answer 1

It gonna be a long answer , grab a drink and read on …

Hashing is all about storing a key-value pair in memory that can be read and written faster. It stores keys in an array and values in a LinkedList .

Lets Say I want to store 4 key value pairs -

{
“girl” => “ahhan” , 
“misused” => “Manmohan Singh” , 
“horsemints” => “guess what”, 
“no” => “way”
}

So to store the keys we need an array of 4 element . Now how do I map one of these 4 keys to 4 array indexes (0,1,2,3)?

So java finds the hashCode of individual keys and map them to a particular array index . Hashcode Formulae is -

1) reverse the string.

2) keep on multiplying ascii of each character with increasing power of 31 . then add the components .

3) So hashCode() of girl would be –(ascii values of  l,r,i,g are 108, 114, 105 and 103) . 

e.g. girl =  108 * 31^0  + 114 * 31^1  + 105 * 31^2 + 103 * 31^3  = 3173020

Hash and girl !! I know what you are thinking. Your fascination about that wild duet might made you miss an important thing .

Why java multiply it with 31 ?

It’s because, 31 is an odd prime in the form 2^5 – 1 . And odd prime reduces the chance of Hash Collision

Now how this hash code is mapped to an array index?

answer is , Hash Code % (Array length -1) . So “girl” is mapped to (3173020 % 3) = 1 in our case . which is second element of the array .

and the value “ahhan” is stored in a LinkedList associated with array index 1 .

HashCollision - If you try to find hasHCode of the keys “misused” and “horsemints” using the formulae described above you’ll see both giving us same 1069518484. Whooaa !! lesson learnt -

2 equal objects must have same hashCode but there is no guarantee if the hashCode matches then the objects are equal . So it should store both values corresponding to “misused” and “horsemints” to bucket 1 (1069518484 % 3) .

Now the hash map looks like –

Array Index 0 –
Array Index 1 - LinkedIst (“ahhan” , “Manmohan Singh” , “guess what”)
Array Index 2 – LinkedList (“way”)
Array Index 3 – 

Now if some body tries to find the value for the key “horsemints” , java quickly will find the hashCode of it , module it and start searching for it’s value in the LinkedList corresponding index 1 . So this way we need not search all the 4 array indexes thus making data access faster.

But , wait , one sec . there are 3 values in that linkedList corresponding Array index 1, how it finds out which one was was the value for key “horsemints” ?

Actually I lied , when I said HashMap just stores values in LinkedList .

It stores both key value pair as map entry. So actually Map looks like this .

Array Index 0 –
Array Index 1 - LinkedIst (<”girl” => “ahhan”> , <” misused” => “Manmohan Singh”> , <”horsemints” => “guess what”>)
Array Index 2 – LinkedList (<”no” => “way”>)
Array Index 3 – 

Now you can see While traversing through the linkedList corresponding to ArrayIndex1 it actually compares key of each entry to of that LinkedList to “horsemints” and when it finds one it just returns the value of it .

Hope you had fun while reading it :)