How HashMap works internally || Popular java interview question on collection (HashMap)

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03 Dec 2019

In this tutorial, we are going to learn how hashmap works internally.This is a very popular java interview question from the collection framework and been asked so many times to check candidates understanding on Map collection.

I am going to cover three things in this series of tutorials which will cover a hashmap internal implementation.

1) We will see a detailed overview of how a map works internally when we use map.put(Key k, Value v )

2)Once we will have an overall idea we are going to debug
everything that we learned in this tutorial

3)In the last tutorial of this series, I will cover how the get method works internally ( map.get(Key K) )

So when we create a map object internally an array of buckets will be created with a size 16 (if we use the default constructor.)Then when we use the map.put method, internally the hashcode and the index will be calculated for the key and the key will be stored in a specific index.

In case of a hash collision, then the key with the same hashcode is going to be stored in the same index as each bucket is a node or a linked list.

Inside each bucket, four things are going to be stored as a form of a linked list.They are hashcode, key, value and the next (which hold the address of the next node with the same hashcode).

So lets understand everything diagrammatically because this is one of the very popular hashmap interview questions.So please put this question on your to-do list and better be prepared for it.

“How hashmap works internally in java with example"

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