Demystifying HashMap in Java: How it Works Under the Hood

How HashMap data structure works internally in Java

Hello guys, one of the popular topic of Java interview is Java collection framework and on that framework, HashMap is probably the most asked class. I have personally seen quetions like HashMap works? How get method of HashMap works, how put method of HashMap works, is HashMap thread-safe etc in my career many times.

In this article, I will try to share everything I know about HashMap so that you can answer such question with confidence.

To start with, HashMap is one of the most widely used data structures in Java, providing an efficient way to store and retrieve key-value pairs. It belongs to the Java Collections Framework and is an implementation of the Map interface.

It represent Hash table data structure, also known as dictionary in python or associative array in Perl. It allows you to map one value to other, so that you can get employee object by storing it against employeeId.

The beauty of this is that it allows get operation in constant time, which means it doesn’t matter how many records you need to store, you will also get the data in constant time. There are some edge cases also which we will discuss in this article