What Is HashMap in Data Structure Can You Provide an Example?


Heather Bennett

HashMap is a crucial data structure in computer science that allows for efficient storage and retrieval of key-value pairs. It is a part of the Java Collections Framework and provides a powerful tool for solving various problems. In this article, we will explore what HashMap is, how it works, and provide an example to illustrate its usage.

What is HashMap?

HashMap is a class in Java that implements the Map interface. It represents a collection of key-value pairs where each key is unique. It allows for quick retrieval of values based on their associated keys, making it suitable for solving problems that involve frequent search operations.

The HashMap class uses an array internally to store the key-value pairs. Each element in this array is known as a bucket. When we insert a key-value pair into the HashMap, it calculates the hash code of the key and uses it to determine the index of the corresponding bucket.

If multiple keys have the same hash code, known as hash collisions, they are stored in the same bucket using a linked list or other similar data structure. This ensures that all values with equal hash codes can be stored and retrieved correctly.

Example Usage

To better understand how HashMap works, let’s consider an example where we want to store information about students in a class. We can use their roll numbers as keys and their names as values.

import java.util.HashMap;

public class StudentDatabase {
public static void main(String[] args) {
// Create a new HashMap instance
HashMap studentDatabase = new HashMap<>();

// Add students to the database
studentDatabase.put(1, “John Doe”);
studentDatabase.put(2, “Jane Smith”);
studentDatabase.put(3, “Alex Johnson”);

// Retrieve values based on keys
String nameOfStudentOne = studentDatabase.get(1);
String nameOfStudentTwo = studentDatabase.get(2);

// Print the retrieved values
System.out.println(“Name of Student 1: ” + nameOfStudentOne);
System.println(“Name of Student 2: ” + nameOfStudentTwo);

In this example, we create a new HashMap called `studentDatabase`. We then use the `put` method to add key-value pairs to the HashMap, where the key represents the roll number and the value represents the name of the student.

We can retrieve values from the HashMap using the `get` method by providing the corresponding key. In this case, we retrieve the names of Student 1 and Student 2 and print them to the console.

When you run this code, you will see:

  • Name of Student 1: John Doe
  • Name of Student 2: Jane Smith

Advantages of Using HashMap

HashMap offers several advantages:

  • Efficient Search: Retrieving values from a HashMap is very fast as it uses hash codes to determine indexes.
  • Flexible Key-Value Pairs: HashMap allows any object as keys and values, providing flexibility in storing different types of data.
  • No Duplicate Keys: Each key in a HashMap must be unique. If you try to insert a duplicate key, it will replace the existing value associated with that key.


In conclusion, HashMap is a versatile data structure that provides efficient storage and retrieval of key-value pairs. It is widely used in various programming scenarios, including database management, caching, and indexing. By understanding how HashMap works and its advantages, you can leverage its power to solve complex problems more effectively.

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