What Is a HashMap Data Structure in Java?


Angela Bailey

HashMap is a widely used data structure in Java that provides an efficient way to store and retrieve key-value pairs. It is part of the Java Collections Framework and is implemented by the HashMap class.

What Is a HashMap?
A HashMap is a data structure that allows you to store elements as key-value pairs. Each element in the HashMap consists of a unique key and its corresponding value. This key-value mapping allows for efficient retrieval of values based on their keys.

How Does a HashMap Work?
Under the hood, a HashMap uses an array of linked lists or buckets to store the key-value pairs. When you add an element to a HashMap, it calculates the hash code of the key using the hashCode() method. The hash code determines which bucket the element should go into.

If multiple elements have the same hash code, they are stored in the same bucket as a linked list. This means that each bucket can contain multiple elements with different keys but potentially identical hash codes.

When you want to retrieve a value from a HashMap, it calculates the hash code of the given key and finds the corresponding bucket. It then iterates through the linked list in that bucket to find the exact key-value pair.

  • Benefits of Using a HashMap:
    • Fast retrieval: Since HashMap uses hashing, it can quickly calculate where an element is stored based on its key.
    • Flexible size: Unlike arrays with fixed sizes, HashMaps can dynamically resize themselves as more elements are added.
    • Efficient storage: A HashMap only allocates memory for its current size, making it memory-efficient.
  • Common Use Cases:
    • Caching: A HashMap can be used for caching frequently accessed data by storing results with their corresponding inputs (keys).
    • Indexing: HashMaps are commonly used in search algorithms and indexing applications where fast retrieval is essential.
    • Associative arrays: HashMaps provide an associative array-like behavior, where you can retrieve a value based on its key.
  • HashMap vs. HashTable:
    • HashMap is not synchronized, whereas HashTable is synchronized. This means that HashMap is not thread-safe by default.
    • HashMap allows null values and only one null key, while HashTable does not allow null keys or values.

Example Usage:

Suppose we want to store the prices of different products in a HashMap:

import java.util.HashMap;

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

// Add key-value pairs to the HashMap
productPrices.put(“Apple”, 0.99);
productPrices.put(“Banana”, 0.5);
productPrices.put(“Orange”, 1.25);

// Retrieve the price of an Apple
double applePrice = productPrices.get(“Apple”);

System.out.println(“The price of an Apple is $” + applePrice);

In the above example, we create a new HashMap called `productPrices`, where the keys are `String` objects representing the product names and the values are `Double` objects representing their prices.

We add three key-value pairs using the `put()` method, specifying the product name as the key and its price as the value.

To retrieve the price of an Apple, we use the `get()` method with “Apple” as the key. The returned value is then printed to the console.

HashMap is a powerful data structure in Java that allows for efficient storage and retrieval of key-value pairs. By leveraging hashing, it provides fast access to elements, making it suitable for various use cases such as caching, indexing, and associative arrays. However, it’s important to note the differences between HashMap and HashTable when considering thread safety and null values.

With a clear understanding of HashMap and its usage, you can now incorporate this versatile data structure into your Java applications.

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