What is the difference between the HashMap and Map objects in Java?

java

collections

best-practices

interface

byNikita Barsukov·Sep 16, 2024

In a nutshell: Map is an interface outlining the shape of a mapping object, while HashMap is the implementation of this interface using a hash table. You'll typically use Map as your reference type for more flexibility and HashMap for its specific features.

// "An apple a day keeps the bugs away" - Anonymous programmer
Map<String, Integer> exampleMap = new HashMap<>(); 
exampleMap.put("apple", 10);  
exampleMap.put("orange", 20);

In the above part, exampleMap harnesses HashMap's power yet remains a Map at heart.

Choosing your battles: Map vs HashMap

When is a Map favorable over HashMap? Good question! The choice between the two often depends on design considerations. If meeting the requirements of an interface like Map, you are making your code less dependent on specific implementations, making it more generic and easier to maintain.

Welcome to the polymorphic world, where you can seamlessly swap different Map implementations like TreeMap, LinkedHashMap or ConcurrentHashMap as needed, promoting code reusability and setting your application up for future changes with minimal fuss.

Performance isn't everything, but it matters. The choice of Map type should also look at functionality and performance needs. HashMap offers constant time performance for basic operations, whereas a TreeMap guarantees order but at a minor premium time cost.

Unleashing the power: Generics and Map

Using a generic interface like Map when declaring a variable abstracts away the specific collection details. You focus on the actions—put, get, remove—rather than the how, adhering to the principle of least knowledge.

This approach to interface based coding reduces coupling, making your code more immune to changes, such as when you would prefer a different Map implementation for your use case.

Embrace the interface: Preparing for the future

In the quest for future-proof code, an interface-first approach has been the golden ticket. Envisage a situation in which your code begins with HashMap, then shifts to needs only TreeMap can satisfy. If only you'd used the Map interface, switches like this would be a piece of cake.

Speak your intention: Utilizing interfaces also helps your code speak its intent, making your code more readable. Code written in this manner relies on the functionality of objects rather than their implementation, keeping it clean and self-documenting.