Java Maps Series
Last updated: June 10, 2026
Mocking is an essential part of unit testing, and the Mockito library makes it easy to write clean and intuitive unit tests for your Java code.
Get started with mocking and improve your application tests using our Mockito guide:
Handling concurrency in an application can be a tricky process with many potential pitfalls. A solid grasp of the fundamentals will go a long way to help minimize these issues.
Get started with understanding multi-threaded applications with our Java Concurrency guide:
Spring 5 added support for reactive programming with the Spring WebFlux module, which has been improved upon ever since. Get started with the Reactor project basics and reactive programming in Spring Boot:
Since its introduction in Java 8, the Stream API has become a staple of Java development. The basic operations like iterating, filtering, mapping sequences of elements are deceptively simple to use.
But these can also be overused and fall into some common pitfalls.
To get a better understanding on how Streams work and how to combine them with other language features, check out our guide to Java Streams:
Explore Spring Boot 3 and Spring 6 in-depth through building a full REST API with the framework:
Yes, Spring Security can be complex, from the more advanced functionality within the Core to the deep OAuth support in the framework.
I built the security material as two full courses - Core and OAuth, to get practical with these more complex scenarios. We explore when and how to use each feature and code through it on the backing project.
You can explore the course here:
Spring Data JPA is a great way to handle the complexity of JPA with the powerful simplicity of Spring Boot.
Get started with Spring Data JPA through the guided reference course:
Refactor Java code safely — and automatically — with OpenRewrite.
Refactoring big codebases by hand is slow, risky, and easy to put off. That’s where OpenRewrite comes in. The open-source framework for large-scale, automated code transformations helps teams modernize safely and consistently.
Each month, the creators and maintainers of OpenRewrite at Moderne run live, hands-on training sessions — one for newcomers and one for experienced users. You’ll see how recipes work, how to apply them across projects, and how to modernize code with confidence.
Join the next session, bring your questions, and learn how to automate the kind of work that usually eats your sprint time.
The Map interface is one of Java’s most powerful data structures, enabling fast key-value lookups with a range of implementations suited to different needs. From the general-purpose HashMap to the ordered TreeMap, insertion-ordered LinkedHashMap, and thread-safe ConcurrentHashMap, choosing the right map type and using it correctly is a core Java skill.
This series covers practical Map operations from fundamentals to advanced patterns, organized by task: from picking the right implementation and initializing maps, through reading and modifying entries, iterating and streaming, sorting and comparing, and converting maps to and from other types.
Map Implementations
- A Guide to Java HashMap
- The Java HashMap Under the Hood
- A Guide to ConcurrentMap
- A Guide to TreeMap in Java
- A Guide to LinkedHashMap in Java
- A Guide to EnumMap
- Immutable Map Implementations in Java
- Collections.synchronizedMap vs. ConcurrentHashMap
Creating and Initializing Maps
- Initialize a HashMap in Java
- Difference Between Map.ofEntries() and Map.of()
- Create an Empty Map in Java
- Copying a HashMap in Java
- HashMap with Multiple Values for the Same Key
- Implementing a Map with Multiple Keys in Java
Reading, Updating, and Removing Entries
- The Map.computeIfAbsent() Method
- Update the Value Associated With a Key in a HashMap
- Difference Between putIfAbsent() and computeIfAbsent() in Java’s Map
- Putting Value Into Map if Not Null in Java
- How to Increment a Map Value in Java
- Remove an Entry from a Java HashMap
- How to Check If a Key Exists in a Map
- Using a Custom Class as a Key in a Java HashMap
Iterating and Streaming Maps
- Iterate Over a Map in Java
- Java Collectors toMap
- Working With Maps Using Streams
- Using the Map.Entry Java Class
- Java Map – keySet() vs. entrySet() vs. values() Methods
- Handle Duplicate Keys When Producing Map Using Java Stream
Sorting, Searching, and Comparing Maps
- Sort a HashMap in Java
- Merging Two Maps with Java
- Comparing Two HashMaps in Java
- Get the Key for a Value from a Java Map
- Finding the Highest Value in a Java Map
- Sorting Java Map in Descending Order
Converting Maps
