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.
Distributed systems often come with complex challenges such as service-to-service communication, state management, asynchronous messaging, security, and more.
Dapr (Distributed Application Runtime) provides a set of APIs and building blocks to address these challenges, abstracting away infrastructure so we can focus on business logic.
In this tutorial, we'll focus on Dapr's pub/sub API for message brokering. Using its Spring Boot integration, we'll simplify the creation of a loosely coupled, portable, and easily testable pub/sub messaging system:
1. Overview
A ClassToInstanceMap<B> is a special kind of map that associates classes with corresponding instances. It makes sure that all the keys and the values are subtypes of the upper bound type B.
ClassToInstanceMap extends Java’s Map interface and provides two additional methods: T getInstance(Class<T>) and T putInstance(Class<T>, T). The advantage of this map is that these two methods can be used to perform type-safe operations and avoid casting.
In this tutorial, we will show how to use the Google Guava’s ClassToInstanceMap interface and its implementations.
2. Google Guava’s ClassToInstanceMap
Let’s have a look at how to use the implementation.
We’ll start by adding the Google Guava library dependency in the pom.xml:
<dependency>
<groupId>com.google.guava</groupId>
<artifactId>guava</artifactId>
<version>32.1.3-jre</version>
</dependency>The latest version of the dependency can be checked here.
ClassToInstanceMap interface has two implementations: a mutable and an immutable one. Let’s have a look at each of them separately.
3. Creating an ImmutableClassToInstanceMap
We can create an instance of ImmutableClassToInstanceMap in a number of ways:
- using the of() method to create an empty map:
ImmutableClassToInstanceMap.of() - using the of(Class<T> type, T value) method to create a single entry map:
ImmutableClassToInstanceMap.of(Save.class, new Save()); - using copyOf() method which accepts another map as a parameter. It will create a map with the same entries as map provided as the parameter:
ImmutableClassToInstanceMap.copyOf(someMap) - using the builder:
ImmutableClassToInstanceMap.<Action>builder() .put(Save.class, new Save()) .put(Open.class, new Open()) .put(Delete.class, new Delete()) .build();
4. Creating a MutableClassToInstanceMap
We can also create an instance of MutableClassToInstanceMap:
- using the create() method which makes an instance backed by HashMap:
MutableClassToInstanceMap.create(); - using the create(Map<Class<? extends B>, B> backingMap) which makes an instance backed by the provided empty map:
MutableClassToInstanceMap.create(new HashMap());
5. Usage
Let’s have a look how to use the two new methods that are added to the regular Map interface:
- first method is <T extends B> T getInstance(Class<T> type):
Action openAction = map.get(Open.class); Delete deleteAction = map.getInstance(Delete.class); - second method is <T extends B> T putInstance(Class<T> type, @Nullable T value):
Action newOpen = map.put(Open.class, new Open()); Delete newDelete = map.putInstance(Delete.class, new Delete());
6. Conclusion
In this quick tutorial, we showed examples how to use ClassToInstanceMap from the Guava library.
