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
In this short article, we’ll be looking at an interesting data structure in the Apache Commons Collections library – the BidiMap.
The BidiMap adds a possibility of looking up the key using the corresponding value on top of the standard Map interface.
2. Dependencies
We need to include the following dependency in our project for us to use BidiMap and its implementations. For Maven based projects, we have to add the following dependency to our pom.xml:
<dependency>
<groupId>org.apache.commons</groupId>
<artifactId>commons-collections4</artifactId>
<version>4.5.0-M2</version>
</dependency>For Gradle-based projects, we have to add the same artifact to our build.gradle file:
compile 'org.apache.commons:commons-collections4:4.5.0-M2'The latest version of this dependency can be found on Maven Central.
3. Implementations and Instantiation
BidiMap itself is just an interface that defines behaviors unique to a bi-directional map – and there are of course multiple implementations available.
It’s important to understand that implementations of BidiMap do not allow key and value duplicates. When a BidiMap gets inverted, any duplicate values will be converted to duplicate keys and will violate the map contract. A map must always have unique keys.
Let’s look at different concrete implementations of this interface:
- DualHashBidiMap: This implementation uses two HashMap instances to implement the BidiMap internally. It provides fast lookup of entries using either an entry’s key or value. However, two instances of HashMap have to be maintained
- DualLinkedHashBidiMap: This implementation uses two LinkedHashMap instances and consequently maintains the insertion order of map entries. If we don’t need the insert order of the map entries to be maintained, we can just use the less expensive DualHashBidiMap
- TreeBidiMap: This implementation is efficient and is realized by a Red-Black tree implementation. The keys and values of TreeBidiMap are guaranteed to be sorted in ascending order using the natural ordering of the keys and values
- There is also DualTreeBidiMap that uses two instances of TreeMap to achieve the same thing as TreeBidiMap. DualTreeBidiMap is obviously more expensive than TreeBidiMap
The BidiMap interface extends the java.util.Map interface and so can serve as a drop-in replacement for it. We can use the no-arg constructor of the concrete implementations to instantiate a concrete object instance.
4. Unique BidiMap Methods
Now that we’ve explored the different implementations let’s look at methods that are unique to the interface.
The put() inserts a new key-value entry into the map. Note that if the value of the new entry matches the value of any existing entry, the existing entry will be removed in favor of the new entry.
The method returns the removed old entry or null if there’s none:
BidiMap<String, String> map = new DualHashBidiMap<>();
map.put("key1", "value1");
map.put("key2", "value2");
assertEquals(map.size(), 2);The inverseBidiMap() reverses the key-value pair of a BidiMap. This method returns a new BidiMap where the keys have become the values and vice-versa. This operation can be very useful in translation and dictionary applications:
BidiMap<String, String> rMap = map.inverseBidiMap();
assertTrue(rMap.containsKey("value1") && rMap.containsKey("value2"));The removeValue() is used to remove a map entry by specifying a value, instead of a key. This is an addition to Map implementations found in the java.util package:
map.removeValue("value2");
assertFalse(map.containsKey("key2"));We can get the key mapped to a particular value in BidiMap with the getKey(). The method returns null if no key is mapped onto the specified value:
assertEquals(map.getKey("value1"), "key1");5. Conclusion
This quick tutorial provided a look into the Apache Commons Collections library – specifically at BidiMap, its implementations and idiosyncratic methods.
The most exciting and distinguishing feature of BidiMap is its ability to look up and manipulate entries via keys as well as values.
