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. Introduction
In this quick guide, we’ll discuss the term “concrete class” in Java.
First, we’ll define the term. Then, we’ll see how it’s different from interfaces and abstract classes.
2. What Is a Concrete Class?
A concrete class is a class that we can create an instance of, using the new keyword.
In other words, it’s a full implementation of its blueprint. A concrete class is complete.
Imagine, for example, a Car class:
public class Car {
public String honk() {
return "beep!";
}
public String drive() {
return "vroom";
}
}Because all of its methods are implemented, we call it a concrete class, and we can instantiate it:
Car car = new Car();Some examples of concrete classes from the JDK are HashMap, HashSet, ArrayList, and LinkedList.
3. Java Abstraction vs. Concrete Classes
Not all Java types implement all their methods, though. This flexibility, also called abstraction, allows us to think in more general terms about the domain we’re trying to model.
In Java, we can achieve abstraction using interfaces and abstract classes.
Let’s get a better look at concrete classes by comparing them to these others.
3.1. Interfaces
An interface is a blueprint for a class. Or, in other words, its a collection of unimplemented method signatures:
interface Driveable {
void honk();
void drive();
}Note that it uses the interface keyword instead of class.
Because Driveable has unimplemented methods, we can’t instantiate it with the new keyword.
But, concrete classes like Car can implement these methods.
The JDK provides a number of interfaces like Map, List, and Set.
3.2. Abstract Classes
An abstract class is a class that has unimplemented methods, though it can actually have both:
public abstract class Vehicle {
public abstract String honk();
public String drive() {
return "zoom";
}
}Note that we mark abstract classes with the keyword abstract.
Again, since Vehicle has an unimplemented method, honk, we won’t be able to use the new keyword.
Some examples of abstract classes from the JDK are AbstractMap and AbstractList.
3.3. Concrete Classes
In contrast, concrete classes don’t have any unimplemented methods. Whether the implementations are inherited or not, so long as each method has an implementation, the class is concrete.
Concrete classes can be as simple as our Car example earlier. They can also implement interfaces and extend abstract classes:
public class FancyCar extends Vehicle implements Driveable {
public String honk() {
return "beep";
}
}The FancyCar class provides an implementation for honk and it inherits the implementation of drive from Vehicle.
As such, it has no unimplemented methods. Therefore, we can create a FancyCar class instance with the new keyword.
FancyCar car = new FancyCar();Or, simply put, all classes which are not abstract, we can call concrete classes.
4. Summary
In this short tutorial, we learned about concrete classes and their specifications.
Additionally, we showed the differences between interfaces and concrete and abstract classes.
