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Partner – Microsoft – NPI EA (cat = Baeldung)
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Azure Container Apps is a fully managed serverless container service that enables you to build and deploy modern, cloud-native Java applications and microservices at scale. It offers a simplified developer experience while providing the flexibility and portability of containers.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, visit the documentation page.

You can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Partner – Microsoft – NPI EA (cat= Spring Boot)
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Azure Container Apps is a fully managed serverless container service that enables you to build and deploy modern, cloud-native Java applications and microservices at scale. It offers a simplified developer experience while providing the flexibility and portability of containers.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, you can get started over on the documentation page.

And, you can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Partner – Orkes – NPI EA (cat=Spring)
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Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.

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Partner – Orkes – NPI EA (tag=Microservices)
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Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.

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eBook – Guide Spring Cloud – NPI EA (cat=Spring Cloud)
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eBook – Mockito – NPI EA (tag = Mockito)
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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.

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eBook – Java Concurrency – NPI EA (cat=Java Concurrency)
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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.

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eBook – Reactive – NPI EA (cat=Reactive)
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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:

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eBook – Java Streams – NPI EA (cat=Java Streams)
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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:

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eBook – Jackson – NPI EA (cat=Jackson)
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Do JSON right with Jackson

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eBook – HTTP Client – NPI EA (cat=Http Client-Side)
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eBook – Persistence – NPI EA (cat=Persistence)
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Building a REST API with Spring?

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Course – LS – NPI EA (cat=Jackson)
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Get started with Spring and Spring Boot, through the Learn Spring course:

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Course – RWSB – NPI EA (cat=REST)
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Explore Spring Boot 3 and Spring 6 in-depth through building a full REST API with the framework:

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Course – LSS – NPI EA (cat=Spring Security)
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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:

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Course – All Access – NPI EA (cat= Spring)
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All Access is finally out, with all of my Spring courses. Learn JUnit is out as well, and Learn Maven is coming fast. And, of course, quite a bit more affordable. Finally.

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Course – LSD – NPI EA (tag=Spring Data JPA)
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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:

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Partner – LambdaTest – NPI EA (cat=Testing)
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End-to-end testing is a very useful method to make sure that your application works as intended. This highlights issues in the overall functionality of the software, that the unit and integration test stages may miss.

Playwright is an easy-to-use, but powerful tool that automates end-to-end testing, and supports all modern browsers and platforms.

When coupled with LambdaTest (an AI-powered cloud-based test execution platform) it can be further scaled to run the Playwright scripts in parallel across 3000+ browser and device combinations:

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Course – Spring Sale 2025 – NPI EA (cat= Baeldung)
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Course – Spring Sale 2025 – NPI (cat=Baeldung)
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1. Overview

Inheritance and composition — along with abstraction, encapsulation, and polymorphism — are cornerstones of object-oriented programming (OOP).

In this tutorial, we’ll cover the basics of inheritance and composition, and we’ll focus strongly on spotting the differences between the two types of relationships.

2. Inheritance’s Basics

Inheritance is a powerful yet overused and misused mechanism.

Simply put, with inheritance, a base class (a.k.a. base type) defines the state and behavior common for a given type and lets the subclasses (a.k.a. subtypes) provide specialized versions of that state and behavior.

To have a clear idea on how to work with inheritance, let’s create a naive example: a base class Person that defines the common fields and methods for a person, while the subclasses Waitress and Actress provide additional, fine-grained method implementations.

Here’s the Person class:

public class Person {
    private final String name;

    // other fields, standard constructors, getters
}

And these are the subclasses:

public class Waitress extends Person {

    public String serveStarter(String starter) {
        return "Serving a " + starter;
    }
    
    // additional methods/constructors
}
public class Actress extends Person {
    
    public String readScript(String movie) {
        return "Reading the script of " + movie;
    } 
    
    // additional methods/constructors
}

In addition, let’s create a unit test to verify that instances of the Waitress and Actress classes are also instances of Person, thus showing that the “is-a” condition is met at the type level:

@Test
public void givenWaitressInstance_whenCheckedType_thenIsInstanceOfPerson() {
    assertThat(new Waitress("Mary", "[email protected]", 22))
      .isInstanceOf(Person.class);
}
    
@Test
public void givenActressInstance_whenCheckedType_thenIsInstanceOfPerson() {
    assertThat(new Actress("Susan", "[email protected]", 30))
      .isInstanceOf(Person.class);
}

It’s important to stress here the semantic facet of inheritance. Aside from reusing the implementation of the Person class, we’ve created a well-defined “is-a” relationship between the base type Person and the subtypes Waitress and Actress. Waitresses and actresses are, effectively, persons.

This may cause us to ask: in which use cases is inheritance the right approach to take?

If subtypes fulfill the “is-a” condition and mainly provide additive functionality further down the classes hierarchy, then inheritance is the way to go.

Of course, method overriding is allowed as long as the overridden methods preserve the base type/subtype substitutability promoted by the Liskov Substitution Principle.

Additionally, we should keep in mind that the subtypes inherit the base type’s API, which is some cases may be overkill or merely undesirable.

Otherwise, we should use composition instead.

3. Inheritance in Design Patterns

While the consensus is that we should favor composition over inheritance whenever possible, there are a few typical use cases where inheritance has its place.

3.1. The Layer Supertype Pattern

In this case, we use inheritance to move common code to a base class (the supertype), on a per-layer basis.

Here’s a basic implementation of this pattern in the domain layer:

public class Entity {
    
    protected long id;
    
    // setters
}
public class User extends Entity {
    
    // additional fields and methods   
}

We can apply the same approach to the other layers in the system, such as the service and persistence layers.

3.2. The Template Method Pattern

In the template method pattern, we can use a base class to define the invariant parts of an algorithm, and then implement the variant parts in the subclasses:

public abstract class ComputerBuilder {
    
    public final Computer buildComputer() {
        addProcessor();
        addMemory();
    }
    
    public abstract void addProcessor();
    
    public abstract void addMemory();
}
public class StandardComputerBuilder extends ComputerBuilder {

    @Override
    public void addProcessor() {
        // method implementation
    }
    
    @Override
    public void addMemory() {
        // method implementation
    }
}

4. Composition’s Basics

The composition is another mechanism provided by OOP for reusing implementation.

In a nutshell, composition allows us to model objects that are made up of other objects, thus defining a “has-a” relationship between them.

Furthermore, the composition is the strongest form of association, which means that the object(s) that compose or are contained by one object are destroyed too when that object is destroyed.

To better understand how composition works, let’s suppose that we need to work with objects that represent computers.

A computer is composed of different parts, including the microprocessor, the memory, a sound card and so forth, so we can model both the computer and each of its parts as individual classes.

Here’s how a simple implementation of the Computer class might look:

public class Computer {

    private Processor processor;
    private Memory memory;
    private SoundCard soundCard;

    // standard getters/setters/constructors
    
    public Optional<SoundCard> getSoundCard() {
        return Optional.ofNullable(soundCard);
    }
}

The following classes model a microprocessor, the memory, and a sound card (interfaces are omitted for brevity’s sake):

public class StandardProcessor implements Processor {

    private String model;
    
    // standard getters/setters
}
public class StandardMemory implements Memory {
    
    private String brand;
    private String size;
    
    // standard constructors, getters, toString
}
public class StandardSoundCard implements SoundCard {
    
    private String brand;

    // standard constructors, getters, toString
}

It’s easy to understand the motivations behind pushing composition over inheritance. In every scenario where it’s possible to establish a semantically correct “has-a” relationship between a given class and others, the composition is the right choice to make.

In the above example, Computer meets the “has-a” condition with the classes that model its parts.

It’s also worth noting that in this case, the containing Computer object has ownership of the contained objects if and only if the objects can’t be reused within another Computer object. If they can, we’d be using aggregation, rather than composition, where ownership isn’t implied.

5. Composition Without Abstraction

Alternatively, we could’ve defined the composition relationship by hard-coding the dependencies of the Computer class, instead of declaring them in the constructor:

public class Computer {

    private StandardProcessor processor
      = new StandardProcessor("Intel I3");
    private StandardMemory memory
      = new StandardMemory("Kingston", "1TB");
    
    // additional fields / methods
}

Of course, this would be a rigid, tightly-coupled design, as we’d be making Computer strongly dependent on specific implementations of Processor and Memory.

We wouldn’t be taking advantage of the level of abstraction provided by interfaces and dependency injection.

With the initial design based on interfaces, we get a loosely-coupled design, which is also easier to test.

6. Conclusion

In this article, we learned the fundamentals of inheritance and composition in Java, and we explored in depth the differences between the two types of relationships (“is-a” vs. “has-a”).

The code backing this article is available on GitHub. Once you're logged in as a Baeldung Pro Member, start learning and coding on the project.
Baeldung Pro – NPI EA (cat = Baeldung)
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Baeldung Pro comes with both absolutely No-Ads as well as finally with Dark Mode, for a clean learning experience:

>> Explore a clean Baeldung

Once the early-adopter seats are all used, the price will go up and stay at $33/year.

Partner – Microsoft – NPI EA (cat = Spring Boot)
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Azure Container Apps is a fully managed serverless container service that enables you to build and deploy modern, cloud-native Java applications and microservices at scale. It offers a simplified developer experience while providing the flexibility and portability of containers.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, visit the documentation page.

You can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Partner – Orkes – NPI EA (cat = Spring)
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Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.

Try a 14-Day Free Trial of Orkes Conductor today.

Partner – Orkes – NPI EA (tag = Microservices)
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Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.

Try a 14-Day Free Trial of Orkes Conductor today.

eBook – HTTP Client – NPI EA (cat=HTTP Client-Side)
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The Apache HTTP Client is a very robust library, suitable for both simple and advanced use cases when testing HTTP endpoints. Check out our guide covering basic request and response handling, as well as security, cookies, timeouts, and more:

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eBook – Java Concurrency – NPI EA (cat=Java Concurrency)
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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:

>> Download the eBook

eBook – Java Streams – NPI EA (cat=Java Streams)
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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:

>> Join Pro and download the eBook

eBook – Persistence – NPI EA (cat=Persistence)
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Working on getting your persistence layer right with Spring?

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Course – LS – NPI EA (cat=REST)

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Get started with Spring Boot and with core Spring, through the Learn Spring course:

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Course – Spring Sale 2025 – NPI EA (cat= Baeldung)
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Yes, we're now running our Spring Sale. All Courses are 25% off until 26th May, 2025:

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Course – Spring Sale 2025 – NPI (All)
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eBook Jackson – NPI EA – 3 (cat = Jackson)