A life committed to learning.

Inheritance in JPA Entities

“..but equals should usually compare state, not identity. This is particularly true for “data-centric” classes which map to database records. […] If you extend a concrete class, and add a new field which contributes to equals, then it is not possible to write a perfectly correct equals method for the new class. Instead, you should use composition instead of inheritance. (See Effective Java by Joshua Bloch for more information.) ”


This can be interpreted as “It’s not a good idea to use inheritance in, for example, JPA Entities. It’s better to use composition”.  I tend to agree. Deep hierarchies of JPA entities,  can be a daunting task to understand, maintain and extends. Having small hierarchies, I think it’s ok. If a JPA model, most of the time models real world concepts, and if real world concepts are better described in terms of composition, instead of inheritance, why complicate things? Take the typical Car exercise, would you model most of your car using inheritance or composition?

But if  you’re  modelling your JPA entities with object oriented principles , not caring about the mapping stuff, for now, everything that you will need to store in a DB are value types, like Integer, Long, String, byte[], or some composition of these and other value types, so there’s no point to inherit just to add values to an object, it’s better to create a new value type that represents the concept.

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Mutation Testing with PITest and JUnit

What? More testing? This time are the tests for your tests. Mutation Testing analyses your code, and based on some heuristics change it to try to “kill” your tests. If if can kill all your tests, then your tests are probably fine, otherwise maybe your coverage it’s not as good as you may think. PIT is a framework worth looking, since it integrates with maven and seems to be maintained.

Just use the maven plugin to try with your maven projects and see the results.

There are other tools for mutation Testing with Java and JUnit, but did not try:

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Programmer testing exercise

Some time ago, I wrote here to leave you with a presentation about programmer testing. This post is a kind of follow up, leaving you now with the exercise.

You can see the exercise here

I also have the presentation in PDF stored in google drive. Find it here.

If you have any feedback/comments or whatever, comment this post.

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An introduction to Maven 2

This is an hands-on workshop about Maven. It will help you to get up and running with Maven if you don’t know Maven.
I have included the exercise that you can use to get your hands on Maven. This information is relevant also for Maven 3, though it was developed for Maven 2.

The presentation is here:

If you prefer the PDF, you can view it here:

The exercise can be downloaded/viewed here

If you any feedback/comment/whatever comment it.

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Quick introduction to REST and JAX-RS

Here are some slides introducing to REST world. Examples are using JAX-RS (JSR 311).

If we’re moving to the cloud-era, the undestanding of this architecture style is essential. In the cloud we are presented with services (Storage, Search, Graph, Geolocation, …) that we can and should use to build applications. Almost all of these services, available  in the cloud, provide some kind of interface and happens that, this  interfaces is, generally, a RESTful Web Service. If I want collaborate with these services I must talk the same language!! Moreover, exploring this architectural style in new applications opens the door for realy distributed and scalable systems, while the simplicity is kept high!

Here the slides:

PDF can be downloaded from here.

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Software Architecture disruption?

:Adrian Colyer is a smart guy and he  put together, in a talk, what software architectures will look like for the next years. True is, most of the services you’re using online today follow these architecture principles. Nothing new, thought.

Nothing in this architecture is can go wrong,  it’s so simple! This type of architecture is the perfect fit for this ubiquitous platform that is Internet.

Btw,  SpringSource makes it really easy to develop applications in this way, just look to Spring Integration project, for example. They have a plenty of projects, built on top of Spring, that makes development of this type of architectures more, and more fun and easy.

(Ignore the sound problems at the beginning 🙂 )

Contrast this with traditional “enterprise” architectures. Which side of the fence you want to be?

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How to violate the Interface Segregation Principle

In my last post, I gave away some slides where OO Design Principles are introduced. I have an though about the big-fail that was the J2EE (before JEE5), in terms of violating the Interface Segregation Principle.

If you you are fortunate enough to have experienced the pain of writing applications with J2EE, then you recall that, for example, when you define an Stateless EJB, you have to implement the SessionBean interface.

Well, this is a clear violation of the Interface Segregation Principle, your EJB component have to implement the EJB lifecycle callback methods, even if you don’t need them. Right?

For instance:

public class MyComponent extends SessionBean {
    public void someCoolBusinessMethod() {
         // Business knowledge is coded here

    public void ejbCreate() {} //Don't have anything to do in this EJB lifecycle callback, but the interface forces me to implement the method

    public void ejbRemove()  {} //Don't have anything to do in this EJB lifecycle callback, but the interface forces me to implement the method

   public void ejbPassivate() { } //Don't have anything to do in this EJB lifecycle callback, but the interface forces me to implement the method

Latest versions of JEE (5 and 6), allows you to bind methods to component lifecycle events declaratively, with annotations of XML. For example:

public void MyOtheComponent {
    public String realBusinessMethod() {
        return "I do really cool stuff";

   private void letMeGo() {
   //Any business that should run before the object is left to the GC

They fixed it, but very behind time…

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OO Design Principles, part 2

This second part is also the last one of this short introduction to OO design principles. I see these principles a the basic conceptual framework to build Object Oriented programs.

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Object Oriented Software Design Principles and guidelines

I had recently to refresh the SOLID principles in Object Oriented Design discipline. SOLID stands for:

Single Responsability Principle
Open Closed Principle
Liskov Substitution Principle
Interface Segragation Principle, and
Dependency Inversion Principle

These are the principles for Object Oriented design, but not following them don’t make me a bad developer, of course. As with any principle, it can be violate if a good reason exists supporting that.

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How to improve productivity when developing JEE based web applications

The title of this post was stolen from Stackoverflow, from this discussion:

I really like and agree with this point:

Being “the enterprise stack”, it is often used to create boring, ugly, “good enough” applications and, in general, enterprises tend not to attract great developers who love programming, and think and care about what they do. The quality of software in the enterprise world is not great.

There are exceptions, of course, but mainly from what I’ve seen from the past 10 years, this is, sadly, true. Why?

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