Qi4j™ is an implementation of Composite Oriented Programming, using the standard Java 5 platform, without the use of any pre-processors or new language elements. Everything you know from Java 5 still applies and you can leverage both your experience and toolkits to become more productive with Composite Oriented Programming today.

Principles

Composite Oriented Programming builds on some principles that are not addressed by Object Oriented Programming at all.

• Behavior depends on Context
• Decoupling is a virtue
• Business Rules matters more.
• Classes are dead, long live interfaces.

Behavior Depends on Context

Many objects has life cycles that are more extensive than the simple model that Object Oriented Programming model wants us to believe. A few simple examples;

• An egg becomes a chicken which in turn becomes food.
• I am a programmer at work, a father+husband at home, a victim in a traffic accident and hunter and pray in the jungle.

But it is more to it than that. The composition of the object may change over time. My home now has a garage and my car have different kind of problems with their own state related to it.
In the programming world, we are constantly faced with change of requirements. These changes are often not related to any real world changes, but people coming to new insights of the problem domain. OOP makes those changes a big deal, and often we have to tear up large chunks of the model and redo the work.

But wait, there is more.

Some objects traverses different scope boundaries to the extreme. For instance, a Person will have its attributes changing slightly over time, new abilities be learnt and so forth, that is mentioned above. But the Person will eventually die, but that doesn't mean that the Person object should be deleted from a system, since the "memory of" that Person may live on for a long time. In a OOP system, we would need to transfer some of the state from a LivingPerson class to a DeadPerson class. In Composite Oriented Programming, it is the same object with different behavior.

We think that one of the the main flaws in OOP is that it is not object oriented at all, but in fact class oriented. Class is the first class citizen that objects are derived from. Not objects being the first-class citizen to which one or many classes are assigned.

Decoupling is Virtue

Decoupling is more important than developers in general think. If you could have every OOP class decoupled from all other classes, it is easy to re-use that class. But when that class references another class and the chain never ends, your chances of re-use diminishes quickly.
Object Oriented Programming is suffering a lot from this, and many mechanisms have been introduced over time to counter this problem. But in reality, the best we can manage is subsystems of functionality, which client code can re-use. And these subsystems tend to be infrastructure related, since domain models are less prone to be similar enough from one project to the next, and since OOP in reality constrains the the re-use of individual domain classes, we need to re-do the domain model from scratch ever time.

Business Rules matters more

Smart developers often think that low-level, infrastructure and framework code is more important and more cool to work with, than the simple domain model. But in reality, it is the Domain Model that reflects the actual need  and pays the bills. Infrastructure is just a necessary evil to get things done.

If most developers could focus on the Business Rules and Domain Model, and not having to worry about any infrastructure issues, such as persistence, transactions, security or the framework housing it all, the productivity would surge. Eric Evans has written an excellent book about Domain Driven Design, where he goes through the real process that makes the money for companies. However, it is very hard to follow that book, since one is constantly caught up in constraints irrelevant to the domain model, introduced by the underlying framework, from the so called smart developers.

You are to embark on a new journey. Enjoy!