### Background

Zest™ is the first Composite Oriented Programming implementation leveraging the Java 5 platform, so that everything you know from Java 5 still applies. You can mix Zest™ with your ordinary Java code as much as you want. All your existing Java tools works just like before, and Zest™ does not introduce any new programming language, no special development tools needed and no XML is required.

#### Purpose

Zest™ addresses the programming problems from the top-down, starting with the Domain Model and Business Rules needs, and let those requirements flow downwards in the software stack and dictate the requirements for underlying layers such as persistence, messaging, querying and more. This means that the business value developer is left to concentrate on the domain models and the actual application bringing the value, instead of creating massive amounts of glue code to tie underlying technologies together.

#### Problem Descriptions

Zest™ didn’t appear out of the blue, when the founders of the project had nothing better to do. It is the result of observation of problems in real applications, and the experience from previous attempts to address or correct these problems, that has led to the Zest™ vision.

#### Object Oriented Programming (OOP)

How can OOP be a problem? We and others have observed that there is a fundamental flaw in the OOP model. In fact, we would like to even state that OOP as it is commonly practiced today is not object oriented at all. The object is not the primary citizen, instead the class is the primary artifact. In most mainstream OOP languages, Objects are derived from classes, not that classes are assigned to created objects. Therefore, we think it should have been called Class Oriented Programming. We can also see this class focus in many of the technologies in Java today: in Spring you declare class names in application contexts, JSP uses class names to declare beans and so forth.

This in turn leads to that there is no good OOP solution for the problem we describe below.

#### Behavior depends on Context

Once you start thinking of "Behavior depends on Context", you have a hard time understanding how people for the last 20 years or so of Object Oriented Programming (OOP) has ignored this fact.

##### Who am I?

When I sitting in front of the computer, I am a software developer, but if I go out in the jungle, I am suddenly hunter-gatherer and prey. A large set of me is the same, but my interaction with the surroundings, i.e. the context, is very different. I need different interfaces, so to speak, in these two different contexts.

Now, the above example is perhaps a bit extreme, but we see it in everyday life of the developer. When an object is stored in the database it is of a different class, than when it is transported to the client and possibly when it is displayed in the GUI. We see the effect of this problem in many of the design patterns and so called "best practices" in Java EE development. Facades, delegation, data transport objects and many more.

#### Coupling

The OOP proponents once proclaimed that classes can be re-used, since the code is encapsulated with the class, so the class is an independent unit which lends itself well to re-use. In reality, however, we have found that classes becomes tightly coupled with many other classes in their neighborhood, leading to impossibilities of single class re-use. Many tricks are introduced to minimize the "Coupling Hell", such as Inversion of Control and Dependency Injection. Although those tools are good, the underlying problem remains.

Why do we end up with large coupled class network graphs?

Essentially, it boils down to "scope". Classes are too large, their scope is too large, and for each small functional unit within the class, there will be additional coupling to other classes. And this often progresses to the full boundary of the entire domain the class remains in.

#### Application layer impedance mismatch

Almost all technologies used in modern software development, starts by looking at an infrastructural problem and try to solve that the best way. This is often done in a vacuum and layers on top will be struggling to map or translate the solution into the higher abstraction, and the higher up we get, the harder it becomes to ignore the assumptions, problems and limitations of the underlying technologies. It is also common that the underlying technologies "bleeds" through the layers all the way into the domain models. The "bleed" combined with the problem of using independently developed technologies, puts a large burden on the application developer, whose job it is to bring business value. And often, the most skilled developers end up doing the bottom layers, leaving the hardest job to the least suitable. Another interesting consequence is that each layer needs to anticipate every single use-case - real, potential or perceived - and deal with it in a specifiable and useful manner. This leads to overly complex solutions, compared to if the system is built from the top layer down, where each layer beneath knows exactly what is expected from it, and only needs to handle those use-cases.

#### Abstracting away meaning.

To paraphrase a famous proverb about a hammer: "If all you have are objects, everything looks like a dependency." We think that increasing abstraction often also increases complexity, and that the abstraction benefits are somewhat linear whereas the complexity negatives are exponential. So, our conclusion is that by making no distinction between different kinds of objects, many sound technologies run into incredibly difficult problems. The implementation of the programming platform (e.g. Java) is of course easier to implement with a hefty amount of scope reduction into as few as possible abstractions. But that is not the situation for the user. The abstraction is then required to be reversed when the rubber hits the road, e.g. ORM mapping must be declared explicitly by the programmer, often using separate tools and languages.

#### Solution

We think the solution was expressed more than 2500 years ago, first by Indian scholars and slightly later by Leucippus and Democritus. We are of course talking about atoms, and by using really small building blocks, we can express arbitrarily complex structures. By reducing the classes into what we in Composite Oriented Programming call Fragments, we limit the coupling network graph substantially. Re-use of Fragments becomes a reality, and by combination of Fragments, we compose larger structures, the Composites.

#### Composition

Composite Oriented Programming also view the object, we call it the Composite instance, as the first class citizen. The Composite instance can be cast to any context, meaning a different behavior can be applied to the Composite instance, without affecting its underlying state. And back. This in turn means that we can for instance create a ServerContextualInvoiceEntity, transport that across to a client, cast it to a GuiContextualInvoiceEntity do the modifications to the underlying state, possibly using extra interfaces and methods for interacting with the GUI environment, and then transport the modified object back to the server, cast it back to the ServerContextualInvoiceEntity, and then persist the changes.

#### Domain Driven Design focus

Composite Oriented Programming is heavily influenced by the book "Domain Driven Design" by Eric Evans. And we are trying to use his analysis of the problem to provide the mechanisms needed to get the job done quicker and more reliably. Mr Evans talks about Applications, Layers, Modules, Specifications, SideEffects and so forth, and all of these should be present in a Composite Oriented Programming implementation, and to a large extent it is in Zest™.