Design Principles from Design Patterns. Leading- Edge Java. Design Principles from Design Patterns. A Conversation with Erich Gamma, Part IIIby Bill Venners.
Design elements and principles at Wikipedia's sister projects. Definitions from Wiktionary: Media from Commons: News stories from Wikinews: Quotations from Wikiquote. Design Principles and Patterns: Enterprise Patterns: Books: Home Home : Bookmark this on Delicious. Books We Recommend. Structural Design Patterns: Click to zoom. Principles of Design Patterns (V). • We’ve now encountered ten OO design principles • Looked at how they are applied in certain patterns. . in the early 2000s that stands for five basic principles of object-oriented programming and design. The principles, when applied together.
Design Principles and Design Patterns. principles and patterns that are exposed here. or design patterns. Principles of Design The principles of design describe the ways that artists use the elements of art in a work of art. If the design was a scale. Discuss this article in the Articles Forum topic, Design Principles from Design Patterns. Resources [1] Erich Gamma is co-author of Design Patterns.
June 6, 2. 00. 5Summary. In this interview, Erich Gamma, co- author of the landmark book, Design Patterns, talks with. Bill Venners about two design principles: program to an interface, not. Erich Gamma lept onto the software world stage in 1. Design Patterns: Elements of Reusable Object- Oriented Software (Addison- Wesley, 1.
This landmark work, often referred to as the Gang of Four (Go. F) book, cataloged 2. In 1. 99. 8, he teamed up with Kent Beck to produce JUnit [2]. Java community. Gamma currently is an IBM Distinguished. Engineer at IBM's Object Technology International (OTI) lab in Zurich, Switzerland. He provides leadership in the Eclipse community, and is responsible. Java development effort for the Eclipse platform [3].
On October 2. 7, 2. Bill Venners met with Erich Gamma at the OOPSLA. Vancouver, Canada. In this interview, which will be published in multiple installments.
Leading- Edge Java on Artima Developer. Gamma gives insights into software design. In Part I: How to Use Design Patterns. Gamma describes gives his opinion on the appropriate. Go. F, and an Alexandrian pattern language. In Part II: Erich Gamma on Flexibility and Reuse, Gamma discusses the importance of reusability.
In this third installment, Gamma discusses. Go. F book: program to an interface, not. Program to an interface, not an implementation. Bill Venners: In the introduction of the Go. F book, you mention two. The first principle is: "Program to an. What's that really mean, and why do it?
Erich Gamma: This principle is really about dependency relationships which. It's easy to add a dependency on a class. It's. almost too easy; just add an import statement and modern Java development tools like. Eclipse even write this statement for you. Interestingly the inverse isn't that easy and. For this reason you have to develop with.
This principle tells us that. Bill Venners: Why?
Erich Gamma: Once you depend on interfaces only, you're decoupled from the implementation. That. means the implementation can vary, and that's a healthy dependency relationship. For. example, for testing purposes you can replace a heavy database implementation with a. Fortunately, with today's refactoring support you no. You can distill an interface from a.
The intended interface is. So this approach gives you flexibility, but it also separates the really valuable part, the. One question is whether you should always use a Java. An abstract class is good as well.
In fact, an abstract. You can add new behavior. Bill Venners: How's that?
Erich Gamma: In Java when you add a new method to an. When you have an abstract class, you. All the clients will.
As always there is a trade- off, an interface gives you freedom with. It isn't always possible to define an interface in an abstract class, but in the light of. Since changing interfaces breaks clients you should consider them as immutable once. As a consequence when adding a new method to an interface you.
In Eclipse we take API stability seriously and for. I*2 interfaces like IMarker. Resolution. 2. or IWorkbench. Part. 2 in our APIs. These interfaces add methods to the. IMarker. Resolution and IWorkbench.
Part. Since the additions are done in separate extension interfaces you do not break the clients. However, there is now some burden on the caller in that they need to determine at run- . Another lesson learned is that you should focus not only on developing version one, but.
This doesn't mean designing in future. API stable for a long time. You want to build to last. That's been an. important theme of Eclipse development since we started. We have built Eclipse as a. We always keep in mind as we design Eclipse that it has to last ten or twenty. This can be scary at times.
We added support for evolution in the base platform when we started. One example of. this is the IAdaptable interface. Classes implementing this interface can. This is an example of the Extension Object pattern. Bill Venners: It's funny nowadays we're so much more advanced, but when we.
When the ancient Egyptians built to last. Erich Gamma: Thousands of years, right? But for Eclipse, ten to twenty years.
Quiet honestly, I don't envision a software archeologist finding an Eclipse. I really mean that. Eclipse should still be able to support an active community in ten or twenty years. The value of interfaces.
Bill Venners: Above you said interfaces are more valuable. What is their value? Why are they more valuable than the implementation? Erich Gamma: An interface distills the collaboration between objects. An. interface is free from implementation details, and it defines the vocabulary of the.
Once I understand the interfaces, I understand most of the system. Why? Because once I understand all the interfaces, I should be able to understand the. Bill Venners: What do you mean by "vocabulary of the problem?". Erich Gamma: What are the method names? What are the abstractions?
The. abstractions plus the method names define the vocabulary. The Java Collections package. The vocabulary for how to work with collections is distilled in. List or Set. There is a rich set of. Bill Venners: I guess the core of my question about "program to an interface, not. In Java, there's a special kind of class called. I'm writing I put in code font—the Java.
But then there's the object- oriented interface concept. If I'm writing client code and need to use an object, that object's class exists in some type.
At the top of the hierarchy, it's very abstract. At the bottom, it's very concrete. The way I think about programming to interfaces is that, as I write client code, I want to. I can go, without. Every single one of those types in the hierarchy has a contract. You're right. And, writing against a type far up in the hierarchy is consistent with the. Bill Venners: How can I write to an implementation?
Erich Gamma: Imagine I define an interface with five. I define an implementation class below that implements these five methods. If only the interface is published as API then if you call. You call a method that is out of. I might break anytime. So it's the difference, as Martin Fowler would say. Something can be public, but that doesn't mean you have.
In Eclipse we have the naming convention that a package which includes the segment. They contain types which we do not consider. So the nice short package. API and the long name is for internals.
Obviously using package private. Java. Bill Venners: Now I understand what you mean. There's public and published. Martin Fowler has nice terms for that difference. Erich Gamma: And in Eclipse we have the conventions for that difference. Actually we even have tool support. In Eclipse 3. 1 we have added support for defining.
API. These access rules are defined on a project's. Once you have these access restrictions defined the Eclipse Java development. For example you get feedback as you type when you add a dependency to a type that isn't. Bill Venners: So if I write code that talks to the interface of a non- published. I am writing to the implementation, and it may break.
Erich Gamma: Yes, and the explanation from the angle of the provider is that I. When to think about interfaces. Bill Venners: On the subject of interfaces, the Go.
F book includes some UML. UML diagrams seem to mix interface and implementation. When you. look at it, you often see the design of the code. It isn't necessarily obvious what's API and. If you look at Java. Doc, by contrast, you see the interfaces. Another place I see the lack of differentiation between interface and implementation is.
XP. XP talks about the code. You're changing this amorphous body of code with test- . When should the designer think about interfaces versus the whole. Erich Gamma: You might think differently when you design an application than.
When you design a platform, you have to care at every. API, and what to keep internal.
Today's. refactoring support makes it trivial to change names so you have to be careful not to. APIs by accident. This goes beyond just defining which types are. You also have to answer questions like: do you allow clients to subclass from.
If you do, it imposes big obligations. If you look at the Eclipse API, we try to. Also with Jim. des Rivières [5] we have an API advocate in our team. Jim helps us to tell. APIs. When it comes to applications, even there you have abstractions that have multiple.
For your design you want to come up with key abstractions, and then you. Then you have flexibility. When a new variation of an abstraction. Regarding XP, as I mentioned earlier, the modern. XP. Bill Venners: So for an application it's the same thought process as for a.
Another difference is that it is easy for me if I have. I can update them if I need to change the. Erich Gamma: Yes, for an application it is the same thought process as for a. You also want to build an application so that it lasts. Reacting to a changing. The fact that you have control over.
Once you have given out your code and you no longer have access to. API business. Bill Venners: Even if those clients are written by a different group in the same. Erich Gamma: Even there, absolutely. Bill Venners: So it sounds like thinking about interfaces becomes more. If the project is just two or three people, it is not quite.
The refactoring support tools.. Erich Gamma: .. will do it all for you.
Bill Venners: But if it is a 1. Different groups will have different areas of responsibility. Erich Gamma: A practice that we follow is to assign a component to a group. The group is responsible for the component and publishes its API.
Dependencies are then. API. We also resist the temptation to define friend relationships, that is. In. Eclipse all components are equal. For example, the Java development tool plug- ins have. APIs as all other plug- ins. Once you have published an API then it is your responsibility to keep them stable. Otherwise you will break the other components and nobody is able to make progress.