How do you go from an abstract project description to actual code?

Question

Maybe its because I've been coding around two semesters now, but the major stumbling block that I'm having at this point is converting the professor's project description and requirements to actual code. Since I'm currently in Algorithms 101, I basically do a bottom-up process, starting with a blank whiteboard and draw out the object and method interactions, then translate that into classes and code.

But now the prof has tossed interfaces and abstract classes into the mix. Intellectually, I can recognize how they work, but am stubbing my toes figuring out how to use these new tools with the current project (simulating a web server).

In my professors own words, mapping the abstract description to Java code is the real trick. So what steps are best used to go from English (or whatever your language is) to computer code? How do you decide where and when to create an interface, or use an abstract class?

Answer 1

So what steps are best used to go from English (or whatever your language is) to computer code?

Experience is what teaches you how to do this. If it's not coming naturally yet (and don't feel bad if it doesn't, because it takes a long time!), there are some questions you can ask yourself:

• What are the main concepts of the system? How are they related to each other? If I was describing this to someone else, what words and phrases would I use? These thoughts will help you decide what classes are useful to think about.

• What sorts of behaviors do these things have? Are there natural dependencies between them? (For example, a LineItem isn't relevant or meaningful without the context of an Order, nor is an Engine much use without a Car.) How do the behaviors affect the state of the other objects? Do they communicate with each other, and if so, in what way? These thoughts will help you develop the public interfaces of your classes.

That's just the tip of the iceberg, of course. For more about this thought process in general, see Eric Evans's excellent book, Domain-Driven Design.

How do you decide where and when to create an interface, or use an abstract class?

There's no hard and fast prescriptions; again, experience is the best guide here. That said, there's certainly some rules of thumb you can follow:

• If several unrelated or significantly different object types all provide the same kind of functionality, use an interface. For example, if the Steerable interface has a Steer(Vector bearing) method, there may be lots of different things that can be steered: Boats, Airplanes, CargoShips, Cars, et cetera. These are completely unrelated things. But they all share the common interface of being able to be steered.

• In general, try to favor an interface instead of an abstract base class. This way you can define a single implementation which implements N interfaces. In the case of Java, you can only have one abstract base class, so you're locked into a particular inheritance hierarchy once you say that a class inherits from another one.

• Whenever you don't need implementation from a base class, definitely favor an interface over an abstract base class. This would also be handy if you're operating in a language where inheritance doesn't apply. For example, in C#, you can't have a struct inherit from a base class.

Answer 2

In general...

1. Read a lot of other people's code. Open source projects are great for that. Respect their licenses though.
2. You'll never get it perfect. It's an iterative process. Don't be discouraged if you don't get it right.
3. Practice. Practice. Practice.
4. Research often. Keep tackling more and more challenging projects / designs. Even if there are easy ones around.

There is no magic bullet, or algorithm for good design.

Nowadays I jump in with a design I believe is decent and work from that.

When the time is right I'll implement understanding the result will have to refactored ( rewritten ) sooner rather than later.

Give this project your best shot, keep an eye out for your mistakes and how things should've been done after you get back your results.

Keep doing this, and you'll be fine.

Answer 3

What you should really do is code from the top-down, not from the bottom-up. Write your main function as clearly and concisely as you can using APIs that you have not yet created as if they already existed. Then, you can implement those APIs in similar fashion, until you have functions that are only a few lines long. If you code from the bottom-up, you will likely create a whole lot of stuff that you don't actually need.

In terms of when to create an interface... pretty much everything should be an interface. When you use APIs that don't yet exist, assume that every concrete class is an implementation of some interface, and use a declared type that is indicative of that interface. Your inheritance should be done solely with interfaces. Only create concrete classes at the very bottom when you are providing an implementation. I would suggest avoiding abstract classes and just using delegation, although abstract classes are also reasonable when two different implementations differ only slightly and have several functions that have a common implementation. For example, if your interface allows one to iterate over elements and also provides a sum function, the sum function is a trivial to implement in terms of the iteration function, so that would be a reasonable use of an abstract class. An alternative would be to use the decorator pattern in that case.

You might also find the Google Techtalk "How to Design a Good API and Why it Matters" to be helpful in this regard. You might also be interested in reading some of my own software design observations.