10 tips on writing reusable code

I have been trying to increase code reuse in the projects I have been doing recently. In my first few years of coding I hardly ever got to reuse any of my code because it was always too coupled together and dependant upon other parts of the code.

So recently I have been trying to write code which I can reuse. It has been interesting that since I have been doing this I have noticed that my library of code is starting to grow. I have started to create more Static Helper classes with useful methods in. I have also been removing the business logic away from any Struts actions or framework work.

To do this I have tried to do a number of things to help this and these are the sort of rules and things I do (in no order) to help me try and achieve this. They are a number of rules and tips I have picked up but can't remember where from

1. Keep the code DRY. Dry means Don't repeat yourself. This is one of the main changes I have tried to bring in. Always try to eradicate duplication and if you find any then move remove the duplication to a relevant place. Sometimes this has lead me to create Static Helper classes or sometimes move it to the class it makes most sense to have it.

2. Make a class/method do just one thing. This is along the lines of the advice of giving the class only one reason to change. This often means creating methods that other methods use but this helps to make the methods/classes simple and less coupled.

3. Write unit tests for your classes AND make it easy to test classes. Writing code that is easy to test is decoupled. If you write code and are thinking about writing a unit test for it then you tend to split up the code into smaller testable chunks.

4. Remove the business logic or main code away from any framework code. Following the rules above will help this. An example I have seen is code that is inside Struts Actions classes, this code is practically impossible to reuse because of all the Struts dependencies that it now linked with.

5. Try to think more abstractly and use Interfaces and Abstract classes. Try to hide dependencies of code behind a more Generic interface/abstract class. The benefit this gives the code is it creates a flexible point in the code where you can then hide future changes behind.

6. Code for extension. Write code that can easily be extended in the future. This is particularly true with the above point. If you write code that uses interfaces then you can extend that interface at a later point.

7. Don't write code that isn't needed. Do the simplest thing possible. Don't waste your time adding methods and classes that might be used in the future. Keep the code simple and focused on what you are trying to deliver. I think I read/heard Josh Bloch say once that "if in doubt, leave it out". Basically who wants to write code that no one (including yourself) is going to use again.

8. Try to reduce coupling. When writing code think about the links and coupling the code is creating, does it need to be linked to those other classes.

9. Be more Modular - make your code more modular, think modular, be modular.

10. Write code like your code is an External API. Imagine the code you are writing is a self contained component.

It wasn't going to be ten until I got to 8 and then thought no one writes 8 tips, lets add two more on. It isn't really a list but it's sort of aims and mental notes I try tell myself when writing code. They are more small bits of code I have written recently that has helped. I would like to hear people's comments and especially their tips on writing reusable code