How to write reusable code? Guide & best practices for reusability in Java

In this post, we dive into code reuse and some key code reusability best practices that help you optimize Java code for reuse.

What is code reusability?

Not all code is one of a kind, highly specific, and single-use. Certain snippets of code may come in handy more than once throughout the development of an application, or may even be used across several applications.

Reusing code helps you avoid investing time and effort in redundantly developing (and testing) the same thing over and over again. You may be able to reuse entire components across projects, boosting efficiency and productivity. Maximizing code reuse is vital for scalability, so it’s a good idea to keep the following tips in mind when you start working on a large application.

Why optimize code for reuse?

As mentioned above, the primary benefit of focusing on reusability is increased efficiency through reducing redundant work. But there’s more to reuse than that.

For one, code optimized for reusability will be easy to read, comprehend, and maintain. Therefore, optimizing for reuse can also help team members collaborate more efficiently. Once you choose to reuse certain code snippets or components, you’ll find that best practices and design patterns are easier to adhere to (simply because code already delivered in high quality will be used again and again). It’s also important to bear in mind the resulting increase in quality: since code is “hardened” and tested in more than one project, you can be sure it is robust and of high quality.

Overall, reusable code saves time, effort, and costs. That’s why more and more teams are looking to implement (or improve) code reuse in their projects. The following best practices help your team reap the benefits.

Best practices for reusability in Java software code

Reusable coding standards & design patterns

When formally deciding to focus on reusability, you’ll need to define the rules for writing reusable code that your whole team will stick to. It’s a good idea to put these rules and resources in writing and make them easily accessible for all team members. While there’s a bit of a learning curve in optimizing code for reuse, these established requirements and internally shared resources will help everyone adopt a similar way of working with code.

Make sure you extend this documentation with stuff that promotes a contextual understanding of reuse. A good example: your team will need to have a decent understanding of concepts like encapsulation, inheritance, and polymorphism for developing easy-to-extend modular applications with code that can be reused across environments. Make sure topics like these are covered in your reuse guidelines and documentation to ensure all team members have a thorough and mutual understanding of code reuse.

It’s also a good idea to rely on design patterns in your code. Design patterns are tried-and-tested solutions to common design problems. More often than not, there’s really no need to reinvent the wheel – just use established best practices or patterns for the development problem at hand.

Modularization & loose coupling: plan for reuse

If you’re going down the reusability road, plan with reuse from the get-go. You’ll achieve the best results by structuring your application from the ground up in a modular way. Building a system of loosely coupled, independent modules helps promote reuse. Here are a few things to bear in mind.

Start by defining what functional modules your application will have and which methods or functions will belong in which component. The smaller the module, the more opportunities for reusing it in other environments.

Use well-defined classes that are responsible for a single functionality. Per Robert C. Martin’s definition of the single responsibility principle:

“A class should have one, and only one, reason to change. […] Gather together the things that change for the same reasons. Separate those things that change for different reasons.”

Make it a priority to exercise decoupling in your classes i.e. aim to reduce dependencies between classes. Use inheritance wisely: you’ll want to move functionality out of class instance methods to reduce the chance of dependency problems arising from class inheritance. Structure your code so that base classes contain common functionality. Use derived subclasses to extend or customize this functionality for the specific use case at hand. This way, parent classes may be easier to reuse for other use cases.

Overall, optimize the architecture for simplicity and reusability. Mark or otherwise keep track of the components that are relevant for reuse, and make sure your team can easily identify these code snippets. This lets the whole team efficiently reuse components/modules as they build the application or when working on a new project.

Program to interfaces rather than classes

Reduce reliance on specific classes. Instead of passing objects to methods, try to make method arguments references to a common interface. You can do this by replacing class-type input parameters with interface types. This helps make sure that objects of any class implementing that interface type can satisfy the parameter (rather than just those of the original class).

Effectively unit test reusable components

You’ll want to make sure that your classes are easy to test and that they are covered by extensive unit test suites. Of course, this is always important, but it is no less than crucial in reusable components since you want these to be very robust and thoroughly tested. As always, your unit tests should be focused: each test case method should test only a single function.

Write comprehensive test suites to validate the correct functioning of your reusable components. An added benefit is that if other team members can see that your code is well-tested, they may be more confident reusing it in their own work. Adequate unit test coverage also serves as documentation for using those components, which leads us over to the next topic.

Document your code and APIs

In addition to writing a robust unit test suite, make sure you adequately document your code to make it fit for reuse. Since these are going to be “standard” components geared for reuse, you’ll want to document the purpose of your code, its dependencies, and any additional information on how to use certain components. Well-documented code is easier to understand and integrate into a different code context.

Summary: how to write reusable Java code?

It’s possible that getting used to all the above best practices for all your team members will take some effort in the beginning. But in the long run, optimizing for reusability goes a long way towards improving the efficiency of not just you as a developer but also your whole team. By reusing high-quality, tested code, you can reduce the effort that goes into developing certain multi-use components. Once you have a library of components intended for reuse built up, you’ll see the benefits accumulate!

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