Reuse

Reuse is a major theme in software engineering practices. By reusing tried-and-tested components, the robustness of a new software system can be enhanced while reducing the manpower and time required. Reusable components come in many forms; a reused component can be a piece of code, a subsystem, or a whole software system.

While you may be tempted to use many libraries/frameworks/platforms that appear regularly and promise to bring great benefits, note that there are costs associated with reuse. Here are some:

• The reused code may be overkill (think using a sledgehammer to crack a nut), increasing the size of, and/or degrading the performance of, your software.
• The reused software may not be mature/stable enough to be used in an important product. That means the software can change drastically and rapidly, possibly in ways that break your software.
• Immature software has the risk of dying off as fast as it emerged, leaving you with a dependency that is no longer maintained.
• The license of the reused software (or its dependencies) restricts how you can use or develop your software.
• The reused software might have bugs, missing features, or security vulnerabilities that are important to your product, but not so important to the maintainers of that software, which means those flaws will not get fixed as fast as you need them to.
• Malicious code can sneak into your product via compromised dependencies.

An Application Programming Interface (API) specifies the interface through which other programs can interact with a software component. It is a contract between the component and its clients.
Example A class has an API (API of the Java String class, API of the Python str class), which is a collection of public methods that you can invoke to make use of the class.
Example The GitHub API is a collection of web request formats that the GitHub server accepts and their corresponding responses. You can write a program that interacts with GitHub through that API.

When developing large systems, if you define the API of each component early, the development team can develop the components in parallel because the future behavior of the other components is now more predictable.

A library is a collection of modular code that is general and can be used by other programs.

These are the typical steps required to use a library:

1. Read the documentation to confirm that its functionality fits your needs.
2. Check the license to confirm that it allows reuse in the way you plan to reuse it. For example, some libraries might allow non-commercial use only.
3. Download the library and make it accessible to your project. Alternatively, you can configure your to do it for you.
4. Call the library API from your code where you need to use the library's functionality.

The overall structure and execution flow of a specific category of software systems can be very similar. This similarity is an opportunity for large-scale reuse.
Example Running example: IDEs for different programming languages are similar in how they support editing code, organizing project files, debugging, etc.

A software framework is a reusable implementation of software (or part thereof) that provides generic functionality that can be selectively customized to produce a specific application.
Example Running example: Eclipse is an IDE framework that can be used to create IDEs for different programming languages.

Some frameworks provide a complete implementation of a default behavior, which makes them immediately usable.
Example Running example: Eclipse is a fully functional Java IDE out-of-the-box.

A framework facilitates the adaptation and customization of some desired functionality.
Example Running example: The Eclipse plugin system can be used to create an IDE for different programming languages while reusing most of the existing IDE features of Eclipse, e.g., https://marketplace.eclipse.org/content/pydev-python-ide-eclipse

Some frameworks cover only a specific component or an aspect.
Example JavaFX is a framework for creating Java GUIs. Tkinter is a GUI framework for Python.

Although both frameworks and libraries are reuse mechanisms, there are notable differences:

• Libraries are meant to be used ‘as is’, while frameworks are meant to be customized/extended. e.g., writing plugins for Eclipse so that it can be used as an IDE for different languages (C++, PHP, etc.), adding modules and themes to Drupal, and adding test cases to JUnit.

• Your code calls the library code while the framework code calls your code. Frameworks use a technique called inversion of control, also known as the “Hollywood principle” (i.e., don’t call us, we’ll call you!). That is, you write code that will be called by the framework, e.g., writing test methods that will be called by the JUnit framework. In the case of libraries, your code calls library code.

A platform provides a runtime environment for applications. A platform is often bundled with various libraries, tools, frameworks, and technologies in addition to a runtime environment, but the defining characteristic of a software platform is the presence of a runtime environment.
Example Technically, an operating system can be called a platform. A Windows PC is a platform for desktop applications, while iOS is a platform for mobile applications.