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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 requirement. Reusable components come in many forms; it can be reusing a piece of code, a subsystem, or a whole software.


While you may be tempted to use many libraries/frameworks/platforms that seem to crop up on a regular basis and promise to bring great benefits, note that there are costs associated with reuse. Here are some:

  • The reused code may be an 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.
  • Non-mature software has the risk of dying off as fast as they emerged, leaving you with a dependency that is no longer maintained.
  • The license of the reused software (or its dependencies) restrict how you can use/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.

A class has an API (e.g., 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.

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 are now more predictable.



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

Java classes you get with the JDK (such as String, ArrayList, HashMap, etc.) are library classes that are provided in the default Java distribution.

Natty is a Java library that can be used for parsing strings that represent dates e.g. The 31st of April in the year 2008

built-in modules you get with Python (such as csv, random, sys, etc.) are libraries that are provided in the default Python distribution. Classes such as list, str, dict are built-in library classes that you get with Python.

Colorama is a Python library that can be used for colorizing text in a CLI.


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. The similarity is an opportunity to reuse at a high scale.

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 a software (or part thereof) providing generic functionality that can be selectively customized to produce a specific application.

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.

Running example:

Eclipse is a fully functional Java IDE out-of-the-box.

A framework facilitates the adaptation and customization of some desired functionality.

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.


Some frameworks cover only a specific component or an aspect.

JavaFX is a framework for creating Java GUIs. Tkinter is a GUI framework for Python.

More examples of frameworks

  • Frameworks for web-based applications: Drupal (PHP), Django (Python), Ruby on Rails (Ruby), Spring (Java)
  • Frameworks for testing: JUnit (Java), unittest (Python), Jest (JavaScript)

Frameworks vs Libraries

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, aka 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 libraries.



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.

Technically, an operating system can be called a platform. For example, Windows PC is a platform for desktop applications while iOS is a platform for mobile applications.

Two well-known examples of platforms are JavaEE and .NET, both of which sit above the operating systems layer, and are used to develop enterprise applications. Infrastructure services such as connection pooling, load balancing, remote code execution, transaction management, authentication, security, messaging etc. are done similarly in most enterprise applications. Both JavaEE and .NET provide these services to applications in a customizable way without developers having to implement them from scratch every time.

  • JavaEE (Java Enterprise Edition) is both a framework and a platform for writing enterprise applications. The runtime used by JavaEE applications is the JVM (Java Virtual Machine) that can run on different Operating Systems.
  • .NET is a similar platform and framework. Its runtime is called CLR (Common Language Runtime) and it is usually used on Windows machines.