In computer science, an application programming interface, or API, is the part of a library of code that is exposed to the public, so an API programmer writes code libraries for other programmers to use.

When programmers use the term API, they are usually talking about a library that can be plugged into any project. For example, the Google Maps API can be plugged into any app to display map information. The Google engineers who maintain this API are responsible for debugging it and updating new releases.

The Anatomy of a Public API

The reason a library of code needs an API instead of simply allowing programmers to have direct access to its data is to prevent unauthorized use of the data. All of the major programming languages use a kind of coding called object-oriented programming, or OOP. This is a form of programming that keeps data protected by storing it inside dynamically allocated memory blocks called objects. Access to data variables is restricted by declaring them to be only privately accessible within the original code, so only the author of the library can change what an object is or does. The code that defines an object’s composition and behavior is called a class, and if your project needs to access data that is created and manipulated within an object, you will have to import the library, instantiate an object and use it according to the public API.

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If your project only needs to manipulate data that is created within your program, you may use static API functions to do operations on the variables in your app. The difference between static and dynamic functions is that dynamic functions are only defined in the program memory when the object that owns them is created. Static functions, on the other hand, are always available. In practice, this means that dynamic objects contain data while static functions are used when the API doesn’t need to create any data. The data that is stored in memory is really the important part of any program, and the way it’s accessed and manipulated depends on many factors, including the choice of programming language, design patterns and public API.

How to Use an Application Programming Interface

Say you need to combine all the images in a directory on your computer into several large image grids. One way is to open them all in Photoshop and copy and paste them into a new image, but if you have to do this dozens of times, it will be extremely tedious and time-consuming. A better way is to write a script that walks through the files in your directory, combines the images into grids and saves new files on your hard drive.

Perhaps the most common scripting language is Python, and the Python Image Library, or PIL, API includes classes and static functions to accomplish your goal. By typing “from PIL import Image” at the beginning of your script, you have access to the “Image” classes and functions in the public API of PIL, including the paste and save functions.

Computer science is one of the fastest-growing fields in the economy, and every major project needs to use API libraries to do operations on program data. If you want to know more about what an API programmer does, you should consider contributing to one of the hundreds of open-source projects on GitHub or Bitbucket.