What is an API? Application programming interfaces explained

API, for application programming interface, is one of those acronyms that is used everywhere from command-line tools to enterprise Java code to Ruby on Rails web apps. Unless you write every single line of code from scratch, you’re going to be interacting with external software components, each with its own API. Even if you do write something entirely from scratch, a well-designed software application will have internal APIs to help organize code and make components more reusable.

Diving a little deeper, an API is a specification of possible interactions with a software component. For example, if a car was a software component, its API would include information about the ability to accelerate, brake, and turn on the radio. It would also include information about how to accelerate: Put your foot on the gas pedal and push. The “what” and “how” information come together in the API definition, which is abstract and separate from the car itself.

One thing to keep in mind is that the name of some APIs is often used to refer to both the specification of the interactions and to the actual software component you interact with. The phrase “Twitter API,” for example, not only refers to the set of rules for programmatically interacting with Twitter, but is generally understood to mean the thing you interact with, as in “We’re doing analysis on the tweets we got from the Twitter API.”

Let’s dig in by looking at the Java API and the Twitter API as examples. First, we’ll get a quick picture of these two APIs and how they fulfill the definition of “what” and “how.” Then, we’ll talk about when you’ll likely use APIs and what goes into a well-designed API.

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