What exactly is the meaning of an API? [closed]_问答_开发者

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I've searched for the definition of an API in a programming language and I am still finding it hard to understand.

Could anyone advice me in simple, layman's terms:

What is an API?
How is it used?
When and where is it used?

Searches should include Wikipedia, which is surprisingly good for a number of programming concepts/terms such as Application Programming Interface:

What is an API?

An application programming interface (API) is a particular set of rules ('code') and specifications that software programs can follow to communicate with each other. It serves as an interface between different software programs and facilitates their interaction, similar to the way the user interface facilitates interaction between humans and computers.

How is it used?

The same way any set of rules are used.

When and where is it used?

Depends upon realm and API, naturally. Consider these:

The x86 (IA-32) Instruction Set (very useful ;-)
A BIOS interrupt call
OpenGL which is often exposed as a C library
Core Windows system calls: WinAPI
The Classes and Methods in Ruby's core library
The Document Object Model exposed by browsers to JavaScript
Web services, such as those provided by Facebook's Graph API
An implementation of a protocol such as JNI in Java

Happy coding.

An API is the interface through which you access someone elses code or through which someone else's code accesses yours. In effect the public methods and properties.

Well, in addition to all the answers, I am just adding an example.

As others said API stands for Application Programming Interface through which softwares can interact with each other. Note, not a human interaction.

Where it is used

An example: You are buying an item online through your credit card. You will provide credit card details and press 'continue' button. It will tell you whether your information is correct or not. To provide these results, there are lot of things in the background.

The application will send your credit card details to a remote application which will validate your information and send the result back to your application. API is used in this scenario.

I hope it helps for the beginners who don't understand really what API is.

ANOTHER EXAMPLE

Weather application

Without API - Weather application must open weather.com site and read the details as a human does.

With API - Weather application will send a message to weather.com and receive the result and then display it.

SOURCE - Various online resources

1) What is an API?

API is a contract. A promise to perform described services when asked in specific ways.

2) How is it used?

According to the rules specified in the contract. The whole point of an API is to define how it's used.

3) When and where is it used?

It's used when 2 or more separate systems need to work together to achieve something they can't do alone.

an API(Application Programming Interface) is a set of defined functions and methods for interfacing with the underlying operating system or another program or service running on the computer.
It is usually used by establishing a reference to a library in your software or importing a function from a dll.
It is used in one form or another in almost all software, being explicitly called in your program or implicitly called by the compiler.

API stands for Application Programming Interface, i.e. API is the way for an application to interact with certain system/application/library/etc.

For example, there are API's for OS (WinAPI), API's for other applications (like databases) and for specific libraries (for example, image processing), etc.

APIs are usually developed in a form consumable by a client application. For C/C++ applications, it a set header files and dynamic/static libraries. For Java - set of jars. And so on.

It is a set of software components that interact with one another. It provides a set of functions, variables, and object classes for the creation of an application, operating system or any other thing.

Conaider this situation:

Mark and Lisa are secretly a couple, and because of age difference they are not allowed to be together. Mark and Lisa meet every night when nobody is watching. They have estabilished their own set of rules how to comunicate when the time comes. He stands in her garden and throws the small rock at her window. Lisa knows that it is time, and responds by waving from the window and opening it afterwards so Mark can climb in. That was example how the API works. The rock is initial request to another end. Another end waves, opens the window which basicaly means "Welcome in!".

API is almost like human language but for computers.

In layman's terms, I've always said an API is like a translator between two people who speak different languages. In software, data can be consumed or distributed using an API (or translator) so that two different kinds of software can communicate. Good software has a strong translator (API) that follows rules and protocols for security and data cleanliness.

I"m a Marketer, not a coder. This all might not be quite right, but it's what I"ve tried to express for about 10 years now...

An API is a set of commands, functions, and protocols which programmers can use when building software for a specific OS or any other software. The API allows programmers to use predefined functions to interact with the operating system, instead of writing them from scratch. All computer operating systems, such as Windows, Unix, and the Mac OS and language such as Java provide an application program interface for programmers.

Source

An API defines the interfaces by which one piece of software communicates with another at the source level. It provides abstraction by providing a standard set of interfaces - usually functions - that one piece of software (typically a higher-level piece) can invoke from another piece of software (usually a lower-level piece).

For example, an API might abstract the concept of drawing text on the screen through a family of functions that provide everything needed to draw the text. The API merely defines the interface; the piece of software that actually provides the API is known as the implementation of the API.

It is common to call an API a "contract". This is not correct, at least in the legal sense of the term, as an API is not a two-way agreement. The API user (generally, the higher-level software) has zero input into the API and its implementation. It may use the API as-is, or not use it at all: take it or leave it!

A real-world example of an API is the interfaces defined by the C standard and implemented by the standard C library. This API defines a family of basic and essential functions, such as memory management and string manipulation routines.

Lets say you are developing a game and you want the game user to login their facebook profile(to get your profile information) before playing it,so how your game is going to access facebook? Now here comes the API.Facebook has already written the program(API) for you to do it, you have to just use those programs in your game application.using Facebook-API you can use their services in your application.Here is a good and detailed look on API... http://money.howstuffworks.com/business-communications/how-to-leverage-an-api-for-conferencing1.htm

Application program interface (API) is a set of routines, protocols, and tools for building software applications. An API specifies how software components should interact and APIs are used when programming graphical user interface (GUI) components. A good API makes it easier to develop a program by providing all the building blocks. A programmer then puts the blocks together.

source: http://www.webopedia.com/TERM/A/API.html