Interface

An interface is a point of contact that enables communication and the exchange of information between two sides — a human and a device, two programs, or components of a larger system. It hides the internal complexity of one side and exposes to the other a structured, predictable way of interacting. As a result the user does not need to know how the system is built, and programs can cooperate without knowing each other's internal implementation.

What an interface involves

In computing the concept appears at several levels:

• user interface (UI) — the graphical or text layer a human interacts with;
• application programming interface (API) — a set of rules through which programs communicate;
• command-line interface (CLI) — controlling a system with text commands;
• hardware interface — physical connectors linking devices, such as USB or HDMI.

The common denominator is a contract: both sides agree on how the exchange looks, so the internals of each can change without breaking cooperation as long as the interface stays the same.

Practical applications

A well-designed user interface determines how comfortable an application is to use and is closely tied to the quality of the experience, or UX. Programming interfaces let services connect — a payment gateway, a map or a shipping system all talk to an application through an API. A stable, well-documented interface is crucial in system architecture, because it lets individual parts be developed and replaced without rebuilding the whole. In practice this principle supports teamwork: when two parts of a system are joined by a clearly defined interface, they can be developed in parallel and independently, as long as both sides honour the contract. Changing an interface, however, can be costly, because it forces updates to everything that relies on it — which is why its design is treated as a long-term decision. A well-designed interface is also intuitive: it hints to the user what can be done and shows the developer how to call a function correctly, minimising the risk of errors.