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A software interface may refer to a range of different types of interface at different "levels": an operating system may interface with pieces of hardware. Applications or programs running on the operating system may need to interact via streams, and in object oriented programs, objects within an application may need to interact via methods.
Software interfaces in practice
A piece of software provides access to computer resources (such as memory, CPU, storage, etc.) by its underlying computer system; the availability of these resources to other software can have major ramifications—sometimes disastrous ones—for its functionality and stability. A key principle of design is to prohibit access to all resources by default, allowing access only through well-defined entry points, i.e. interfaces.
The types of access that interfaces provide between software components can include: constants, data types, types of procedures, exception specifications and method signatures. In some[which?] instances, it may be useful to define public variables as part of the interface. It often also specifies the functionality of those procedures and methods, either by comments or (in some[which?] experimental languages) by formal logical assertions and preconditions.
The interface of a software module is deliberately kept separate from the implementation of that module. The latter contains the actual code of the procedures and methods described in the interface, as well as other "private" variables, procedures, etc.. Any other software module (that can be referred to as a client to A) that interacts with is forced to do so only through the interface. One practical advantage of this arrangement is that replacing the implementation of by another one that meets the same specifications of the interface should not cause to fail—as long as its use of complies with the specifications of the interface. (See also Liskov substitution principle.)
Software interfaces in object-oriented languages
In object-oriented languages the term "interface" is often used to define an abstract type that contains no data, but exposes behaviors defined as methods. A class having all the methods corresponding to that interface is said to implement that interface. Furthermore, a class can implement multiple interfaces, and hence can be of different types at the same time.
An interface is hence a type definition; anywhere an object can be exchanged (in a function or method call) the type of the object to be exchanged can be defined in terms of an interface instead of a specific class. This allows later code to use the same function exchanging different object types; hence such code turns out to be more generic and reusable.