Interfaces (The Java™ Tutorials > Learning the Java Language ...

Interfaces as APIs

The robotic car example shows an interface being used as an industry standard Application Programming Interface (API). APIs are also common in commercial software products. Typically, a company sells a software package that contains complex methods that another company wants to use in its own software product. An example would be a package of digital image processing methods that are sold to companies making end-user graphics programs. The image processing company writes its classes to implement an interface, which it makes public to its customers. The graphics company then invokes the image processing methods using the signatures and return types defined in the interface. While the image processing company's API is made public (to its customers), its implementation of the API is kept as a closely guarded secret—in fact, it may revise the implementation at a later date as long as it continues to implement the original interface that its customers have relied on.

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objects can have multiple types: the type of their own class and the types of all the interfaces that they implement. This means that if a variable is declared to be the type of an interface, its value can reference any object that is instantiated from any class that implements the interface.

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There are a number of situations in software engineering when it is important for disparate groups of programmers to agree to a "contract" that spells out how their software interacts. Each group should be able to write their code without any knowledge of how the other group's code is written. Generally speaking, interfaces are such contracts.

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interfaces are such contracts.

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In the Java programming language, an interface is a reference type, similar to a class, that can contain only constants, method signatures, and nested types.

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Interfaces cannot be instantiated—they can only be implemented by classes or extended by other interfaces.

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Note that the method signatures have no braces and are terminated with a semicolon.

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objects can have multiple types:

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This means that if a variable is declared to be the type of an interface, its value can reference any

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object that is instantiated from any class that implements the interface.

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