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Trail: Learning the Java Language
Lesson: Interfaces and Packages

## Creating Interfaces

> The Java programming language supports interfaces that you use to define a protocol of behavior that can be implemented by any class anywhere in the class hierarchy.
> What Is an Interface?
>   This section defines the term "interface," shows you an example of an interface and how to use it, and talks about why you may need interfaces in your programs.

> Defining an Interface
>   Defining an interface is similar to creating a new class. An interface definition has two components: the interface declaration and the interface body.

>   >     interfaceDeclaration {
>   >         interfaceBody
>   >     }

>   The interfaceDeclaration declares various attributes about the interface such as its name and whether it extends another interface. The interfaceBody contains the constant and method declarations within the interface.

> Implementing an Interface
>   To use an interface, you write a class that implements the interface. When a class claims to implement an interface, the class is claiming that it provides a method implementation for all of the methods declared within the interface (and its superinterfaces).

> Using an Interface as a Type
>   When you define a new interface you are in essence defining a new reference data type. You can use interface names anywhere you'd use any other type name: variable declarations, method parameters and so on.

> Warning! Interfaces Cannot Grow
>   If you ship public interfaces to other programmers, here's a limitation of interfaces that you should be aware of: Interfaces cannot grow. Let's look at why this is the case.

> Summary
>   A summary of important concepts covered in this section.

> Questions and Exercises: Interfaces

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