> For the complete documentation index, see [llms.txt](https://boxlang.ortusbooks.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://boxlang.ortusbooks.com/boxlang-language/classes/interfaces.md).

# Interfaces

Interfaces are a type of class that have a set of signatures for specific functions and it can even have some implemented functions. You can basically call an interface a signature map for the type of classes you want to create. In statically typed languages, they make a lot of sense since they can allow you to add/modify behavior of classes that the compiler can understand on how to link and compile. In a dynamic language, where functions can mutate or even be removed or injected at runtime, interfaces don't make soooo much sense. However, interfaces are a great way to provide documented signatures for developers to follow.

![](/files/sK4XfLFWazxNS4IPpo7e)

If you are developing frameworks, libraries or structured domain models where implementations can be done at a later point of time, or different strategies adapted; interfaces are king.

![](/files/GTVkTDExrxICcCAwjxzI)

## Declaration

Interfaces are defined in a file template with a `.bx` extension. For best practice you can start the name of the interface with a capital `I,` example: `IAnimal.bx, ILogger.bx, IAdapter.bx`

```java
interface extends="other_interfaces"{

    any function returnAny( required numeric obj, boolean why=false )
    function sayHello()

    ILogger function logEvent( required logEvent )

    // You can implement default behavior
    function getCacheKey(){
        return "default_key";
    }
}
```

## Implementation

Interfaces can extend other interfaces and classes that implement them can also implement many interfaces:

```java
class implements="ILogger,IAdapter"{
    any function returnAny( required numeric obj, boolean why=false ){
        // implementation here.
    }
    function sayHello(){
        return "Hola";
    }

    ILogger function logEvent( required logEvent ){
       // log this...
       return this;
    }
}
```


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