I've read these questions:

• http://programmers.stackexchange.com/q/232359/95744
• http://programmers.stackexchange.com/q/131332/95744
• http://programmers.stackexchange.com/q/283762/95744

I don't understand how to "program to an interface" if you are using methods in concrete classes that aren't part of the interface.

I realize the most common example of this design principle is the List vs ArrayList in Java because it is easy to understand and illustrate the point.

Here is a somewhat silly example to illustrate what my question is asking (the code is in PHP but it applies to most OOP languages):

interface ResponseInterface {
    public function getStatusCode();
}

class Response implements ResponseInterface {
    private $status;

    public function getStatusCode() {
        return $this->status;
    }
}

class AwesomeResponse implements ResponseInterface {
    private $status;
    private $message = ['200' => 'OK', '500' => 'Internal Server Error'];

    public function getStatusCode() {
        return $this->status;
    }

    public function getStatusMessage() {
        return $this->message[$status];
    }
}

class Server {
    public function sendResponse(ResponseInterface $response) {
        // this seems wrong -----^

        header(vsprintf('HTTP/1.1 %d %s', [
            $response->getStatusCode(),
            $response->getStatusMessage()
        ]), true, $response->getStatusCode());
    }
}

As you can see, the sendResponse method takes a ResponseInterface parameter but it calls getStatusMessage() which isn't part of the interface but only something implemented in AwesomeResponse, which implements ResponseInterface.

The application crashes at runtime when a Response object is passed as it tries to call the non-existent method getStatusMessage(). Therefore, the proper implementation would be:

public function sendResponse(AwesomeResponse $response) {
    // ...stuff
}

But AwesomeResponse isn't an interface so how do I program to an interface?

I've read these questions:

• Understanding "programming to an interface"
• What is the point of an interface?
• Does it always make sense to "program to an interface" in Java?

I don't understand how to "program to an interface" if you are using methods in concrete classes that aren't part of the interface.

I realize the most common example of this design principle is the List vs ArrayList in Java because it is easy to understand and illustrate the point.

Here is a somewhat silly example to illustrate what my question is asking (the code is in PHP but it applies to most OOP languages):

interface ResponseInterface {
    public function getStatusCode();
}

class Response implements ResponseInterface {
    private $status;

    public function getStatusCode() {
        return $this->status;
    }
}

class AwesomeResponse implements ResponseInterface {
    private $status;
    private $message = ['200' => 'OK', '500' => 'Internal Server Error'];

    public function getStatusCode() {
        return $this->status;
    }

    public function getStatusMessage() {
        return $this->message[$status];
    }
}

class Server {
    public function sendResponse(ResponseInterface $response) {
        // this seems wrong -----^

        header(vsprintf('HTTP/1.1 %d %s', [
            $response->getStatusCode(),
            $response->getStatusMessage()
        ]), true, $response->getStatusCode());
    }
}

As you can see, the sendResponse method takes a ResponseInterface parameter but it calls getStatusMessage() which isn't part of the interface but only something implemented in AwesomeResponse, which implements ResponseInterface.

The application crashes at runtime when a Response object is passed as it tries to call the non-existent method getStatusMessage(). Therefore, the proper implementation would be:

public function sendResponse(AwesomeResponse $response) {
    // ...stuff
}

But AwesomeResponse isn't an interface so how do I program to an interface?