Question

I would like to have an interface that allows for a generic type

public function persist($object);

But my concrete implementations to have a type

public function persist(User $user);

From what I understand of PHP this is not possible. From an object oriented design point of view could someone explain to me why what I am doing is misguided and wrong.

Edit: I should clarify, I’m aware of type hinting and how it works my question is really trying to understand from an OO perspective where I am going wrong when I want my concrete implementation to take a different type to the interface.

Answer 1

The interface’s purpose is to be a contract between classes. An interface would be useless if multiple concrete classes implemented it, but all expected different inputs. By looking at the interface, you would not know what type of inputs that the implementing classes expect, thus making the interface basically useless. You could not interchange different concrete classes that all use the same interface, as they all expect different inputs (have different interfaces).

I could not replace classA with classB with the assurance that they would both work, since they both have the same interface. This would basically make interfaces useless for every OOP pattern known to man.

EDIT EXAMPLE

class CommandList {

    public function addCommand(Command $command) {
        $this->commands[] = $command;
    }

    public function runCommands() {
        foreach ($this->commands as $command) $command->run($this);
    }
}

interface Command {
    public function run(CommandList $commandList);
}

class Hop implements Command {
    public function run(CommandList $commandList) {
        // hop here
    }
}

class Skip implements Command {
    public function run(CommandList $commandList) {
        // skip here
    }
}

See how the interface acts as a contract? If you break that contact, then things that implement Command would not be interchangeable.