Should we avoid to use Object as the input parameter/ output value of a method?

Question

Take Java syntax as an example, though the question itself is language independent. If the following snippet takes an object MyAbstractEmailTemplate as input argument in the method setTemplate, the class MyGateway will then become tightly-coupled with the object MyAbstractEmailTemplate, which lessens the re-usability of the class MyGateway.

A compromise is to use dependency-injection to ease the instantiation of MyAbstractEmailTemplate. This might solve the coupling problem to some extent, but the interface is still rigid, hardly providing enough flexibility to other developers/ applications.

So if we only use primitive data type (or even plain XML in web service) as the input/ output of a method, it seems the coupling problem no longer exists. So what do you think?

public class MyGateway {

    protected MyAbstractEmailTemplate template;

    public void setTemplate(MyAbstractEmailTemplate template) {
        this.template = template;
    }
}

Answer 1

It's pretty difficult to understand what you are really asking, but going the route of typing everything to Object does not lead to loose coupling because you can't do anything with the input without downcasting, which would break the Liskov Substituion Principle.

Taken to the extreme it leads you here:

public class MyClass
{
    public object Invoke(object obj);
}

This is not loose coupling, it's just obscure and hard-to-maintain code.

Answer 2

IMHO there is nothing inherently wrong in using objects (wth small cap, not Objects) as method parameters and/or class members. Yes, these create dependencies. You can manage this in (at least) two ways:

• acknowledge that by creating this dependency, the two classes become tightly coupled. This is entirely appropriate in many cases, where two (or more) classes in fact form a component, which is a meaningful unit of reuse in itself, and its parts may not make much sense or be interchangeable.
• if there are multiple interchangeable candidates for a method parameter, these are obvious candidates to form a class hierarchy. Then you program for the interface and can pass any object of any class implementing that interface as parameter to your method.
  Note that the phrase "there are multiple interchangeable candidates for a method parameter" is a loose rephrasing of the Liskov Substitution Principle, which is the foundation of polymorphism.
• in some languages, e.g. C++, the third way would be using templates. Then you need no common interface, only specific methods/members need to resolvable when the template is instantiated. However, since instantiation happens at compile time, this is entirely static binding.

Answer 3

MyGateway has to assume something about the inputs. Even if it used XML, it would have to assume something about the structure and content of the XML. Coupling isn't an evil in its own right; expresses the contract between two pieces of code. The oft-repeated advice to avoid tight coupling is really just saying that coupling should express the essence of a contract, not more and not less. Passing a specific type (particularly an interface type) is a very good way to achieve this balance.

Answer 4

The name MyAbstractEmailTemplate makes me believe that you are talking about an abstract class.

You should always program against interfaces, so instead of having MyGateway depend on MyAbstractEmailTemplate, it should depend on an EmailTemplate interface, where MyAbstractEmailTemplate implements EmailTemplate. Then, you can pass your custom implementations around as you want to, without further tight coupling.

Combine this with DI and you've got yourself a pretty decent solution.

Not exactly sure what you mean with "the interface is still rigid", but obviously you should design your interface in such a way that it provides the functionality you need.

Answer 5

The first problem you will run into is that a lot of types are simply not representable by a primitive data type (It's a Java problem that there are primitive types at all.).

The coupling should be reduced by using a proper inheritance hierarchy. What means proper? The method should take exactly that part of the interface as a parameter that is need. Not more not less.

After all you won't be able to avoid dependencies. Methods have to know about what they can do with their input or have to able to make assumptions (see C++ concepts) about the capabilities of the input.

Answer 6

sThe problem is I would say, that the best java can offer are interfaces and people start to see that they are too rigid. It would be interesting to use something like what is in Go language, that allows flexible checking for all methods of an interface to be present in the type, you do not have to be explicit about implementing some interface. We also need something better than interfaces to specify the constraints - maybe some sort of contracts. Another thing is the interface evolution.