Do we tell the object to do something or do we do something (on/with/to/etc.) the object?

Question

Take the (unprecedented and groundbreaking) example of the calculator:

public class Calculator
{
    public double Add(double augend, double addend) 
    {
        return augend + addend;
    }
}

When I write calculator.Add(val1, val2) which of the following am I doing?

• Telling the calculator to add the inputs
• Adding the inputs with the calculator

(One could also ask: "Is it a method of the object or is it a method on the object?")

Other examples: do we tell the repository to add the entity or do we add the entity to the repository? Do we tell the message service to deliver a message or do we send a message through the message service? Do we tell the can of paint to mix itself or do we mix the can of paint?

Answer 1

is it a method of the object or is it a method on the object?

It is a method of the class.

You create instances of classes to encapsulate state, not code. All of the code in that method is the same code across all of the objects; only the state (private variables) varies between objects.

Calling methods is a form of message passing. You send a message to an object by calling a method. The method can send back a message of its own by returning some value. The method may also cause side effects by altering the state of the object, writing to a log, etc.

Do we tell the can of paint to mix or do we mix the can of paint?

If you call

canOfPaint.Mix();

You are telling the can of paint to mix.

If you call

Mix(canOfPaint)

You are mixing the can of paint.

Answer 2

In my opinion it's a matter of the context in the system, and sometimes depends on the conceptual role of the component in the system.

For example in a game, a character might have a method attack(). Is it meant to allow other characters to attack this character? Or is it a method used to tell the character to attack? Totally depends on your system.

Answer 3

Generally speaking, it doesn't matter.

If you're telling a Calculator object to add, your code looks like this:

class Calculator {
  public Double add(Double a, Double b) {
    return a + b;
  }
}

If you're adding two number with a Calculator, it looks like this:

class Calculator {
  public Double add(Double a, Double b) {
    return a + b;
  }
}

The implementation is the same. And I suspect your code will be the same in most cases regardless of which preposition you use to describe it. In most cases, your object is both the subject and direct object of the verb. In cases where it won't be the same, you should try to clarify the method name or include adequate documentation to indicate what role the object plays in the action.