Does this method call violate the Law Of Demeter?

Question

Say you have something like the following (sadly, I'm not allowed to post the original code):

public void foo() {
    MyObject obj = getMyObject();

    bar(obj);
}

public void bar(MyObject obj) {
   Type type = new Type(obj.getOtherObject());
}

foo calls bar, passes in obj. But instead of using obj,it calls a getter on it to retrieve the needed information. Does this violate the Law Of Demeter?

Would it be better to write something like this:

public void foo() {
    MyObject obj = getMyObject();

    bar(obj.getOtherObject());
}

public void bar(MyOtherObject otherObj) {
   Type type = new Type(otherObj);
}

Answer 1

Indeed according to the wiki on the Law of Demeter:

The fundamental notion is that a given object should assume as little as possible about the structure or properties of anything else...

Your bar assumes that a given MyObject (a concrete type so strongly coupled, again against LoD) has a method called getOtherObject, so your proposed solution sorts the assumption and moves the code closer to adhering to LoD. You can go even further and instead provide the type that bar wants:

bar(new Type(obj.getOtherObject());

Depending on your language, can you not pass an interface/contract instead of a solid type? This would turn the strong coupling into a looser coupling.

Of course, if this is all internal to a given object then perhaps it isn't breaking LoD because it's a "close friend":

• Each unit should have only limited knowledge about other units: only units "closely" related to the current unit.
• Each unit should only talk to its friends; don't talk to strangers.
• Only talk to your immediate friends.

In OO I think your original code is breaking LoD based on this argument:

...an object A can request a service (call a method) of an object instance B, but object A cannot "reach through" object B to access yet another object, C, to request its services. Doing so would mean that object A implicitly requires greater knowledge of object B's internal structure.

To me it seems that you are using obj in order to call getOtherObj. Your proposed code is a potential solution.