The Law of Demeter indicates that you should only speak to objects that you know about directly. That is, do not perform method chaining to talk to other objects. When you do so, you are establishing improper linkages with the intermediary objects, inappropriately coupling your code to other code.
That's bad.
The solution would be for the class you do know about to essentially expose simple wrappers that delegate the responsibility to the object it has the relationship with.
That's good.
But, that seems to result in the class having low cohesion. No longer is it simply responsible for precisely what it does, but it also has the delegates that in a sense, making the code less cohesive by duplicating portions of the interface of its related object.
That's bad.
Does it really result in lowering cohesion? Is it the lesser of two evils?
Is this one of those gray areas of development, where you can debate where the line is, or are there strong, principled ways of making a decision of where to draw the line and what criteria you can use to make that decision?
Grady Booch in "Object Oriented Analysis and Design":
"The idea of cohesion also comes from structured design. Simply stated, cohesion measures the degree of connectivity among the elements of a single module (and for object-oriented design, a single class or object). The least desirable form of cohesion is coincidental cohesion, in which entirely unrelated abstractions are thrown into the same class or module. For example, consider a class comprising the abstractions of dogs and spacecraft, whose behaviors are quite unrelated. The most desirable form of cohesion is functional cohesion, in which the elements of a class or module all work together to provide some well-bounded behavior. Thus, the class Dog is functionally cohesive if its semantics embrace the behavior of a dog, the whole dog, and nothing but the dog."
Subsitute Dog with Customer in the above and it might be a bit clearer. So the goal is really just to aim for functional cohesion and to move away from coincidental cohesion as much as possible. Depending on your abstractions, this may be simple or could require some refactoring.
Note cohesion applies just as much to a "module" than to a single class, ie a group of classes working together. So in this case the Customer and Order classes still have decent cohesion because they have this strong relationshhip, customers create orders, orders belong to customers.
Martin Fowler says he'd be more comfortable calling it the "Suggestion of Demeter" (see the article Mocks aren't stubs):
"Mockist testers do talk more about avoiding 'train wrecks' - method chains of style of getThis().getThat().getTheOther(). Avoiding method chains is also known as following the Law of Demeter. While method chains are a smell, the opposite problem of middle men objects bloated with forwarding methods is also a smell. (I've always felt I'd be more comfortable with the Law of Demeter if it were called the Suggestion of Demeter .)"
That sums up nicely where I'm coming from: it is perfectly acceptable and often necessary to have a lower level of cohesion than the strict adherence to the "law" might require. Avoid coincidental cohesion and aim for functional cohesion, but don't get hung up on tweaking where needed to fit in more naturally with your design abstraction.
If you are violating the Law of Demeter by having
int price = customer.getOrder().getPrice();
the solution is not to create a getOrderPrice() and transform the code into
int price = customer.getOrderPrice();
but instead to note that this is a code smell and make the relevant changes that hopefully both increase cohesion and lower coupling. Unfortunately there is no simple refactoring here that always applies, but you should probably apply tell don't ask
I think you may have misunderstood what cohesion means. A class that is implemented in terms of several other classes does not necessarily have low cohesion, as long as it represents a clear concept, and has a clear purpose. For example, you may have a class Person, which is implemented in terms of classes Date (for date of birth), Address, and Education (a list of schools the person went to). You may provide wrappers in Person for getting the year of birth, the last school the person went to, or the state where he lives, to avoid exposing the fact that Person is implemented in terms of those other classes. This would reduce coupling, but it would make Person no less cohesive.
It’s a grey area. These principals are meant to help you in your work, if you find you’re working for them (i.e. they’re getting in your way and/or you find it over complicates your code) then you’re conforming too hard and you need to back off.
Make it work for you, don’t work for it.
I don't know if this actually lowers cohesion.
Aggregation/composition are all about a class utilising other classes to meet the contract it exposes through its public methods. The class does not need to duplicate the interface of it's related objects. It's actually hiding any knwowledge about these aggregated classes from the method caller.
To obey the law of Demeter in the case of multiple levels of class dependency, you just need to apply aggregation/composition and good encapsulation at each level.
In other words each class has one or more dependencies on other classes, however these are only ever dependencies on the referenced class and not on any objects returned from properies/methods.
In the situations where there seems to be a tradeoff between coupling and cohesion, I'd probably ask myself "if somebody else had already written this logic, and I were looking for a bug in it, where would I look first?", and write the code that way.
来源：https://stackoverflow.com/questions/163071/coupling-cohesion-and-the-law-of-demeter