Several days ago, I published a tweet where I asked which of these two statements was better:

dog.expressHappiness();
dog.getBody().getTail().wag();

Many things can be said in favor of the first statement.

A thread 1/6

#SoftwareEngineering #oop #100DaysOfCode

The first statement:

Increases readability: it expresses the intent of the code clearly.
Reduces complexity: it contains only one method call.
Hides implementation details: it expresses "what" the code does, not "how". 2/6

The second statement makes changes difficult.

What happens if happiness must now be expressed by "jumping and barking"?

We have to apply changes in all places where statement 2 (or a similar version of it) appears.

This is how systems become *rigid* and hard to change. 3/6

The "Law of Demeter" says that a module should not know about the internals of the objects it manipulates.

Statement 2 violates this principle because it asks the dog about its internal structure (body and tail).

Statement 2 breaks encapsulation and increases coupling. 4/6

We should not be forced to traverse the graph of objects, searching for the method we want to call. We should be able to directly tell an object what to do:

object.doSomething();

This is called the "Tell Don't Ask" principle. 5/6

The first statement facilitates testing.

If we want to unit test a piece of code that calls the "expressHappiness" method, we can mock the implementation of this method.

We do not need the whole dog object implemented. 6/6