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A Concrete Example
Listing 1 defines an Order class to which you can apply the LCOM metrics. Figure 1 displays a high-level UML model of an imaginary Order Processing system.
Figure 1: This shows a UML class diagram for an Order Processing Sytem.
Note: The example UML diagram and the source code below is just an example and present a very much rough code sample. Please do not try to reuse this class for any real project.
To apply the LCOM3 metric to the Order class, define the metric variables as follows:
m(number of methods in the class) = 4
a(number of attributes in the class)=5
m[prod] = number of methods accessing the attribute prod = 3
m[orderNumber] = number of methods accessing the attribute orderNumber= 2
m[totalAmount] = number of methods accessing the attribute totalAmount = 4
m[deliveryType] = number of methods accessing the attribute deliveryType = 2
m[underPromotion] =number of methods accessing the attribute underPromotion=3
Therefore:
LCOM3 = ( m - sum(mA)/a ) / (m - 1)
= ( 4 - 14/5 ) / (4-1)
= 0.73
Remember to also count the constructor as a method of the class in this calculation.
The LCOM analysis on the Order class revealed that it had a reasonably high level of non-cohesiveness. Although, it is not "alarming" (as values >1 would be alarming according to LCOM3), you would be wise to improve its design.
A little inspection of all the methods in the class will immediately reveal that the method checkIfProductExists() is not a good fit for this class. The Order class must encapsulate the behaviours that an Order will need to exhibit. Basically, assume that when an instance of Order is created, the product within it is a valid and existing product. The responsibility of assigning a valid product to the Order object is better placed as part of the OrderManager class.
After removing the checkIfProductExists() method from the Order class, recalculate the value of LCOM3:
m(number of methods in the class) = 3
a(number of attributes in the class)=5
Therefore:
LCOM3 = ( m - sum(mA)/a ) / (m - 1)
= ( 3 - 13/5 ) / (3-1) ***Note: sum(mA) is now reduced by one.
= .20
This class is now more cohesive, with an LCOM value of 0.20—compared to the previous value of 0.73. That simple change made a world of difference in peace of mind.
Measuring Coupling
Coupling is a word that's usually used in a derogatory manner in design review meetings. Even so, it's not possible to design a functional OO application without coupling. Whenever one object interacts with another object, that is a coupling. In reality, what you need to try to minimise is coupling factors. Strong coupling means that one object is strongly coupled with the implementation details of another object. Strong coupling is discouraged because it results in less flexible, less scalable application software. However, coupling can be used so that it enables objects to talk to each other while also preserving the scalability and flexibility.
Though this seems like a difficult task, OO metrics can help you to measure the right level of coupling.
Demeter's Law
Ian Hollaand first proposed the Law of Demeter. The class form of Demeter's Law has two versions: a strict version and a minimization version. The strict form of the law states that every supplier class of a method must be a preferred supplier. The minimization form is more permissive than the first version and requires only minimizing the number of acquaintance classes of each method.
- Definition 1 (Client): Method M is a client of method f attached to class C, if inside M message f is sent to an object of class C, or to C. If f is specialized in one or more subclasses, then M is only a client of f attached to the highest class in the hierarchy. Method M is a client of some method attached to C.In Listing 1, the constructor of the Order class is a client to the Product class because it calls the getPrice() method of the Product class.
- Definition 2 (Supplier): If M is a client of class C then C is a supplier to M. In other words, a supplier class to a method is a class whose methods are called in the method. In Listing 1, the Product class is a supplier class to the client class Order.
- Definition 3 (Acquaintance Class): A class C1 is an acquaintance class of method M attached to class C2, if C1 is a supplier to M and C1 is not one of the following:
- The same as C2;
- A class used in the declaration of an argument of M
- A class used in the declaration of an instance variable of C2
In Listing 1, Product is an acquaintance class of the Order class, because Product is declared as an instance variable of the Order class and also passed as an argument to the constructor.
- Definition 4 (Preferred-supplier class): Class B is called a preferred-supplier to method M (attached to class C) if B is a supplier to M and one of the following conditions holds:
- B is used in the declaration of an instance variable of C
- B is used in the declaration of an argument of M, including C and its superclasses
- B is a preferred acquaintance class of M.
In Figure 1, the OrderLine, Order class is a preferred supplier class to the OrderManager class as they are used as instance variables inside the OrderManager class.
A Concrete Example
In the imaginary Order Processing example, imagine you've got the following form of code in the OrderManager class.
Class OrderManager {
Private Order order;
public void getOrderByCustomer() {
int productPrice = order.prod.price
}
You can see that the Product class becomes a supplier to the OrderManager class. This is not allowed according to the strict form of Demeter law because Product is not a preferred supplier to the OrderManager class.
Try the following:
Class OrderManager {
Private Order order;
public void getOrderByCustomer() {
int productPrice = order.getProductPrice();
}
Note that the above code makes the assumption that OrderManager will deal with only one Order at one time and also that one Order can have only one Product.
In the modified code, the client OrderManager class does not need to know how the object model is between Order and Product. Also, you never used or initialised the Product class within the OrderManager class. Having to reference to the Product class indirectly to get the price is a violation of Demeter Law.
Figure 2 presents a corrected UML diagram with all the changes that have been made so far.
Figure 2: The corrected UML diagram.
What It All Adds Up To
All these coupling metrics really serve the same function: to reduce dependencies between several classes. The less dependency, the better the chance of a more flexible solution. But in OO programming, coupling is unavoidable. Therefore, the goal is to reduce unnecessary dependencies and make necessary dependencies coherent. In this article's example, the OrderManager class is still coupled to the OrderLine and Order classes, but its unnecessary coupling with the Product class has been eliminated.
Make no mistake, these metrics are vital to measuring the quality of an application.
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