Minimalist Abstract Observer | 289applied to all subclasses, a change to the superclass will do the trick. All changes are
inherited by the subclasses.
The biggest disadvantage attributed to inheritance isbreaking encapsulation. This
occurs where a subclass is changed because a parent class changes. The change may
break certain functionality of the subclass. (Other actions unrelated to inheritance
can break encapsulation, but they’re not relevant here.) Another disadvantage of
inheritance is the inheritance of unwanted features. Subclasses are stuck with the full
set of features from the parent class, and, as was seen in Chapter 4, overrides are
used to solve this problem. Finally, because inheritance occurs at compile time, you
cannot change the implementations from the parent class at runtime. All of these dis-
advantages reduce flexibility.
One advantage of object composition is the flip side of the main disadvantage of
inheritance—maintaining encapsulation. Object composition focuses on what each
object in the object set does, and its relationship to other objects. By focusing on
clear and limited tasks for each class (object), keeping encapsulation intact is better
and simpler. The focus changes from how something works to the relationship
between objects, and because each object comes to be dependent on others, object
encapsulation and reliability are more important and central.
By having securely encapsulated and focused objects, your number of classes is likely
to be larger, and that fact is a disadvantage of object composition. However, these
small focused classes also mean small hierarchies, and instead of inheriting andbeing
a feature of the object, composition allows one object tohave afeature of another
object.
The Observer design pattern uses object composition instead of inheritance. The two
major interfaces and concrete classes are eithersubjectorobserver. Together, these
objects arecomposed to create the overall model for the software design.
Minimalist Abstract Observer
To launch an Observer application using the minimum essentials, we need only two
interfaces and two classes, reflecting the model in the class diagram (Figure 8-2):
- A subject interface
- An observer interface
- A concrete subject
- A concrete observer
Keeping the overall goal in mind to centrally distribute state information, the con-
structs boil down to a subscribing method, an unsubscribing method, a notification
method for the subject, and an update method for the observer. The key to making