UML Weekend Crash Course™ - To Parent Directory

Session 3—How to Approach the UML 31
been driving for two years and still doesn’t know about them. The information that the car
exposes so that someone can use the car is called the car’s interface. The interface is how
you communicate to the car that you want to use one or more of the car’s behaviors. For
example, when you press on the gas pedal, you’re telling the car to go faster.
So in order to use an object, you need to expose the interface of the object, like the car
interface in Figure 3-6.
Figure 3-6
The interface for a car allows us to use the car.
To make the object work properly
I remember as a kid our neighbors had the old shell of a car in a field behind the house. It
had brake and gas pedals, a steering wheel, a gearshift, and so on. But wish as we might,
the car never responded. An interface without an implementation doesn’t do much. We could
communicate with the object, but the object didn’t have any way to respond.
In order to make the object work, you need to provide the mechanisms that respond to
the interface. Here is where it all starts to come together. Remember back to the part about
what an object knows When you press the gas pedal, the behavior (the implementation
behind that interface) needs to know how the car is put together and the state of the car.
An object knows about itself, so the car would know how it is put together and its current
state.
More than that, the knowledge must not be altered inappropriately or the behavior won’t
work properly. Consequently, encapsulation tells you that the information has to be inside
the object with the behavior so that you can control access to it and protect its integrity.
This is why encapsulation is often called information hiding. You hide the information inside
the object, where the object has complete control. For example, Figure 3-7 illustrates the
hidden, internal mechanisms that make an engine work properly.
In order to make the object work properly, you need to place inside the object:
The implementations for each interface
The data that describes the structure of the object
The data that describes the current state of the object