0.2 Encapsulation - the big picture  (Page 3/12)

Create instances of the new type

Once the new type (class) is defined, one or more objects of that type can be brought into being (instantiated, caused to occupy memory) .

Objects have state and behavior

Once instantiated, the object is said to have state and behavior . The state of an object is determined by the current values of the data that it contains and the behavior of an object is determined by its methods.

The state and behavior of a GUI Button object

For example, if we think of a GUI Button as an object, it is fairly easy to visualize the object's state and behavior.

A GUI Button can usually manifest any of a number of different states: size, position, depressed image, not depressed image, label, etc. Each of these statesis determined by data stored in the instance variables of the Button object at any given point in time. (The combination of one or more instance variables that determine a particular state is often referred toas a property of the object.)

Similarly, it is not too difficult to visualize the behavior of a GUI Button . When you click it with the mouse, some specific action usually occurs.

An ActionScript class named Button

If you dig deeply enough into the ActionScript class library, youwill find that there is a class named Button . Each individual Button object in a Flex application is an instance of the ActionScript class named Button .

The state of Button objects

Each Button object has instance variables, which it does not share with other Button objects. The values of the instance variables define the state of the button at any given time. Other Button objects in the same scope can have different values in their instance variables.Hence they can have a different state.

The behavior of a Button object

Each Button object also has certain fundamental behaviors such as responding to a mouse click event or responding to a mouseOver event.

The ActionScript programmer has control over the code that is executed in response to the event. However, the ActionScript programmer has no controlover the fact that a Button object will respond to such an event. The fact that a Button will respond to certain event types is an inherent part of the type specification for the Button class and can only be modified by modifying the source code for the Button class.

Encapsulation

If abstraction is the design or specification of a new type, then encapsulation is its definition and implementation.

A programmer defines the data representation and the behavior of an abstract data type into a class, thereby defining its implementation and its interface.That data representation and behavior is then encapsulated in objects that are instantiated from the class.

Expose the interface and hide the implementation

According to good object-oriented programming practice, an encapsulated design usually exposes the interface and hides the implementation. This isaccomplished in different ways with different languages.

Just as most of us don't usually need to care about how the steering mechanism of a car is implemented, a user of a class should not need to careabout the details of implementation for that class.