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The end of the class
Listing 6 also signals the end of the class named Driver .
The class named MyShape is shown in its entirety in Listing 7, and also in Listing 11 near the end of the lesson.
package CustomClasses{
public class MyShape{public function area():String{
return "General Shape\n" +"Unable to compute area.";
}// end area method}//end class}//end package The sole purpose
The sole purpose of the class named MyShape is to serve as the root of a hierarchy of more specialized shape classes and to provide a default versionof the area method that can be overridden in the subclasses.
The class accomplishes those things well, and beyond that, there isn't much to say about the code in Listing 7. Note that the class doesn't evendefine a constructor but instead uses the default constructor that is provided by the compiler.
The class named MyCircle is shown in its entirety in Listing 8, and also in Listing 12 near the end of the lesson.
package CustomClasses{
public class MyCircle extends MyShape{private var radius:Number;public function MyCircle(radius:Number){//constructor
this.radius = radius;}//end constructoroverride public function area():String{
return "Circle\n" +"Radius = " + radius + "\n" +
"Area = " + Math.PI * radius * radius;}//end area
}//end class}//end package The class named MyCircle is only slightly more complicated than the class named MyShape .
The constructor
The constructor for the class named MyCircle receives an incoming value for the radius and saves that value in a private instance variable named radius . Saving the value in an instance variable makes it available to the area method that will be executed later.
Type coercion
Recall from Listing 4 and Listing 5 that the value that is actually passed to the constructor is of type uint , which is an unsigned integer. That value is coerced to type Number by passing it as an argument that is defined to be of type Number .
An overridden area method
Note that the method named area is declared to be an override. This syntax is required by ActionScript when a method in a subclass overrides an inheritedmethod.
Return a String object
The code in the area method concatenates several individual strings (including the computed area of the circle) into a single String object and returns a reference to that object. A sample of the returned value is shown displayed in the text area of Figure 4.
Concatenation of strings with numeric values
The computed value of the area and the stored value of the radius are both numeric values. When a numeric value is concatenated with a string, thenumeric value is coerced into a string of characters, decimal points, etc., and the two strings are concatenated into a single string.
Computation of the area
As you can see in Listing 8, the area of the circle is computed as the square of the radius multiplied by the constant PI. (Hopefully you recall that formula from your high school geometry class.) As mentioned earlier, the resulting area value is concatenated with the string to its left where itbecomes part of the larger String object that is returned by the method.
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