Lecture 8 : programming language

Constructors and Inheritance
In a hierarchy, it is possible for both super classes and subclasses to have their own constructors. This raises an important question: what constructor is responsible for building an object of the subclass—the one in the superclass, the one in the subclass, or both? The answer is this: the constructor for the superclass constructs the superclass portion of the object, and the constructor for the subclass constructs the subclass part. This makes sense because the superclass has no knowledge of or access to any element in a subclass. Thus, their construction must be separate.
The preceding examples have relied upon the default constructors created automatically by Java, so this was not an issue. However, in practice, most classes will have explicit constructors.
Here you will see how to handle this situation.
When only the subclass defines a constructor, the process is straightforward: simply
construct the subclass object. The superclass portion of the object is constructed automatically using its default constructor. For example, here is a reworked version of Triangle that defines a constructor. It also makes style private since it is now set by the constructor.
// Add a constructor to Triangle.
// A class for two-dimensional objects.
class TwoDShape {
private double width; // these are
private double height; // now private
// Accessor methods for width and height.
double getWidth() { return width; }
double getHeight() { return height; }
void setWidth(double w) { width = w; }
void setHeight(double h) { height = h; }
void showDim() {
System.out.println("Width and height are " +
width + " and " + height);
}
}
// A subclass of TwoDShape for triangles.
class Triangle extends TwoDShape {
private String style;
// Constructor
Triangle(String s, double w, double h) { //Initialize TwoDShape portion of object.
setWidth(w);
setHeight(h);
style = s;
}
double area() {
return getWidth() * getHeight() / 2;