Relationship between Superclass Objects & Subclass Objects

• If we have a class of Quadrilateral with subclasses of Rhombus, Rectangle, and Trapezoid, we can create an array of Quadrilateral whose elements can be objects in any of the subclasses.

Rhombus rhomex = new Rhombus(3);
Rectangle rectex = new Rectangle(3,5);
Quadrilateral arrayOfQuadrilaterals[] = new Quadrilateral[ 10 ];

arrayOfQuadrilaterals[0] = rhomex;
arrayOfQuadrilaterals[1] = rectex;

An object of a subclass can be treated as an object of its superclass.

• Use a cast to convert a superclass reference to a subclass reference.

  This can be done only when the superclass reference is actually referencing a subclass object;

  otherwise Java will throw a ClassCastException runtime exception.

• The accompanying applet (Fig. 7.4) illustrates both:
  • treating a sublcass's objects as objects of its superclass (a Circle object treated as a Point object)
  • treating a superclass's object as an object of one of its subclasses, when explicitly cast (a Point object treated as a Circle object).
• In the Point class definition, its instance variables x and y are protected, meaning that its subclasses, such as Circle, can refer to them directly.

  If they were declared private, Circle's methods would have to use Point's public get methods to access them.

• Since Circle inherits from Point, it inherits all of Point's non-private members, both instance variables and methods.
• The Circle constructors must invoke the Point constructor to initialize the Point part of the Circle object (its center point).

  If this were not done, the compiler would try to use the no-arguments Point constructor. (Why?)

  But, there is none, so the compiler would generate an error.

• Circle overrides the toString method of Point.

  References by Circle methods or clients to the toString method will refer to the overridden method.

  A Circle method or client can refer to the Point toString method by indicating its intention with the super. prefix: super.toString().

• Nota bene: When pointRef is sent the toString message, Java knows that the object really is a Circle, so it chooses the Circle toString method, not the Point toString method.

  Java waits until execution-time to decide which toString method to use (called dynamic binding).

• The applet's last statement incorrectly casts p as a Circle; it's an Point.

  At execution time, Java checks this, noticing that p is not a Circle object.

  It consequently throws an exception, terminating the applet (because no provision was made to handle this exception).