Inheritance

What is inheritance?

• A class can be defined in terms an existing class.
• The new class gets all of the methods and state variables of the old class by default.
• The new class can add new methods and state variables.
• The new class can redefine methods and state variables of its parent.

For the nerds:
• No multiple inheritance
• Interfaces provide the good features of multiple inheritance.

Diagram of the bicycle inheritance. Lifted from The Java Tutorial (Sun Microsystems).

An simple example of inheritance

class Point3D extends Point {
        double z;
        
        Point3D (double x, double y, double z) {
                this.x = x;
                this.y = y:
                this.z = z;
        }
        
        public double length() {
                return Math.sqrt(x*x + y*y + z*z);
        }
        
        ...
}

• The x and y values of the point values come from Point automatically.
• The getX and setX methods from Point do not have to be rewritten.
• The length method needs to be redefined.
• The addPoints method can be retained, but an addPoints3D needs to be added.

Inheritance and instantiation

• Inheritance and instantiation are unrelated.
• Class (object blue prints) have a strict hierarchy (no multiple inheritance).
• Objects can instantiate other objects irrespective of the class hierarchy.
• The instantiating object points to the instantiated object, but other objects may also point to the instantiated object so there is no strict hierarchy of objects.
• Objects become garbage when they are no longer pointed to.