Method Overriding

COMP122 Lectures 2021-03-08

Overriding a Superclass Method

A subclass inherits the public or protected attributes and methods in its superclass.

It can override an inherited method (identified by its signature) by re-implementing an inherited method.

Shapes Example

classDiagram
	Shape <|-- Circle
	class Shape{
		+colour String
		+toString() String
	}
	class Circle{
		+radius double
		+area() double
	}

Consider that the Shape.toString() function returns a statement like: “I am a colour shape.”

You may want to re-implement this for the subclass to be more specific.

Example in Java

The following code would emulate the behaviour above:

public class Shape {
	public String colour;
	public String toString() {
		return "I'm a " + colour + " shape!";
	}
}

public class Circle extends Shape {
	public double radius;
	public String toString() {
		return "I'm a " + colour + " circle!";
	}
	// Show print functions
	public static void main(String[] args) {
		Shape s = new Shape();
		s.colour = "red";
		System.out.println(s.toString());
		Circle c = new Circle();
		c.colour = "red";
		System.out.println(c.toString());
	}
}

Object Class

As all classes are subclasses of the Object class we can draw the following hierarchy:

classDiagram
	Shape <|-- Circle
	Object <|-- Shape
	Object: +toString() String
	class Shape{
		+colour String
		+toString() String
	}
	class Circle{
		+radius double
		+area() double
		+toString() String
	}

The Object class has a toString() method of its own so we have also overwritten it using Shape. This is the reason why you can always print a given object by calling:

className.toString();

Overriding vs. Overloading

• Overriding - Involves providing several methods with the same name and parameter list, but declared in classes which are in a subclass/superclass relationship.
• Overloading - Involves providing several methods with the same name, but different parameter lists.