Polymorphism

COMP122 Lectures 2021-03-08

Class Hierarchy = Type Hierarchy

Every class defines a data type.

• Subclasses therefore define sub-types.

Example

Every circle is also a shape and thus can be assigned to a variable of type Shape.

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

Shape s = new Circle();
s.colour = "red";
s.radius = 2.0; // this will fail 
s.to.String(); // this will call Circle.toString()

Assigning Variables Between Subclasses

Circle c = new Circle("red");
Triangle t = new Triangle ("blue");

Shape s = c; // makes a new shape initialised to c
System.out.println(s); // calls s.toString() from circle

s = t; // reassigns to triangle
System.out.println(s); // calls s.toString() from triangle

You can see that the Shape s inherits the methods from it’s assignment when assigned a variable of a subclass.

Type Casting

This is telling the compiler that an object is from a particular subclass and not of a superclass.

Shape s = new Circle();
Circle c = s; // will fail as not every Shape is Circle
Circle c = (Circle)s; // will pass as we know s is Circle