Polymorphism

The term polymorphism literally means "having many forms" A polymorphic reference is a variable that can refer to different types of objects at different points in time The method invoked through a polymorphic reference can change from one invocation to the next All object references in Java are potentially polymorphic

Example Perfectly legal: Shape s1; Rectangle r = new Rectangle(…); s1=r; Shape s2 = new Rectangle(…); Object o = new Circle(…); Not legal: Shape s = new Circle; Rectangle r = s;

What Gets Checked at Compile-Time? Validity of assignments Can cause non-obvious restrictions: Shape s = new Rectangle(…); Rectangle r = s; \\ is this still a rectangle? Checking validity of assignments at compile time lets programs run faster

Casting If you really need a more specific type, you can do this: Shape s = new Rectangle(…); Rectangle r = (Rectangle) s; But this can be dangerous, and should generally be avoided

Late Binding: Which Method? Consider: Shape s = new Rectangle(…); String myOutput = s.toString(); If toString was defined in Shape then overridden in Rectangle, which definition should be used?  s is a Shape reference: Use the one from Shape ?  s actually refers to a Rectangle Use the one from Rectangle ?

Which Method? Java follows the reference to the actual instance  the reference is just pointing to the thing we use The instance method is what gets used  Rectangle This is called late binding… Java doesn’t decide which method gets called until the method is encountered at run-time

Why? Let’s you use a more generic reference when needed, but still get the right reference public void draw(Collection shapes) { for(Shape s: shapes) s.draw(); } Different shapes will have different draw methods  … but this will use the right one in each case

Also Works with Interfaces public boolean startsWith(List list, int val) { return list.get(0)==val; } Whether the argument is an ArrayList, Vector, or LinkedList, this method works  uses the.get(…) method from the appropriate underlying implementation

Late Binding Works Everywhere Interface, Abstract Class, Non-Abstract Class  can declare a reference to any of these  any object that inherits/implements the reference type can be used for the reference  any method that is defined by the reference can be used  the implementation in the actual instance will be called

A Caveat from the Text Consider interface Speaker, with method speak() Suppose Dog and Philosopher both implement Speaker Speaker guest; guest = new Philosopher(); guest.speak(); guest = new Dog(); guest.speak();

A Caveat from the Text Now suppose Philosopher also includes a method pontificate() Then this does not work: Speaker special = new Philosopher(); special.pontificate(); Method called must be in the interface

Example: The Coffee Bar Ordering at a coffee bar  espressos, coffee, low-fat milk, etc…  lots of structure, lots of subtypes, lots of polymorphism We want to represent orders taken, these can be passed on to make the drinks

How Hard is This? © Starbucks 2005, “Make it Your Drink”

Stuff to Represent Cup (to-go, for-here, iced, personal cup) Size (small, medium, large) Shots (1 or more espresso, caf/decaf)  default determined by size, but can be changed Syrups (0 or more flavour shots) Milk (whole, 2%, skim, soy) Toppings (whipped cream, caramel) Drink (espresso shot, Americano, mocha…)

Class Hierarchy

Implementation ….