AbstractClassesInterfacesPolymorphism 1

Abstract Classes, Interfaces, Polymorphism

Barb EricsonGeorgia Tech

ericson@cc.gatech.eduApril 2010

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Learning Goals

• Understand at a conceptual and practical level– Inheritance– Abstract Classes– Interfaces– Polymorphism

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Object-Oriented Principles

• Objects with data (fields) and operations (methods)– Usually classes too

• Inheritance– Hierarchy of types– Generalization / Specialization

• Polymorphism– Executing the right method based on the type

of the object at run-time

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Abstract Classes

• Abstract classes are classes that can’t be instantiated. Abstract classes can only be subclassed.

• Create an abstract class by using the keyword abstract in the class declaration.– public abstract class Food

Hamburger Coke

Foodprice

calories

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Why use an Abstract Class?

• Represents an abstract idea (like a Shape or a List)

• Holds methods common to several related classes

• Holds attributes common to several related classes

• Enforce naming convention by abstract methods that must be overridden by children

• Allows for general algorithms based on abstract methods with customization by children

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Interfaces

• Interfaces are a description of behavior. – They are a special kind of abstract class that

only has public abstract methods and constants.public interface ShapeInterface{

public void setShape(int shape);

}– You don’t have to declare the methods as

abstract or public• They automatically are

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Classes Implement Interfaces

• Classes that implement interfaces must provide the implementations for the methods specified in the interface.– Or be declared abstract as well

public class ShapeCanvas implements ShapeInterface

{

public void setShape(int shape)

{ code to handle set shape }

}

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Why use an Interface?

• Separates what from who– I don’t care who you are I just need a way to

talk to you– Choose from several implementers

• A class can implement many interfaces but inherit from only one class– like multiple inheritance but easier to use– thinner than inheritance

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Interfaces Versus Inheritance

• When a class inherits from a parent class it inherits all the object attributes and methods.– With inheritance it inherits the structure and behavior

of the parent class.– With an interface it inherits only the method names

and parameter lists.

• A class can inherit from only one parent class– public class Person extends Object

• A class can implement more than one interface.– public class ShapeCanvas implements

Interface1,Interface2,…

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Comparable Interface

• How would you compare any two objects?– And decide if one is less than, equal too, or

greater than the other

• It would depend on the Class of the objects being compared– For String objects compare the letters in the

string

• Implement the Comparable interface– public int compareTo(Object object)

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Comparable Exercise

• How would you compare two Person objects?– Implement the Comparable interface

public class Person implements Comparable<Person>

– Add a compareTo methodpublic int compareTo(Person comparePerson)• Compare the last names first

– If they are equal compare the first names

• The String class implements Comparable so you can use the results of comparing the last name and first name

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Collections - java.util

• Used to hold objects – Uses wrapper classes to hold primitive values

int numItems = 3;

Integer numItemsInt = new Integer(numItems);

• Three basic types– List - ordered list of objects

• Can have duplicate objects

– Set - group of objects without an order• No duplicate objects allowed

– Map - map of keys to objects

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List and Set Interfaces and Classes<<interface>>

Collection

<<interface>>List

<<interface>>Set

<<interface>>SortedSetArrayList Vector LinkedList

TreeSet

HashSet

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Collection Methods

• Add an object to a collectionboolean add(Object object); // optional

• Remove an object from a collectionboolean remove(Object object); //optional

• See if the collection has the object in itboolean contains(Object object);

• Add all objects in another collectionboolean addAll(Collection collection); // optional

• Get the intersection of two collectionsboolean retainAll(Collection collection); // optional

• Empty a collectionVoid clear();

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Use Interface Name as Type

• Declare the type of the collection variable to be one of the main interface types– List– Set

• SortedSet

– Map• SortedMap

• This allows you to change the implementation without changing much code

Map addressMap = new HashMap();

Map addressMap = new Hashtable();

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Polymorphism

• Literally: many forms• In Object-Oriented

development it means that what happens when a message is sent to an object depends on the type (class) of the object at runtime

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How Does Polymorphism Work?

• If a class is declared to be final – then the compiler can figure out the location

of a method that matches the message

• If a class can be subclassed– then a variable declared to be of the parent

type can point to an object of the parent class or any subclass at run-time

– the compiler can’t determine the method to invoke

– the method to invoke is figured out at run-time

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Shape Panel Exercise

• Execute the main method of ShapePanel

• Click the Rectangle button and then click and drag to position the rectangle

• Click the Oval button and click and drag to position the oval

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Class Diagram for ShapePanel

Shape

draw()

Oval

draw()

Rectangle

draw()

ShapeCanvas

paint()

ShapePanel

ButtonPanel1

1

*

ShapeInterface

11

1

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Polymorphism - Many Forms

• Polymorphism is overloading that is resolved at execution time, and is also called dynamic or run-time binding.

• Say you have an array of Shapes that actually holds objects that are subclasses of shape.– When you ask a shape to draw itself what gets drawn depends

on the run-time type.Shape

draw()

Oval

draw()

Rectangle

draw()

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Add Abstract Class Subclass Exercise

– Create a new class Line which is a subclass of Shape.

• Use Oval.java or Rectangle.java as starting points

– Add “Line” to the array of shapeNames in ButtonPanel

– Compile and run ShapePanel to try it out.

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Advantages to Polymorphism

• Used to create general algorithms that work on objects of different types– Collections that hold Objects

• List, Set, Stack, Queue, Map

• Makes it easy to add new types– Just create the new class and implement the

required operations– Don’t change existing code

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Summary

• Class fields are on an object of the class Class– Not on objects of the class

• Class methods can only work on class fields– Not object fields

• Objects inherit fields and methods from a parent class– But need to use public methods to access private

inherited fields

• Polymorphism allows you to write general methods based on a common parent or interface