CIS3023: Programming Fundamentals for CIS Majors IISummer 2010

Ganesh Viswanathan

Interfaces (Part I)

Course Lecture Slides9 June 2010

“ As far as the customer is concerned, the interface is the product. ” --Jef Raskin

ANIMALpicturefoodsleep()roam()makeNoise()eat()

FELINE

roam()

CANINE

roam()

LIONmakeNoise()eat()

CATmakeNoise()eat()

WOLFmakeNoise()eat()

DOGmakeNoise()eat()

Problem

Need to add following methods to model Pet behaviors to the pet animal classes

• play()• beFriendly()

Option 1

Add concrete methods: play() and beFriendly() to Animal class

Animal

Feline Canine

Lion Cat Wolf Dog

Option 2Add abstract methods: play() and

beFriendly() to Animal class

Provide empty body for them in Lion and Wolf classes

Provide non-empty body for them in Cat and Dog classes

Animal

Feline Canine

Lion Cat Wolf Dog

Option 3

Add concrete methods: play() and beFriendly() only in Cat and Dog classes

Animal

Feline Canine

Lion Cat Wolf Dog

What do we need?Pet behaviors just in Pet classes like Dog and Cat

A way to guarantee that all Pet classes use the same signature for these Pet methods

A way to take advantage of Polymorphism

What do we need?

Animal

Feline Canine

Lion Cat Wolf Dog

Pet

But Java does not allow multiple inheritance! Why?

Multiple Inheritance Issues

DigitalRecorder

CDBurner

burn()DVDBurner()

burn()

ComboDrive

Solution: Interfaces

Animal

Feline Canine

Lion Cat Wolf Dog

Petabstract play();abstract beFriendly();

Make Pet an interface and put all pet behaviors in it as abstract methods

InterfacesAn interface is a classlike construct that

• contains only constants and abstract methods. • cannot contain variables or concrete methods.

Syntax for declaring interfaces

public interface <InterfaceName> { // constant declarations; // method signatures;}

Pet Interface

public interface Pet {public void

beFriendly();public void play();

}

Interfaces, cont.To implement an interface, a class must:

1. include the phrase

implements <InterfaceName>

at the start of the class definition

2. Provide body for all the methods listed in the definition of the interface or it must be declared abstract

Implementing Pet Interfaceclass Dog extends Canine implements Pet{ public void beFriendly(){

System.out.println(“Wag tail”); }

public void play(){ System.out.println(“Catch ball”);

} }class Cat extends Feline implements Pet{ public void beFriendly(){

System.out.println(“Curl up ”); }

public void play(){ System.out.println(“Roll ball”);

} }

class Main{ public static void main(String[] args){

Pet[] pets = new Pet[2];pet[0] = new Dog();pet[1] = new Cat();for(int i = 0; i < 2; i++) pet[i].play();

}}

Using the Pet Interface

Characteristics of Interfaces• Interfaces are defined in their own file

• All methods of an interface are abstract. They do not have implementations. You may omit the abstract keyword.

• Interfaces do not have instance fields.

• Interfaces can have constant fields. These are automatically treated as public, static, final; you may omit any of these keywords

Characteristics of Interfaces, cont.• A class can implement any number of

interfaces by providing implementations for all the methods of all the interfaces it claims to implement.

Example:public class A implements I1, I2, I3 {

…}

Characteristics of Interfaces, cont.• Interfaces can not be instantiated

• But can be used as a data type for a variable

Animal

Feline Canine

Lion Cat Wolf Dog

Petabstract play();abstract beFriendly();

Robot

RoboDog Roomba

Classes from different inheritance hierarchy can implement same interface!

Interfaces vs. Abstract Classes

Variables Constructors Methods

Abstract class

No restrictions Constructors are invoked by subclasses through constructor chaining.

An abstract class cannot be instantiated using the new operator.

No restrictions.

Interface All variables must be public static final

No constructors.

An interface cannot be instantiated using the new operator.

All methods must be public abstract instance methods

Interfaces vs. Abstract Classes

• You can not inherit from multiple classes but a class can implement multiple interfaces

• Interfaces is used to model “can do” type relationships while Inheritance used to model “is a” type of relationship

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Get more info!

• Interfaces @Java-doc: http://java.sun.com/docs/books/tutorial/java/IandI/createinterface.html

• Why use interfaces?

http://www.codestyle.org/java/faq-Interface.shtml

• Characteristics of interfaces:http://mindprod.com/jgloss/interface.html

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