class heir arc hies and interfaces

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Class Hierarchies and Interfaces

Java MethodsA & AB

Object-Oriented Programmingand Data Structures

Maria Litvin Gary Litvin

Copyright 2006 by Maria Litvin, Gary Litvin, and Skylight Publishing. All rights reserved.

11

Chapter

Chap Apt

A


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11-2

Objectives:

Understand class hierarchies andpolymorphism

Learn about abstract classes

Learn the syntax for calling superclasssconstructors and methods

Understand interfaces


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Inheritance

Inheritance represents the IS-A relationshipbetween objects: an object of a subclassIS-A(n) object of the superclass.

Superclass(Base class)

Subclass(Derived class)

Subclass extendsSuperclass


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Class Hierarchies

Using inheritance, a programmer can define ahierarchy of classes.

Biped

Walker Hopper Dancer

ToedInWalker CharlieChaplin


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Class Hierarchies (contd)

Help reduce duplicationof code by factoring outcommon code from

similar classes into acommon superclass.

BipedConstructorAccessorsturnLeftturnRightturnArounddraw

Walker

ConstructorfirstStepnextStepstopdistanceTraveled

Hopper

ConstructorfirstStepnextStepstopdistanceTraveled


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Class Hierarchies (contd)

Help reduce duplication of code by letting youwrite more general methods in client classes.

public void moveAcross(Walker creature, int distance)

{creature.firstStep();while (creature.distanceTraveled() < distance)

creature.nextStep();creature.stop();

}

public void moveAcross(Hopper creature, int distance)



}

public void moveAcross(Biped creature, int distance)



}

Works for eitherWalker or Hopper

due to polymorphism


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Polymorphism

Ensures that the correct method is called foran object of a specific type, even when thatobject is disguised as a reference to a more

generic type, that is, the type of the objectssuperclass or some ancestor higher up theinheritance line.

Once you define a common superclass,polymorphism is just there

no need to do

anything special.


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Polymorphism (contd)

public void moveAcross (Biped creature, int distance){creature.firstStep();while (creature.distanceTraveled () < distance)


}

The actual parameter passed to thismethod can be a Walker, a Hopper,etc. any subclass of Biped.

Correct methods will be called automatically forany specific type of creature: Walkers methodsfor Walker, Hoppers forHopper, etc.


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Abstract Classes

Some of the methods in a class can bedeclared abstract and left with only signaturesdefined

A class with one or more abstract methodsmust be declared abstract

public abstract class Biped{

...public abstract void firstStep();public abstract void nextStep();public abstract void stop();...public void draw(Graphics g) { ... }

}

Abstractmethods


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Abstract Classes (contd)

Abstract classes serve as commonsuperclasses for more specific classes

An abstract method provides an opportunity

for the compiler to do additional errorchecking

Abstract classes and methods are needed for

polymorphism to work Abstract classes are closer to the root of the

hierarchy; they describe more abstractobjects


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Object

... Component

... Button TextComponent Container

... JComponent Window

... JTextComponent AbstractButton JPanel ...

... JTextArea JTextField ... JMenuItem JButton ...

A fragment of Java library GUI class hierarchy(abstract classes are boxed)


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Java does not allow us to instantiate (that is,create objects of) abstract classes

Still, an abstract class can have constructors

they can be called from constructors of

subclasses

A class with no abstract methods is calledconcrete


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Class Object

In Java every class by default extends alibrary class Object (fromjava.lang)

Object is a concrete class

public class Object{

public String toString {...}public boolean equals (Object other) {... }public int hashCode() { ... }

// a few other methods...

}

Methodsredefined(overridden)as necessary


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Calling Superclasss

Constructorspublic class Walker extends Biped{

// Constructorpublic Walker(int x, int y, Image leftPic, Image rightPic){

super(x, y, leftPic, rightPic);...

}}

Biped

Walker

Calls Bipedsconstructor

If present, must bethe first statement

The number / types of parameters passed tosuper must match parameters of one of thesuperclasss constructors.


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Calling Superclasss

Constructors (contd) One of the superclasss constructors is

always called, but you dont have to have an

explicit super statement. If there is no explicit call to super, then

superclasss no-args constructor is called bydefault.

Must be defined then. If not defined syntax error:cannot find symbol : constructor ...


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Calling Superclasss

Constructors (contd) Superclasss constructor calls its superclasss

constructor, and so on, all the way up to

Objects constructor.

Biped

Walker

Objectsuper( )

super(...)


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Calling Superclasss

Methodspublic class CharlieChaplin

extends Walker

{...public void nextStep (){

turnFeetIn();

super.nextStep();turnFeetOut();}

...}

Walker

CharlieChaplin

Calls Walkers

nextStep

super.someMethodrefers to someMethod inthe nearest class, up the inheritance line, wheresomeMethod is defined.


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Calling Superclasss

Methods (contd) super. calls are often used in subclasses

of library classes:

public class Canvas extends JPanel{

...public void paintComponent (Graphics g)

{super.paintComponent (g);...

}...


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Interfaces

DanceFloor

DanceGroup

ControlPanel

Band

Dancer

Aerobics

Waltz

Rumba

Cha-Cha-Cha

Salsa

Dance

Interface


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Interfaces (contd)

An interface in Java is like an abstract class,but it does not have any fields or constructors,and all its methods are abstract.

public abstract is not written because all themethods are public abstract.

public interface Dance{

DanceStep getStep (int i);int getTempo ();int getBeat (int i);

}


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Interfaces (contd)

We must officially state that a classimplements an interface.

A concrete class that implements an interface

must supply all the methods of that interface.public class Waltz implements Dance{

...// Methods:public DanceStep getStep (int i) { ... }public int getTempo () { return 750; }public int getBeat (int i) { ... }...

}


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Interfaces (contd)

A class can implement several interfaces.

Like an abstract class, an interface supplies asecondary data type to objects of a class that

implements that interface. You can declare variables and parameters of

an interface type.

Polymorphism fully applies to objectsdisguised as interface types.

Dance d = new Waltz( );


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Interfaces (contd)public interface Edible{

String getFoodGroup();int getCaloriesPerServing();

}

public class Breakfast{

private int myTotalCalories = 0;...public void eat (Edible obj, int servings)

{myTotalCalories +=

obj.getCaloriesPerServing () * servings;

}...

}

Polymorphism:the correctmethod is calledfor any specifictype of Edible,e.g., a Pancake

public class Pancakeimplements Edible

{...

}


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Classes Interfaces

A superclass provides asecondary data type to

objects of itssubclasses.

An abstract classcannot be instantiated.

An interface provides asecondary data type to

objects of classes thatimplement thatinterface.

An interface cannot beinstantiated.

Similarities


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Classes Interfaces

A concrete subclass ofan abstract class mustdefine all the inherited

abstract methods.

A class can extend

another class. Asubclass can addmethods and overridesome of its superclasss

methods.

A concrete class thatimplements an interfacemust define all the

methods specified bythe interface.

An interface can extend

another interface (calledits superinterface) byadding declarations ofabstract methods.

Similarities


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Classes Interfaces

A class can extend onlyone class.

A class can have fields.

A class defines its ownconstructors (or gets adefault constructor).

A class can implementany number ofinterfaces.

An interface cannothave fields (except,possibly, some publicstatic final constants).

An interface has noconstructors.

Differences


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Classes Interfaces

A concrete class has allits methods defined. Anabstract class usually

has one or moreabstract methods.

Every class is a part of

a hierarchy of classeswith Object at the top.

All methods declared inan interface areabstract.

An interface may

belong to a smallhierarchy of interfaces,but this is not ascommon.

Differences


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DanceStudio

DanceStudio

DanceFloor

DanceGroup

ControlPanel

interfaceDanceDancerBiped

Foot

CoordinateSystem

DanceStep

A B

extends

depends on

has

implements

Band

DanceLesson

interfaceStudentGroup

< various dances>

AbstractDance


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Review

Describe two ways for eliminating duplicatecode using class hierarchies.

What is an abstract class?

Why is it better to use an abstract methodrather than an empty method?

Define concreteclass.

What happens when a constructor of asubclass does not have a super statement?Is superclasss constructor called?


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Review (contd)

Can an abstract class be instantiated?

Can someMethod1 have a callsuper.someMethod2 ( )?

What happens if, by mistake, a programmerputs in his paintComponent method a call

paintComponent(g);

instead ofsuper.paintComponent(g);

?


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Review (contd)

What is the main difference between anabstract class and an interface?

Can a class implement several interfaces?

Suppose you declare a variable of aninterface type. What type of value can beassigned to that variable?

What is the main advantage of interfaces overabstract classes?

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