Class Hierarchies and Interfaces

Java MethodsJava MethodsA & ABA & 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

11-2

Objectives:

• Understand class hierarchies and polymorphism

• Learn about abstract classes

• Learn the syntax for calling superclass’s constructors and methods

• Understand interfaces

11-3

Inheritance

• Inheritance represents the IS-A relationship between objects: an object of a subclass IS-A(n) object of the superclass.

Superclass(Base class)

Subclass(Derived class)

Subclass extends Superclass

11-4

Class Hierarchies

• Using inheritance, a programmer can define a hierarchy of classes.

Biped

Walker Hopper Dancer

ToedInWalker CharlieChaplin

11-5

Class Hierarchies (cont’d)

• Help reduce duplication of code by factoring out common code from similar classes into a common superclass.

Biped Constructor Accessors turnLeft turnRight turnAround draw

Walker Constructor firstStep nextStep stop distanceTraveled

Hopper Constructor firstStep nextStep stop distanceTraveled

11-6

Class Hierarchies (cont’d)• Help reduce duplication of code by letting you

write 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) { creature.firstStep(); while (creature.distanceTraveled() < distance) creature.nextStep(); creature.stop(); }

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

Works for either Walker or Hopperdue to polymorphism

11-7

Polymorphism

• Ensures that the correct method is called for an object of a specific type, even when that object is disguised as a reference to a more generic type, that is, the type of the object’s superclass or some ancestor higher up the inheritance line.

• Once you define a common superclass, polymorphism is just there no need to do anything special.

11-8

Polymorphism (cont’d)

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

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

Correct methods will be called automatically for any specific type of creature: Walker’s methods for Walker, Hopper’s for Hopper, etc.

11-9

Abstract Classes• Some of the methods in a class can be

declared abstract and left with only signatures defined

• A class with one or more abstract methods must be declared abstract

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

Abstract methods

11-10

Abstract Classes (cont’d)

• Abstract classes serve as common superclasses for more specific classes

• An abstract method provides an opportunity for the compiler to do additional error checking

• Abstract classes and methods are needed for polymorphism to work

• Abstract classes are closer to the root of the hierarchy; they describe more abstract objects

11-11

Abstract Classes (cont’d) 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)

11-12

Abstract Classes (cont’d)

• 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 called concrete

11-13

Class Object• In Java every class by default extends a library class

Object (from java.lang)

• Object is a concrete class

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

// a few other methods ...}

Methods redefined (overridden) as necessary

11-14

Calling Superclass’s Constructors

public class Walker extends Biped{ // Constructor public Walker(int x, int y, Image leftPic, Image rightPic) { super(x, y, leftPic, rightPic); ... }}

Biped

Walker

Calls Biped’s constructor

If present, must be the first statement

The number / types of parameters passed to super must match parameters of one of the superclass’s constructors.

11-15

Calling Superclass’s Constructors (cont’d)• One of the superclass’s constructors is

always called, but you don’t have to have an explicit super statement.

• If there is no explicit call to super, then superclass’s no-args constructor is called by default.

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

11-16

Calling Superclass’s Constructors (cont’d)• Superclass’s constructor calls its superclass’s

constructor, and so on, all the way up to Object’s constructor.

Biped

Walker

Objectsuper( )

super(...)

11-17

Calling Superclass’s Methods

public class CharlieChaplin extends Walker{ ... public void nextStep () { turnFeetIn(); super.nextStep(); turnFeetOut(); } ...}

Walker

CharlieChaplin

Calls Walker’s nextStep

super.someMethod refers to someMethod in the nearest class, up the inheritance line, where someMethod is defined.

11-18

Calling Superclass’s Methods (cont’d)• super. calls are often used in subclasses

of library classes:

public class Canvas extends JPanel{ ... public void paintComponent (Graphics g) { super.paintComponent (g); ... } ...

11-19

Interfaces

DanceFloor

DanceGroup

ControlPanel

Band

Dancer

Aerobics

Waltz

Rumba

Cha-Cha-Cha

Salsa

Dance

Interface

11-20

Interfaces (cont’d)

• 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 the methods are public abstract.

public interface Dance{ DanceStep getStep (int i); int getTempo (); int getBeat (int i);}

11-21

Interfaces (cont’d)• We must “officially” state that a class

implements 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) { ... } ...}

11-22

Interfaces (cont’d)

• A class can implement several interfaces.

• Like an abstract class, an interface supplies a secondary 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 objects disguised as interface types.

Dance d = new Waltz( );

11-23

Interfaces (cont’d)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 correct method is called for any specific type of Edible, e.g., a Pancake

public class Pancake implements Edible{ ...}

11-24

Classes Interfaces

• A superclass provides a secondary data type to objects of its subclasses.

• An abstract class cannot be instantiated.

• An interface provides a secondary data type to objects of classes that implement that interface.

• An interface cannot be instantiated.

Similarities

11-25

Classes Interfaces

• A concrete subclass of an abstract class must define all the inherited abstract methods.

• A class can extend another class. A subclass can add methods and override some of its superclass’s methods.

• A concrete class that implements an interface must define all the methods specified by the interface.

• An interface can extend another interface (called its superinterface) by adding declarations of abstract methods.

Similarities

11-26

Classes Interfaces

• A class can extend only one class.

• A class can have fields.

• A class defines its own constructors (or gets a default constructor).

• A class can implement any number of interfaces.

• An interface cannot have fields (except, possibly, some public static final constants).

• An interface has no constructors.

Differences

11-27

Classes Interfaces

• A concrete class has all its methods defined. An abstract class usually has one or more abstract methods.

• Every class is a part of a hierarchy of classes with Object at the top.

• All methods declared in an interface are abstract.

• An interface may belong to a small hierarchy of interfaces, but this is not as common.

Differences

11-28

DanceStudio

DanceStudio

DanceFloor

DanceGroup

ControlPanel

«interface» Dance Dancer Biped

Foot

CoordinateSystem

DanceStep

A B

extends

depends on

has

implements

Band

DanceLesson

«interface» StudentGroup

< various dances >

AbstractDance

11-29

Review

• Describe two ways for eliminating duplicate code using class hierarchies.

• What is an abstract class?

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

• Define concrete class.

• What happens when a constructor of a subclass does not have a super statement? Is superclass’s constructor called?

11-30

Review (cont’d)

• Can an abstract class be instantiated?

• Can someMethod1 have a call super.someMethod2 ( )?

• What happens if, by mistake, a programmer puts in his paintComponent method a call

paintComponent(g);

instead of

super.paintComponent(g);

?

11-31

Review (cont’d)

• What is the main difference between an abstract class and an interface?

• Can a class implement several interfaces?

• Suppose you declare a variable of an interface type. What type of value can be assigned to that variable?

• What is the main advantage of interfaces over abstract classes?