Chapter 12 Advanced Chapter 12 Advanced InheritanceInheritance

Jim BurnsJim Burns

Creating and Using Abstract Creating and Using Abstract ClassesClasses

Child classes are more specific than their Child classes are more specific than their parentsparents

When you create a child class, it inherits all the When you create a child class, it inherits all the general attributes you needgeneral attributes you need

Thus you must create only the new, more Thus you must create only the new, more specific attributesspecific attributes

Superclasses contain the features that are Superclasses contain the features that are shared by their subclassesshared by their subclasses For example, the members of the For example, the members of the DogDog class are class are

shared by the shared by the PoodlePoodle and and Spaniel Spaniel subclassessubclasses

Parent classes…Parent classes…Are sometimes so general that you never Are sometimes so general that you never

intend to create any specific instances of intend to create any specific instances of the classthe class

An An abstract classabstract class is one from which you is one from which you cannot create any concrete instantiations, cannot create any concrete instantiations, but from which you can inheritbut from which you can inherit

Abstract classes usually have one or more Abstract classes usually have one or more empty abstract methodsempty abstract methods

Abstract ClassesAbstract ClassesYou use the keyword ______ when you You use the keyword ______ when you

declare an abstract classdeclare an abstract class

Abstract methodsAbstract methodsHave no body…Have no body…

No braces, and no method statements.No braces, and no method statements.When you create an abstract method, you use When you create an abstract method, you use

the keyword _____the keyword _____

public abstract class Animalpublic abstract class Animal{{

private String nameOfAnimal;private String nameOfAnimal;public abstract void speak();public abstract void speak();public String getAnimalName()public String getAnimalName(){{

return nameOfAnimal;return nameOfAnimal;}}public void setAnimalName(String name)public void setAnimalName(String name){{

nameOfAnimal = name;nameOfAnimal = name;}}

}}

Note…Note… If you declare any method to be an If you declare any method to be an

abstract method, you must also declare its abstract method, you must also declare its class to be _______.class to be _______.

For ____ classesFor ____ classesYou cannot place a statement such asYou cannot place a statement such asAnimal myPet = new Animal(“Murphy”);Animal myPet = new Animal(“Murphy”);

You create an You create an abstractabstract class such as class such as Animal only so you can extend it.Animal only so you can extend it.

Example of extending an abstract classExample of extending an abstract classpublic class Dog extends Animalpublic class Dog extends Animal{{

public void speak()public void speak(){{

System.out.println(“Woof!”);System.out.println(“Woof!”);}}

}}The speak() method within the Dog class The speak() method within the Dog class

is required because the abstract, parent is required because the abstract, parent Animal class contains an abstract speak() Animal class contains an abstract speak() method. You can code any statements method. You can code any statements you want within the Dog speak() method.you want within the Dog speak() method.

Public class Cow extends animalPublic class Cow extends animal{{

public void speak()public void speak(){{

System.out.println(“Moo!”);System.out.println(“Moo!”);}}

}}

THE COW CLASSTHE COW CLASS

Public class Snake extends animalPublic class Snake extends animal{{

public void speak()public void speak(){{

System.out.println(“Ssss!”);System.out.println(“Ssss!”);}}

}}

THE SNAKE CLASSTHE SNAKE CLASS

public class UseAnimalspublic class UseAnimals{{ public static void main(String[] args)public static void main(String[] args) {{ Dog myDog = new Dog();Dog myDog = new Dog(); Cow myCow = new Cow();Cow myCow = new Cow(); Snake mySnake = new Snake();Snake mySnake = new Snake(); myDog.setAnimalName("My dog Murphy");myDog.setAnimalName("My dog Murphy"); myCow.setAnimalName("My cow Elsie");myCow.setAnimalName("My cow Elsie"); mySnake.setAnimalName("My snake Sammy");mySnake.setAnimalName("My snake Sammy"); System.out.print(myDog.getAnimalName() + " says ");System.out.print(myDog.getAnimalName() + " says "); myDog.speak();myDog.speak(); System.out.print(myCow.getAnimalName() + " says ");System.out.print(myCow.getAnimalName() + " says "); myCow.speak();myCow.speak(); System.out.print(mySnake.getAnimalName() + " says ");System.out.print(mySnake.getAnimalName() + " says "); mySnake.speak();mySnake.speak(); }}}}

The above code produces the The above code produces the following at the command prompt:following at the command prompt:

C:\Java>java UseAnimalsC:\Java>java UseAnimalsMy dog Murphy says Woof!My dog Murphy says Woof!My cow Elsie says Moo!My cow Elsie says Moo!My snake Sammy says Ssss!My snake Sammy says Ssss!

C:\Java>C:\Java>

Using Dynamic Method BindingUsing Dynamic Method BindingAn instantiation of a subclass “is a” An instantiation of a subclass “is a”

superclass object. superclass object. Every SalariedEmployee “is an” employee; Every SalariedEmployee “is an” employee;

every Dog “is an” animalevery Dog “is an” animalThe opposite is not true—an Animal is not The opposite is not true—an Animal is not

a Doga DogBecause every subclass object “is a” Because every subclass object “is a”

superclass member, you can convert superclass member, you can convert subclass objects to superclass objectssubclass objects to superclass objects

More on dynamic method bindingMore on dynamic method binding Even though you cannot instantiate any objects Even though you cannot instantiate any objects

of an abstract class, you can indirectly create a of an abstract class, you can indirectly create a referencereference to a superclass abstract object to a superclass abstract object

A reference is not an object, but it points to a A reference is not an object, but it points to a memory addressmemory address

When you create a reference, you do not use When you create a reference, you do not use the keyword new; instead, you create a variable the keyword new; instead, you create a variable name in which you can hold the memory name in which you can hold the memory address of a concrete objectaddress of a concrete object

Even though a reference to an abstract Even though a reference to an abstract superclass object is not concrete, you can store superclass object is not concrete, you can store a reference to a concrete subclass object therea reference to a concrete subclass object there

An example of the use of referenceAn example of the use of reference

public class AnimalReferencepublic class AnimalReference{{ public static void main(String[] args)public static void main(String[] args) {{ Animal ref;Animal ref; ref = new Cow();ref = new Cow(); ref.speak();ref.speak(); ref = new Dog();ref = new Dog(); ref.speak();ref.speak(); }}}}

Recall from chapter 11…Recall from chapter 11…That you can use the That you can use the instanceofinstanceof keyword keyword

to determine whether an object is an to determine whether an object is an instance of any class in its hierarchy.instance of any class in its hierarchy.

For example, using the For example, using the AnimalAnimal and and DogDog classes, both of the following are true if classes, both of the following are true if myPoodlemyPoodle is a dog object: is a dog object:

myPoodle instanceof AnimalmyPoodle instanceof AnimalmyPoodle instanceof DogmyPoodle instanceof Dog

Polymorphic behaviorPolymorphic behaviorThe example above demonstrates The example above demonstrates

polymorphic behaviorpolymorphic behaviorAfter the variable After the variable refref is assigned an is assigned an

address, the address, the ref.speak()ref.speak() method calls the method calls the correct correct speak()speak() method method

An application’s ability to select the correct An application’s ability to select the correct subclass method is known as subclass method is known as dynamic dynamic method bindingmethod binding

When the application executes, the correct When the application executes, the correct method is attached to the application method is attached to the application based on the current changing contentbased on the current changing content

Using a Superclass as a Method Using a Superclass as a Method ParameterParameter

Dynamic method binding is most useful when Dynamic method binding is most useful when you want to create a method that has one or you want to create a method that has one or more parameters that might be one of several more parameters that might be one of several typestypes

In the following, the header for the In the following, the header for the talkingAnimal()talkingAnimal() method in Figure 12-9 accepts method in Figure 12-9 accepts any type of any type of AnimalAnimal argument argument

The The talkingAnimal()talkingAnimal() method can be used in method can be used in programs that contain programs that contain DogDog objects, objects, CowCow objects objects or objects of any class that descends from or objects of any class that descends from AnimalAnimal

public class TalkingAnimalDemopublic class TalkingAnimalDemo{{ public static void main(String[] args)public static void main(String[] args) {{ Dog dog = new Dog();Dog dog = new Dog(); Cow cow = new Cow();Cow cow = new Cow(); dog.setAnimalName("Ginger");dog.setAnimalName("Ginger"); cow.setAnimalName("Molly");cow.setAnimalName("Molly"); talkingAnimal(dog);talkingAnimal(dog); talkingAnimal(cow);talkingAnimal(cow); }} public static void talkingAnimal(Animal animal)public static void talkingAnimal(Animal animal) {{ System.out.println("Come one come all");System.out.println("Come one come all"); System.out.println("See the amazing talking animal!");System.out.println("See the amazing talking animal!"); System.out.println(animal.getAnimalName() + " says");System.out.println(animal.getAnimalName() + " says"); animal.speak();animal.speak(); System.out.println("***************");System.out.println("***************"); }}}}

The above code produces the The above code produces the following at the command prompt:following at the command prompt:

C:\Java>java TalkingAnimalDemoC:\Java>java TalkingAnimalDemoCome one come allCome one come allSee the amazing talking animal!See the amazing talking animal!Ginger saysGinger saysWoof!Woof!********************************Come one come allCome one come allSee the maxing talking animal!See the maxing talking animal!Molly saysMolly saysMoo!Moo!**********************************

C:\Java>C:\Java>

Creating Arrays of Sublcass Creating Arrays of Sublcass ObjectsObjects

It can be convenient to create an array of generic It can be convenient to create an array of generic AnimalAnimal referencesreferences

An An AnimalAnimal array might contain individual elements that array might contain individual elements that are are DogDog, , CowCow, or , or SnakeSnake objects objects

The following statement creates an array of three Animal The following statement creates an array of three Animal referencesreferences

Animal[] ref = new Animal[3];Animal[] ref = new Animal[3]; This statement reserves enough computer memory for This statement reserves enough computer memory for

three three AnimalAnimal objects named objects named ref[0],ref[0], ref[1],ref[1], and and ref[2]ref[2] The statement does not actually instantiate The statement does not actually instantiate AnimalAnimals as s as

AnimalAnimals are abstracts are abstract

public class AnimalArrayDemopublic class AnimalArrayDemo{{ public static void main(String[] args)public static void main(String[] args) {{ Animal[] ref = new Animal[3];Animal[] ref = new Animal[3]; ref[0] = new Dog();ref[0] = new Dog(); ref[1] = new Cow();ref[1] = new Cow(); ref[2] = new Snake();ref[2] = new Snake(); for(int x = 0; x < 3; ++x)for(int x = 0; x < 3; ++x) ref[x].speak();ref[x].speak(); }}}}

The above code produces the The above code produces the following at the command prompt:following at the command prompt:

C:\Java>java AnimalArrayDemoC:\Java>java AnimalArrayDemoWoof!Woof!Moo!Moo!Ssss!Ssss!

C:\Java>C:\Java>

Using the Object Class and Its Using the Object Class and Its MethodsMethods

The toString() MethodThe toString() MethodThe equals() MethodThe equals() Method

Using the Object Class and Its Using the Object Class and Its MethodsMethods

Every class in Java is actually a subclass, Every class in Java is actually a subclass, except oneexcept one

When you define a class, if you do not When you define a class, if you do not explicitly extend another class, your class explicitly extend another class, your class is an extension of the Object classis an extension of the Object class

The Object class is defined in the The Object class is defined in the java.lang package, which is imported java.lang package, which is imported automatically every time you write a automatically every time you write a program; it includes methods that you can program; it includes methods that you can use or override as you see fituse or override as you see fit

Using the Using the toString()toString() Method MethodThe Object class The Object class toString()toString() method method

converts an Object into a string that converts an Object into a string that contains information about the Objectcontains information about the Object

If you do not create a If you do not create a toString()toString() method for method for a class, you can use the superclass a class, you can use the superclass version of the version of the toString()toString() method method

public abstract class Animalpublic abstract class Animal{{ private String nameOfAnimal;private String nameOfAnimal; public abstract void speak();public abstract void speak(); public String getAnimalName()public String getAnimalName() {{ return nameOfAnimal;return nameOfAnimal; }} public void setAnimalName(String name)public void setAnimalName(String name) {{ nameOfAnimal = name;nameOfAnimal = name; }}}}public class Dog extends Animalpublic class Dog extends Animal{{ public void speak()public void speak() {{ System.out.println("Woof!");System.out.println("Woof!"); }}}}Public class DisplayDogPublic class DisplayDog{{

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

Dog myDog = new Dog();Dog myDog = new Dog();string dogString = myDog.toString();string dogString = myDog.toString();System.out.println(dogString);System.out.println(dogString);

}}}}

Public class DisplayDogPublic class DisplayDog{{

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

Dog myDog = new Dog();Dog myDog = new Dog();string dogString = myDog.toString();string dogString = myDog.toString();System.out.println(dogString);System.out.println(dogString);

}}}}

The above produces the following The above produces the following output at the command Promptoutput at the command Prompt

C:\Java>java DisplayDogC:\Java>java DisplayDogDog@19821fDog@19821f

C:\Java>C:\Java>

The output is the word Dog ‘at’ 19821f, The output is the word Dog ‘at’ 19821f, which is a hexadecimal number which is a hexadecimal number representing the referencerepresenting the reference

It is better to write your…It is better to write your…own toString() method to override the one own toString() method to override the one

supplied so you can, among other supplied so you can, among other purposes, debug a programpurposes, debug a program

The following example shows a The following example shows a BankAccount class that contains a mistake BankAccount class that contains a mistake in the pink linein the pink line

public class BankAccountpublic class BankAccount{{ private int acctNum;private int acctNum; private double balance;private double balance; public BankAccount(int num, double bal)public BankAccount(int num, double bal) {{ acctNum = num;acctNum = num; balance = num; // Mistake! Should be balance = balbalance = num; // Mistake! Should be balance = bal }} public String toString()public String toString() {{ String info = "BankAccount acctNum = " + acctNum +String info = "BankAccount acctNum = " + acctNum + " Balance = $" + balance;" Balance = $" + balance; return info;return info; }} public boolean equals(BankAccount secondAcct)public boolean equals(BankAccount secondAcct) {{ boolean result;boolean result; if(acctNum == secondAcct.acctNum &&if(acctNum == secondAcct.acctNum && balance == secondAcct.balance)balance == secondAcct.balance) result = true;result = true; elseelse result = false;result = false; return result;return result; }}

}}

public class TestBankAccountpublic class TestBankAccount{{ public static void main(String[] args)public static void main(String[] args) {{ BankAccount myAccount = new BankAccount myAccount = new

BankAccount(123, 4567.89);BankAccount(123, 4567.89); System.out.println(myAccount.toString());System.out.println(myAccount.toString()); }}}}

The above produces the following The above produces the following output at the Command Promptoutput at the Command Prompt

C:\Java>java TestBankAccountC:\Java>java TestBankAccountBankAccount accNum = 123 BankAccount accNum = 123 Balance = $123.00Balance = $123.00

C:\JavaC:\Java

The advantage of using the The advantage of using the toString()toString()

toString()toString() is Java’s universal name for a is Java’s universal name for a method that converts an object’s relevant method that converts an object’s relevant details into details into StringString format format

As you use other programmers classes, As you use other programmers classes, you can hope that they have provided a you can hope that they have provided a toString()toString() method that provides output that method that provides output that you would want to see.you would want to see.

Using the Using the equals()equals() Method MethodThe The ObjectObject class class equals()equals() method returns method returns

a boolean value indicating whether the a boolean value indicating whether the objects are equalobjects are equal

if(someObject.equals(someOtherObjectOfThif(someObject.equals(someOtherObjectOfTheSameType)) system.out.println( “The eSameType)) system.out.println( “The objects are equal”);objects are equal”);

Two objects are equal only if they have the Two objects are equal only if they have the same memory addresssame memory address

public class CompareAccountspublic class CompareAccounts{{ public static void main(String[] args)public static void main(String[] args) {{ BankAccount acct1 = new BankAccount(1234, BankAccount acct1 = new BankAccount(1234,

500.00);500.00); BankAccount acct2 = new BankAccount(1234, BankAccount acct2 = new BankAccount(1234,

500.00);500.00); if(acct1.equals(acct2))if(acct1.equals(acct2)) System.out.println("Accounts are equal");System.out.println("Accounts are equal"); elseelse System.out.println("Accounts are not equal");System.out.println("Accounts are not equal"); }}}}

Command Prompt outputCommand Prompt output

C:\Java>java CompareAccountsC:\Java>java CompareAccountsAccounts are not equalAccounts are not equal

C:\Java>C:\Java>

If you want two bank accounts…If you want two bank accounts…

To be equal if they have the same account To be equal if they have the same account number and balance, then you must write number and balance, then you must write your own your own equals()equals() method… method…

public class BankAccountpublic class BankAccount{{ private int acctNum;private int acctNum; private double balance;private double balance; public BankAccount(int num, double bal)public BankAccount(int num, double bal) {{ acctNum = num;acctNum = num; balance = num; // Mistake! Should be balance = balbalance = num; // Mistake! Should be balance = bal }} public String toString()public String toString() {{ String info = "BankAccount acctNum = " + acctNum +String info = "BankAccount acctNum = " + acctNum + " Balance = $" + balance;" Balance = $" + balance; return info;return info; }} public boolean equals(BankAccount secondAcct)public boolean equals(BankAccount secondAcct) {{ boolean result;boolean result; if(acctNum == secondAcct.acctNum &&if(acctNum == secondAcct.acctNum && balance == secondAcct.balance)balance == secondAcct.balance) result = true;result = true; elseelse result = false;result = false; return result;return result; }}

}}

Example of your own Example of your own equals()equals() public boolean equals(BankAccount secondAcct)public boolean equals(BankAccount secondAcct) {{ boolean result;boolean result; if(acctNum == secondAcct.acctNum &&if(acctNum == secondAcct.acctNum && balance == secondAcct.balance)balance == secondAcct.balance) result = true;result = true; elseelse result = false;result = false; return result;return result; }}

Command Prompt outputCommand Prompt output

C:\Java>java CompareAccountsC:\Java>java CompareAccountsAccounts are equalAccounts are equal

C:\Java>C:\Java>

Using Inheritance to Achieve Good Using Inheritance to Achieve Good Software DesignSoftware Design

When new car models are created, not When new car models are created, not every feature and function is redesigned every feature and function is redesigned from scratchfrom scratch

With inheritance, you can reuse existing With inheritance, you can reuse existing superclasses to create good designs superclasses to create good designs quicklyquickly

Advantages of InheritanceAdvantages of InheritanceSubclass creators save development time Subclass creators save development time

because much of the code needed for the because much of the code needed for the class has already been writtenclass has already been written

Subclass creators save testing time Subclass creators save testing time because the superclass code has already because the superclass code has already been tested and used in a variety of been tested and used in a variety of situationssituations

More advantages…More advantages…Programmers who create or use new Programmers who create or use new

subclasses already understand how the subclasses already understand how the superclass works, so the time it takes to superclass works, so the time it takes to learn the new class features is reducedlearn the new class features is reduced

When you create a new subclass in Java, When you create a new subclass in Java, neither the superclass source code nor the neither the superclass source code nor the superclass bytecode is changedsuperclass bytecode is changed

Creating and Using InterfacesCreating and Using InterfacesSome programming languages like C++ Some programming languages like C++

allow for a subclass to inherit from more allow for a subclass to inherit from more than one superclassthan one superclass

The ability to inherit from more than one The ability to inherit from more than one class is called class is called multiple inheritancemultiple inheritance

Multiple inheritance is a difficult concept—Multiple inheritance is a difficult concept—to which superclass should to which superclass should supersuper refer to? refer to?

Sooo, multiple inheritance is disallowed in Sooo, multiple inheritance is disallowed in JavaJava

InterfacesInterfaces Java provides an alternative to multiple Java provides an alternative to multiple

inheritance—an inheritance—an InterfaceInterface An interface looks much like a class, except that An interface looks much like a class, except that

all of its methods are implicitly all of its methods are implicitly publicpublic and and abstractabstract, and all of its data items are implicitly , and all of its data items are implicitly publicpublic, , staticstatic and and finalfinal

When you create a class that uses an interface, When you create a class that uses an interface, you include the keyword implements and the you include the keyword implements and the interface name in the class headerinterface name in the class header

This notation requires class objects to include This notation requires class objects to include code for every method in the interface that has code for every method in the interface that has been implementedbeen implemented

More on InterfacesMore on InterfacesWhereas using Whereas using extendsextends allows a subclass allows a subclass

to use nonprivate, nonoverriden members to use nonprivate, nonoverriden members of its parent’s class, of its parent’s class, implementsimplements requires requires the subclass to implement its own version the subclass to implement its own version of each method in the interfaceof each method in the interface

In the following, a In the following, a workerworker interface is interface is defined that contains the single method defined that contains the single method called called work()work()

When any class implements When any class implements workerworker, it , it must also include a must also include a work()work() method method

Public abstract class animalPublic abstract class animal{{

private String nameOfAnimal;private String nameOfAnimal;public abstract void speak();public abstract void speak();public String getAnimalName()public String getAnimalName(){{

return nameOfanimal;return nameOfanimal;}}public void setAnimalName(String name)public void setAnimalName(String name){{

nameOfanimal = name;nameOfanimal = name;}}

}}Public class Dog extends AnimalPublic class Dog extends Animal{{

public void speak()public void speak(){{

System.out.println(“woof!”);System.out.println(“woof!”);}}

}}Public interface WorkerPublic interface Worker{{

public void work();public void work();}}

public class WorkingDog extends Dog implements Workerpublic class WorkingDog extends Dog implements Worker{{ private int hoursOfTraining;private int hoursOfTraining; public void setHoursOfTraining(int hrs)public void setHoursOfTraining(int hrs) {{ hoursOfTraining = hrs;hoursOfTraining = hrs; }} public int getHoursOfTraining()public int getHoursOfTraining() {{ return hoursOfTraining;return hoursOfTraining; }} public void work()public void work() {{ speak();speak(); System.out.println("I am a dog who works");System.out.println("I am a dog who works"); System.out.println("I have " + hoursOfTraining +System.out.println("I have " + hoursOfTraining + " hours of professional training!");" hours of professional training!"); }}}}

public class DemoWorkingDogspublic class DemoWorkingDogs{{ public static void main(String[] args)public static void main(String[] args) {{ WorkingDog aSheepHerder = new WorkingDog();WorkingDog aSheepHerder = new WorkingDog(); WorkingDog aSeeingEyeDog = new WorkingDog();WorkingDog aSeeingEyeDog = new WorkingDog(); aSheepHerder.setAnimalName("Simon, the Border Collie");aSheepHerder.setAnimalName("Simon, the Border Collie"); aSeeingEyeDog.setAnimalName("Sophie, the German Shepherd");aSeeingEyeDog.setAnimalName("Sophie, the German Shepherd"); aSheepHerder.setHoursOfTraining(40);aSheepHerder.setHoursOfTraining(40); aSeeingEyeDog.setHoursOfTraining(300);aSeeingEyeDog.setHoursOfTraining(300); System.out.println(aSheepHerder.getAnimalName() + " says ");System.out.println(aSheepHerder.getAnimalName() + " says "); aSheepHerder.speak();aSheepHerder.speak(); aSheepHerder.work();aSheepHerder.work(); System.out.println();System.out.println(); System.out.println(aSeeingEyeDog.getAnimalName() + " says ");System.out.println(aSeeingEyeDog.getAnimalName() + " says "); aSeeingEyeDog.speak();aSeeingEyeDog.speak(); aSeeingEyeDog.work();aSeeingEyeDog.work(); }}}}

Command Prompt outputCommand Prompt outputC:\Java>java demoWorkingDogsC:\Java>java demoWorkingDogsSimon, the Border Collie saysSimon, the Border Collie saysWoof!Woof!Woof!Woof!I am a dog who worksI am a dog who worksI have 40 hours of professional training!I have 40 hours of professional training!

Sophie, the German Shepherd saysSophie, the German Shepherd saysWoof!Woof!Woof!Woof!I am a dog who worksI am a dog who worksI have 300 hours of professional training!I have 300 hours of professional training!

C:\Java>C:\Java>

More on InterfacesMore on Interfaces Abstract classes and interfaces are similar in Abstract classes and interfaces are similar in

that you cannot instantiate concrete objects from that you cannot instantiate concrete objects from either oneeither one

Abstract classes differe from interfaces because Abstract classes differe from interfaces because abstract classes can contain nonabstract abstract classes can contain nonabstract methods, but all methods within and interface methods, but all methods within and interface must be abstract.must be abstract.

A class can inherit from only one abstract A class can inherit from only one abstract superclass, but it can implement any number of superclass, but it can implement any number of iterfacesiterfaces

Creating Interfaces to Store Creating Interfaces to Store Related ConstantsRelated Constants

Interfaces can contain data fields, but the Interfaces can contain data fields, but the must be public, static and finalmust be public, static and final

The purpose in creating an interface The purpose in creating an interface containing constants is to provide a set of containing constants is to provide a set of data that a number of classes can use data that a number of classes can use without having to re-declare the valueswithout having to re-declare the values

An example followsAn example follows

public interface PizzaConstantspublic interface PizzaConstants {{ public static final int SMALL_DIAMETER = 12;public static final int SMALL_DIAMETER = 12; public static final int LARGE_DIAMETER = 16;public static final int LARGE_DIAMETER = 16; public static final double TAX_RATE = 0.07;public static final double TAX_RATE = 0.07; public static final String COMPANY = public static final String COMPANY =

"Antonio's Pizzeria";"Antonio's Pizzeria"; }}

public class PizzaDemo implements PizzaConstantspublic class PizzaDemo implements PizzaConstants {{ public static void main(String[] args)public static void main(String[] args) {{ double specialPrice = 11.25;double specialPrice = 11.25; System.out.println("Welcome to " + COMPANY);System.out.println("Welcome to " + COMPANY); System.out.println("We are having a special offer:\na System.out.println("We are having a special offer:\na

" +" + SMALL_DIAMETER + " inch pizza with four SMALL_DIAMETER + " inch pizza with four

ingredients\nor a " +ingredients\nor a " + LARGE_DIAMETER + " inch pizza with one LARGE_DIAMETER + " inch pizza with one

ingredient\nfor only $" +ingredient\nfor only $" + specialPrice);specialPrice); System.out.println("With tax, that is only $" +System.out.println("With tax, that is only $" + (specialPrice + specialPrice * TAX_RATE));(specialPrice + specialPrice * TAX_RATE)); }} }}

Command Prompt outputCommand Prompt outputC:\Java>javaPizzaDemoC:\Java>javaPizzaDemoWelcome to Antonio’s PizzeriaWelcome to Antonio’s PizzeriaWe are having a special offer:We are having a special offer:A 12 inch pizza with four ingredientsA 12 inch pizza with four ingredientsOr a 16 inch pizza with one ingredientOr a 16 inch pizza with one ingredientFor only $11.25For only $11.25With tax, that is only $12.0375With tax, that is only $12.0375

C:\Java>C:\Java>

Creating and Using PackagesCreating and Using Packages You know how to import packages into your You know how to import packages into your

programsprograms Java.lang package is automatically imported into Java.lang package is automatically imported into

every program your writeevery program your write A A packagepackage is a named collection of classes is a named collection of classes When you create your own classes, you can When you create your own classes, you can

place them in packages so that you can easily place them in packages so that you can easily import related classes into new programsimport related classes into new programs

More on packagesMore on packagesCreating packages encourages others to Creating packages encourages others to

reuse software because it makes it reuse software because it makes it convenient to import many related classes convenient to import many related classes at onceat once

When you create classes for others to use, When you create classes for others to use, you do not want to provide the users with you do not want to provide the users with your source code in the files with .java your source code in the files with .java extensionsextensions

So you compile your classesSo you compile your classesYou are creating compiled classes with the You are creating compiled classes with the

.class extensions.class extensionsThe .class files are the files you place in a The .class files are the files you place in a

package so other programmers can import package so other programmers can import themthem

You include a package statement at the You include a package statement at the beginning of your class file to place the beginning of your class file to place the compiled code into the indicated foldercompiled code into the indicated folder

The package statementThe package statement

package com.course.animals;package com.course.animals;

Indicates that the compiled file should be placed in Indicates that the compiled file should be placed in a folder named com.course.animalsa folder named com.course.animals

The compiled program goes into an animals folder The compiled program goes into an animals folder within the course folder within the com folderwithin the course folder within the com folder

The package statement should be the first such The package statement should be the first such statement and should be outside the class statement and should be outside the class definitiondefinition