Chapter four
Interfaces and Packages
Contents;
 abstract classes and methods,
 interfaces,

Final methods and classes,
 inner classes
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Organizing Classes into a Package
• A package is, essentially, a grouping of classes.
• A package provides a mechanism by which related pieces of a
program can be organized as a unit.
• Classes defined within a package can be made private to that
package and not accessible by code outside the package.
• Thus, the package provides a means by which classes can be
encapsulated
• All classes in Java belong to some package. When no package
statement is specified, the default (global) package is used.
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To create a package, put a package command at the top of a Java source file
This is the general form of the package statement:
package pkg;
Here, pkg is the name of the package.(Lowercase is often used for package
names.)
For example, the following statement creates a package called mypack:
package mypack;
Example
// A short package demonstration.
package bookpack;
This file is part of the bookpack package
class Book {
Thus, Book is part of bookpack
private String title;
private String author;
private int pubDate;
Book(String t, String a, int d) {
title = t;
author = a;
pubDate = d;
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}
void show() {
System.out.println(title);
System.out.println(author);
System.out.println(pubDate);
System.out.println();}}
BookDemo is also part of bookpack.
class BookDemo {
public static void main(String args[]) {
Book books[] = new Book[5];
books[0] = new Book("Java: A Beginner's Guide",
"Schildt", 2014);
books[1] = new Book("Java: The Complete Reference",
"Schildt", 2014);
books[2] = new Book("The Art of Java",
"Schildt and Holmes", 2003);
books[3] = new Book("Red Storm Rising",
"Clancy", 1986);
books[4] = new Book("On the Road",
"Kerouac", 1955);
for(int i=0; i < books.length; i++)
books[i].show();
}
}
4
Abstract Classes
An abstract class is one that cannot be instantiated.
If a class is abstract and cannot be instantiated, the class does not have much
use unless it is subclassed.
Use the abstract keyword to declare a class abstract.
A class is abstract if the class contains an abstract method or does not
provide an implementation of an inherited abstract method.
We say a method is implemented if it has a method body.
Abstract methods consist of a method signature, but no method body.
The following points about abstract classes are worth noting:
• An abstract method cannot be contained in a nonabstract class. If a subclass
of an abstract superclass does not implement all the abstract methods, the
subclass must be defined as abstract.
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In other words, in a nonabstract subclass extended from an abstract class, all
the abstract methods must be implemented. Also note that abstract methods
are nonstatic and noneprivate.
• An abstract class cannot be instantiated using the new operator, but you can
still define its constructors, which are invoked in the constructors of its
subclasses.
• A class that contains abstract methods must be abstract. However, it is
possible to define an abstract class that doesn’t contain any abstract
methods.
• A subclass can override a method from its superclass to define it as abstract.
In this case, the subclass must be defined as abstract.
• A subclass can be abstract even if its superclass is concrete.
• You cannot create an instance from an abstract class using the new
operator, but an abstract class can be used as a data type.
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Example
public abstract class Employee
The keyword abstract
{
here denotes an abstract
private String name;
class.
private String address;
private int number;
public Employee(String name, String address, int number)
{
System.out.println(“Constructing an Employee”);
this.name = name;
Abstract method has no
this.address = address;
method body, just a
this.number = number;
semicolon.
}
public abstract double computePay();
The keyword abstract
here denotes an
public void mailCheck()
abstract method.
{
System.out.println(“Mailing a check to “
+ this.name + “ “ + this.address);
}
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public String toString()
{
return name + “ “ + address + “ “ + number;
}
public String getName()
{
return name;
}
public String getAddress()
{
return address;
}
public void setAddress(String newAddress)
{
address = newAddress;
}
public int getNumber()
{
return number;
}
}
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public class Contractor extends Employee
{
private double dailyRate;
private int daysWorked;
public Contractor(String name, String address, int number,
double dailyRate)
{
super(name, address, number);
setDailyRate(dailyRate);
}
public double computePay()
{
System.out.println(“Computing contractor pay for “
+ getName());
return dailyRate * daysWorked;
}
public void setDailyRate(double newRate)
{
if(newRate >= 0.0 && newRate <= 2000.00)
{
dailyRate = newRate;
}
}
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public double getDailyRate()
{
return dailyRate;
}
public void setDaysWorked(int daysWorked)
{
if(daysWorked >= 0)
{
this.daysWorked = daysWorked;
}
}
}
public class SmartBoss
{
public void payEmployee(Employee e)
{
double pay = 0.0;
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if(e instanceof Salary)
{
pay = ((Salary) e).computePay();
}
else if(e instanceof Hourly)
{
pay = ((Hourly) e).computePay();
}
System.out.println(“Pay = “ + pay);
e.mailCheck();
}
}
public class PayEmployees
{
public static void main(String [] args)
{
Salary s = new Salary(“Tsehay”, “CS3”,3, 2600.00);
Hourly h = new Hourly(“Nuri A”, “MU”, 2,2.50);
h.setHoursWorked(40);
Contractor c = new Contractor(“Belay M”, “KA”,44, 1000.00);
c.setDaysWorked(5);
SmartBoss boss = new SmartBoss();
boss.payEmployee(s);
boss.payEmployee(h);
boss.payEmployee(c);
}
}
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Interfaces
An interface is a class-like construct that contains constants and abstract methods.
Suppose that a person is calling her pets to dinner by whistling. Each animal responded
in its own way: Some ran, some flew, and some swam.
Let’s specify some behaviors for these pets. For example, suppose our pets are able to
• Be named
• Eat
• Respond to a command
We could specify the following method headings for these behaviors:
public void setName(String petName)
public boolean eat(String food)
public String respond(String command)
These method headings can form a class interface
Now imagine that each of the three classes Dog, Bird, and Fish implements all of these
methods. The objects of these classes then have the same behaviors—that is, each
object can be named, can eat, and can respond.
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The nature of these behaviors, however, can be different among the objects.
Although dogs, birds, and fish respond to a command, for example, the way
they respond differs.
Java interface begins like a class definition, except that you use the reserved
word interface instead of class. That is, an interface begins with
public interface Interface_Name
rather than
public class Class_Name.
By convention,
• An interface name begins with an uppercase letter, just as class names do.
• You store an interface in its own file, using a name that begins with the name
of the interface, followed by .java.
• An interface does not declare any constructors for a class.
• Methods within an interface must be public, so you can omit public from their
headings.
• An interface can also define any number of public named constants. It
contains no instance variables, however, nor any complete method
definitions—that is, methods cannot have bodies
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As with an abstract class, you cannot create an instance from an interface using the new
operator.
Here is an example of an interface:
public interface Edible {
/** Describe how to eat */
public abstract String howToEat();
}
You can use the Edible interface to specify whether an object is edible.
This is accomplished by letting the class for the object implement this interface using the
implements keyword.
To implement more than one interface, just list all the interface names, separated by
commas, as in implements MyInterface, YourInterface
Example
public interface Measurable
{
/** Returns the perimeter. */
public double getPerimeter();
/** Returns the area. */
public double getArea();
}
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/**
A class of rectangles.
*/
public class Rectangle implements Measurable
{
private double myWidth;
private double myHeight;
public Rectangle(double width, double height)
{
myWidth = width;
myHeight = height;
}
public double getPerimeter()
{
return 2 * (myWidth + myHeight);
}
public double getArea()
{
return myWidth * myHeight;
}
}
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/**
A class of circles.
*/
public class Circle implements Measurable
{
private double myRadius;
public Circle(double radius)
{
myRadius = radius;
}
public double getPerimeter()
{
return 2 * Math.PI * myRadius;
}
This method is not
public double getCircumference()
declared in the interface.
{
return getPerimeter();
Calls another method instead
}
of repeating its body.
public double getArea()
{
return Math.PI * myRadius * myRadius;
}
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}
An interface is a reference type. Thus, you can write a method that has a
parameter of an interface type, such as a parameter of type Measurable.
For example, suppose that your program defines the following method:
public static void display(Measurable figure)
{
double perimeter = figure.getPerimeter();
double area = figure.getArea();
System.out.println("Perimeter = " + perimeter +
"; area = " + area);
}
Your program can invoke this method, passing it an object of any class that
implements the interface Measurable.
For instance, your program might contain the following statements:
Measurable box = new Rectangle(5.0, 5.0);
Measurable disc = new Circle(5.0);
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Thus, the invocation
display(box);
displays
Perimeter = 20.0; area = 25.0
while the invocation
display(disc);
displays
Perimeter = 31.4; area = 78.5
Example 2 (let us implement the Edible interface)
abstract class Animal {
/** Return animal sound */
public abstract String sound();
}
class Chicken extends Animal implements Edible {
@Override
public String howToEat() {
return "Chicken: Fry it";
}
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@Override
public String sound() {
return "Chicken: cock-a-doodle-doo";
}
}
class Tiger extends Animal {
@Override
public String sound() {
return "Tiger: RROOAARR";
}
}
abstract class Fruit implements Edible {
// Data fields, constructors, and methods omitted here
}
class Apple extends Fruit {
@Override
public String howToEat() {
return "Apple: Make apple cider";
}
}
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class Orange extends Fruit {
@Override
public String howToEat() {
return "Orange: Make orange juice";
}
}
public class TestEdible {
public static void main(String[] args) {
Object[] objects = {new Tiger(), new Chicken(), new
Apple()};
for (int i = 0; i < objects.length; i++) {
if (objects[i] instanceof Edible)
System.out.println(((Edible)objects[i]).howToEat());
if (objects[i] instanceof Animal) {
System.out.println(((Animal)objects[i]).sound());
}
}
}
}
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Output
Tiger: RROOAARR
Chicken: Fry it
Chicken: cock-a-doodle-doo
Apple: Make apple cider
An interface is different from a class in several ways, including:
• You cannot instantiate an interface.
• An interface does not contain any constructors.
• An interface cannot contain instance fields. The only fields that can appear
in an interface must be declared both static and final.
• An interface is not extended by a class; it is implemented by a class.
• An interface can extend multiple interfaces.
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Inner Classes
An inner class, or nested class, is a class defined within the scope of another
class.
Inner classes are useful for defining handler classes.
Example
// OuterClass.java: inner class demo
public class OuterClass {
private int data;
/** A method in the outer class */
public void m() {
// Do something
}
// An inner class
class InnerClass {
/** A method in the inner class */
public void mi() {
// Directly reference data and method
// defined in its outer class
data++;
m();
}
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}
Normally, you define a class as an inner class if it is used only by its outer
class. An inner class has the following features:
• An inner class is compiled into a class named
OuterClassName$InnerClassName.class.
• An inner class can reference the data and the methods defined in the outer
class in which it nests, so you need not pass the reference of an object of
the outer class to the constructor of the inner class. For this reason, inner
classes can make programs simple and concise.
• An inner class can be defined with a visibility modifier subject to the same
visibility rules applied to a member of the class.
• An inner class can be defined as static.
• A static inner class can be accessed using the outer class name.
• A static inner class cannot access nonstatic members of the outer class.
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• Objects of an inner class are often created in the outer class. But you can
also create an object of an inner class from another class.
If the inner class is nonstatic, you must first create an instance of the outer
class, then use the following syntax to create an object for the inner class:
OuterClass.InnerClass innerObject = outerObject.new InnerClass();
• If the inner class is static, use the following syntax to create an object for it:
OuterClass.InnerClass innerObject = new OuterClass.InnerClass();
Example
// Use an inner class.
class Outer {
int nums[];
Outer(int n[]) {
nums = n;
}
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void analyze() {
Inner inOb = new Inner();
System.out.println("Minimum: " +
System.out.println("Maximum: " +
System.out.println("Average: " +
}
// This is an inner class.
class Inner {
int min() {
int m = nums[0];
for(int i=1; i < nums.length;
if(nums[i] < m) m = nums[i];
return m;
}
int max() {
int m = nums[0];
for(int i=1; i < nums.length;
if(nums[i] > m) m = nums[i];
return m;
}
inOb.min());
inOb.max());
inOb.avg());
i++)
i++)
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int avg() {
int a = 0;
for(int i=0; i < nums.length; i++)
a += nums[i];
return a / nums.length;
}
}
}
class NestedClassDemo {
public static void main(String args[]) {
int x[] = { 3, 2, 1, 5, 6, 9, 7, 8 };
Outer outOb = new Outer(x);
outOb.analyze();
}
}
Output
Minimum: 1
Maximum: 9
Average: 5
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it is possible to nest a class within a block scope. Doing so simply creates a
localized class that is not known outside its block.
Example 2
class LocalClassDemo {
public static void main(String args[]) {
// An inner class.
class ShowNEven {
int numevens;
ShowNEven(int n) {
numevens = n;
}
void show() {
int cnt = 2;
for(int i =1;i<= numevens; i++) {
System.out.print(cnt+" ");
cnt+=2;
if(i==numevens) System.out.println();
}
}
}
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for(int b=1; b <= 10; b++){
ShowNEven byteval = new ShowNEven(b);
System.out.print(b +(b==1?" even:":"
evens:"));
byteval.show();
}
}
}
run:
1 even:2
2 evens:2 4
3 evens:2 4 6
4 evens:2 4 6 8
5 evens:2 4 6 8 10
6 evens:2 4 6 8 10 12
7 evens:2 4 6 8 10 12 14
8 evens:2 4 6 8 10 12 14 16
9 evens:2 4 6 8 10 12 14 16 18
10 evens:2 4 6 8 10 12 14 16 18 20
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Anonymous Inner Class Handlers
An anonymous inner class is an inner class without a name.
It combines defining an inner class and creating an instance of the class into
one step.
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