programming with java ics201 university of hail1 chapter 13 interfaces
TRANSCRIPT
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University Of Hail 1
Chapter 13
Interfaces
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1Introduction
• In English, an interface is a device or system that unrelated entities use to interact. – A remote control is an interface between you
and a television set– English language is an interface between two
people
• Java interface is a system that unrelated objects use to interact with one another.
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Interface
o An interface is something like an abstract class
However, an interface is not a class
o The syntax for defining an interface is similar to that of
defining a class
Except the word interface is used in place of class
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Interface
o An interface specifies a set of methods that any class that
implements the interface must have
It contains method headings and constant definitions
It contains neither instance variables nor any complete
method definitions
Example: interface A
{
int x = 10;
public void display( String s);
}
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Interface
o Interface variable is implicitly public, static and final
o Interface method is implicitly public and abstract (is not
implemented by this class)
o A class can implement one or more interfaces
o An interface can be implemented by several classes
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1Interface Vs. Class
An interface is similar to a class in :
• An interface can contain any number of methods.
• An interface is written in a file with a .java extension.
However, an interface is different from a class in:
• You cannot instantiate an interface.
• An interface does not contain any constructors.
• All of the methods in an interface are abstract.
• An interface is not extended by a class; it is
implemented by a class.
• An interface can extend multiple interfaces.
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1Why do we use Interfaces?
To have unrelated classes implement similar methods● Example:– Class Line and class MyInteger
They are not related through inheritance● You want both to implement comparison methods
– checkIsGreater(Object x, Object y)– checkIsLess(Object x, Object y)– checkIsEqual(Object x, Object y)
– Define Comparison interface which has the threeabstract methods above
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1Why do we use Interfaces?
To model multiple inheritance
– A class can implement multiple interfaces while it can extend only one class
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1Defining Interfaces
Use the interface keyword
public interface Vehicle {public void Method1();public void Method2();
} Like abstract methods, the signature is terminated
with a semi-colon
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1Example
Write a set of Circle, Rectangle, and Triangle classes.
• Certain operations that are common to all shapes.
perimeterarea
• Every shape has them but computes them differently.
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1Shape area, perimeter
• Rectangle (as defined by width w and height h):area = w h
perimeter = 2w + 2h
• Circle (as defined by radius r):area = r2
perimeter = 2 r
• Triangle (as defined by side lengths a, b, and c)area = √(s (s - a) (s - b) (s - c))
where s = ½ (a + b + c)
perimeter = a + b + c
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1Shape interface
public interface Shape { public double area(); public double perimeter();}
– This interface describes the features common to all shapes.(Every shape has an area and perimeter.)
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1Implementing an interface
public class name implements interface { ...}
• This means the class must contain each of the abstract methods in that interface. (Otherwise, it will not compile.)
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1Interface diagram
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1Complete Circle class
// Represents circles.public class Circle implements Shape { private double radius; //Constructs a new circle with the given radius. public Circle(double radius) { this.radius = radius; } // Returns the area of this circle. public double area() { return Math.PI * radius * radius; } // Returns the perimeter of this circle. public double perimeter() { return 2.0 * Math.PI * radius; }}
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1Complete Rectangle class
// Represents rectangles.public class Rectangle implements Shape { private double width; private double height; // Constructs a new rectangle with the given dimensions. public Rectangle(double width, double height) { this.width = width; this.height = height; }
// Returns the area of this rectangle. public double area() { return width * height; } // Returns the perimeter of this rectangle. public double perimeter() { return 2.0 * (width + height); }}
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1Complete Triangle class
// Represents triangles.public class Triangle implements Shape { private double a; private double b; private double c; // Constructs a new Triangle given side lengths. public Triangle(double a, double b, double c) { this.a = a; this.b = b; this.c = c; } public double area() { double s = (a + b + c) / 2.0; return Math.sqrt(s*(s–a)*(s-b)*(s-c)); } // Returns the perimeter of this triangle. public double perimeter() { return a + b + c; }}
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1Interfaces Versus Classes
• An interface type is similar to a class,
but there are several important
differences:
– All methods in an interface type are
abstract; they don't have an
implementation
– All methods in an interface type are
automatically public
– An interface type cannot have instance
variables, although they can have
constants.
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1Syntax: Defining an Interface
public interface InterfaceName { // constants
// method signatures }
Example: public interface Measurable {
double getMeasure(); }
• To define an interface and its method signatures. – The methods are automatically public. – Variables are automatically public , static, or final.
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1Syntax: Implementing an Interface
public class ClassName implements InterfaceName, InterfaceName, ... { // methods // instance variables }
Example: public class BankAccount implements Measurable { // Other BankAccount methods public double getMeasure() { // Method implementation } }
Purpose:To define a new class that implements the methods of an interface
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We can define the speak() method as part of the Speakable interface.
public interface Speakable { public String speak(); // Abstract method}
public class Animal implements Speakable { protected String kind; // Cow, dog, cat, etc. public Animal() { } public String speak() { return ". . . "; }}
• Because speak() is no longer defined in Animal, the class Animal should implement the Speakable interface.
Syntax: Defining an Interface (Cont’d)
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• Subclasses of Animal can now implement the Speakable interface in their own distinct ways. public class Cat extends Animal implements Speakable { public Cat() { kind = "cat"; } public String speak() { return "meow"; }}
public class Cow extends Animal implements Speakable { public Cow() { kind = "cow"; } public String speak() { return "moo"; }}
Inheritance: A Cat is both an Animal and a Speakable!!!
Syntax: Defining an Interface
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Example (Interface)interface Communicate{
int LOUD = 0;int SOFT = 1;int OFF = 2;void talk();void listen();
}class Telephone implements Communicate {
public void talk() { … } //implementation of interfacepublic void listen() { … } // other methods implementedpublic void call ( String number) { … } //method member implemented
}class Professor implements Communicate{
public void talk() { … } //implementation of interfacepublic void listen() { … } // other methods implementedvoid Lecture( String topic) { … }
}
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Example (Interface)
o The keyword implements indicates that the class
implements one or more interfaces.
o Using Objects with common interfaces methods
Professor prof = new Professor(" XXXXXX" );
Telephone tel = new Telephone(" 111" );
prof.listen();
tel.listen();
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Exercise (Interface)
interface B{
void display();}class D0 {}class D1 implements B{
public void display(){
System.out.println( "D1" );}
}class D2 implements B{
public void display() {
System.out.println( "D2" );}
}
class InterfaceRefVariable{ public static void main( String [] args) {
B b = new D0(); b.display();b = new D1(); b.display();b = new D2(); b.display();
}}
Incompatible types Class D0 does not implement
the requested interface B
// What compile-time error generated for this program?
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1Derived Interfaces
• Like classes, an interface may be derived from a base interface– This is called extending the interface– The derived interface must include the phrase
extends BaseInterfaceName
• If more than one interface is implemented, each is listed, separated by commas.
– The concrete class must implement all the method headings listed in the definition(s) of any methods in the derived interface as well as any methods in the base interface
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Example (Interface Reference)
interface J { int i=200;
int J1();}interface K {
double K1();}interface L extends J, K{
boolean L1();}class I implements L {
public int J1(){
return 4;}
public double K1(){
return 7.98; } public boolean L1() {
return true; }}class InterfaceInheritance{ public static void main( String[] args) {
I a = new I();System.out.println(a.i);System.out.println(a.J1());System.out.println(a.K1());System.out.println(a.L1());
} }
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// Interface extends one or more interfaces
2004
7.98true
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The instanceof Operator
o The instanceof operator is used to determine if an object is of a particular class or implements a specific interface.
o Syntax: varName instanceof type
o varName is an object reference variable
o type is the name of either a class or an interface
o The expression evaluates to true if varName is a type. Otherwise, it evaluates to false.
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Example (The instanceof Operator)abstract class Fish{
abstract void display(); }abstract class SaltWtrFish extends Fish{}abstract class FreshWtrFish extends Fish{}class Trout extends FreshWtrFish{
void display(){
System.out.println( "Trout" ); }
}class Tuna extends SaltWtrFish{
void display(){
System.out.println( "Tuna" );}
}
class InstantofOperator
{
public static void main( String[] arg)
{
Fish f[] = new Fish[3];
f[0] = new Trout();
f[1] = new Tuna();
f[2] = new Trout();
for(int j = 0; j < 3; j++)
if ( f[j] instanceof FreshWtrFish )
f[j].display();
}
}
Output:
Trout
Trout
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Example (The instanceof Operator)interface Vehicle{
void drive();}abstract class Mammal { }class Bear extends Mammal { }class Elephant extends Mammal implements Vehicle{
public void drive(){
System.out.println( "Elephant: Drive" );}
}class Horse extends Mammal implements Vehicle{
public void drive(){
System.out.println( "Horse:Drive" );}
}class Lion extends Mammal{ }
class InstantofInterface{
public static void main( String[] ar){
Mammal m[] = new Mammal[4];m[0]=new Elephant();m[1]=new Bear(); m[2]=new Horse();m[3]=new Lion();for( int j = 0; j < 4; j++){ if ( m[j] instanceof Vehicle) { Vehicle v =
(Vehicle)m[j]; v.drive(); }
}}
}Output:
Elephant: Drive
Horse: Drive
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