This document discusses interfaces in Java, including defining interfaces, implementing interfaces, and providing examples. It covers that an interface defines a collection of methods that a class implements, concrete classes that implement interfaces must provide code bodies for all interface methods, and interfaces allow for multiple inheritance in Java by having classes implement multiple interfaces.

1. Recap
• An interface is a collection of method
declarations
• Constants can also be defined as part of
interface
• Interface feature in Java supports
• Implementation should be separated
from interface
• Main benefit of interface is
• Implement multiple inheritance in Java
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2. Defining Interfaces
• To define an interface,
public interface [InterfaceName] {
//some methods without the body
}
User defined
interface name
Interface Keyword
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3. Defining Interfaces contd..
Let us create an interface Shape
// Definition of interface Shape
public interface Shape {
public double area();
public double volume();
public String getName();
}
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4. Defining Interfaces contd..
• Another example
• Interface to define relationships between two
objects according to the “natural order” of the
objects
public interface Relation {
public boolean isGreater( Object a, Object b);
public boolean isLess( Object a, Object b);
public boolean isEqual( Object a, Object b);
}
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5. Implementing Interface
• To implement an interface means
• To create a concrete class that implements
an interface
• Use the implements keyword
A class which can be instantiated
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6. Implementing Interface contd..
Syntax is :
class concreteClass implements InterfaceName {
• // Write code for all methods of interface
• // Write any code for any methods related to class
}
• Remember
Concrete class must implement ALL methods
of the interface
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7. Implementing Interface Contd..
• Implementing class can have its own methods
• Implementing class can extend a single super
class or abstract class
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8. Example Program
public class Point implements Shape {
protected int x, y; // coordinates
// no-argument constructor
public Point() { setPoint( 0, 0 ); }
Usage of implements
// constructor keyword
public Point( int a, int b ) {
setPoint( a, b ); } Point class provides code
public void setPoint( int a, int b ) { for all methods of Shape
interface
x = a; area()
y = b; volume()
} getName()
public int getX() { return x; }
Point class has some
public int getY() { return y; } additional methods also
public String toString() { getX()
return "[" + x + ", " + y + "]"; } getY()
toString()
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9. Example Program Contd..
public double area() {
return 0.0;
}
public double volume(){
return 0.0;
}
public String getName() {
return "Point";
}
}
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10. Example Program Contd..
import javax.swing.JOptionPane;
public class TestPoint {
public static void main(String args[]) {
Point point1 = new Point( 7, 11 );
Point point2 = new Point( 12, 24 );
Point point3 = new Point( 4, 10 );
Shape arrayOfShapes[];
arrayOfShapes = new Shape[ 3 ];
arrayOfShapes[ 0 ] = point1;
arrayOfShapes[ 1 ] = point2;
arrayOfShapes[ 2 ] = point3;
String output = "";
// Loop through arrayOfShapes and
//print the name of object.
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11. Example Program Contd..
for ( int i = 0; i <
arrayOfShapes.length; i++ ) {
output += "nn" +
arrayOfShapes[ i ].getName() + “: “
+ arrayOfShapes[ i ].toString() ;
}
JOptionPane.showMessageDialog
(null, output,"Implementing Interface",
JOptionPane.INFORMATION_MESSAGE Output
);
System.exit( 0 );
}
}
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12. Another Example Program
public class Line implements Relation {
Concrete class is Line
private double x1; Interface is Relation
private double x2;
private double y1;
private double y2;
public Line(double x1, double x2, double y1, double y2){
this.x1 = x1;
this.x2 = x2;
this.y1 = y1;
this.y2 = y2; The methods of class Line is
} getLength()
public double getLength(){
double length = Math.sqrt((x2-x1)*(x2-x1) + (y2-y1)* (y2-y1));
return length;
}
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13. Another Example Program
public boolean isGreater( Object a, Object b){
double aLen = ((Line)a).getLength();
double bLen = ((Line)b).getLength();
return (aLen > bLen); The methods of interface
Relation are
} isGreater()
isLess()
public boolean isLess( Object a, Object b){ isEqual()
double aLen = ((Line)a).getLength();
double bLen = ((Line)b).getLength();
return (aLen < bLen);
}
public boolean isEqual( Object a, Object b){
double aLen = ((Line)a).getLength();
double bLen = ((Line)b).getLength();
return (aLen == bLen);
}
}
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14. Another Example Program Contd..
import javax.swing.JOptionPane;
class TestLine{
public static void main(String[] args){
Line line1 = new Line(7.0,3.0,15.0, 23.0);
Line line2 = new Line(2.0,4.0,12.0, 53.0);
String output = "";
output += "Line1 is Greater than Line2 is " +line1.isGreater(line1,line2)
+"n";
output += "Line1 is Less than Line2 is " +line1.isLess(line1,line2) +"n";
output += "Line1 is Equal to Line2 is " +line1.isEqual(line1,line2) +"n";
JOptionPane.showMessageDialog( null, output, "Implementing
Interface", JOptionPane.INFORMATION_MESSAGE ); Output
System.exit( 0 );
}
Line1 is less than Line2
}
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15. Summary
• In this class we have discussed
• How to define an interface
• How to implement an interface
• Simple programs using interfaces
• In the next lesson we look at
• How to perform multiple inheritance
• Scope of variable and some more concepts of
interfaces
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16. Frequently Asked Questions
1. Write the syntax for defining an interface
2. How to define an interface?
3. How to implement an interface ?
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17. Quiz
1.Which keyword is useful for implementing an
interface ?
1. implement
2. implements
3. interface
4. extends
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18. Quiz Contd..
2. Which of the following statements is false ?
1. Implementing class cannot have its own
methods
2. Implementing class can have its own
methods
3. Implementing class can extend from another
class
4. All

19. Assignments
• Write Java program to define an interface called
Operator that does add, subtract and multiply
operations
• Write Java program to Implement Operator
interface for arithmetic operations
• Write Java program to Implement Operator
interface for matrix operations
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