Interfaces

1. Interfaces

2. Interfaces • An interface describes a set of methods: • no constructors • no instance variables • The interface must be implemented by some class. • 646 java classes implement one or more interfaces • Let's begin with a simple interface.

3. Old MacDonald had a farm • Interfaces have public method headings followed by semicolons. • no { } • No methods are implemented: • Some other will do it public interface BarnyardAnimal { public String sound( ); }

4. One or classes will implement an interface • To implement an interface, you must have all method specified exactly as written in the interface. public class Cow implements BarnyardAnimal { public String sound( ) { return "moo"; } } public class Chicken implements BarnyardAnimal { public String sound( ) { return "cluck"; } }

5. What is the output? public class OldMacDonald { public static void main(String[] args) { BarnyardAnimal animalOne = new Cow( ); BarnyardAnimal animalTwo = new Chicken( ); System.out.println( "With a " + animalOne.sound() + " " + animalOne.sound() + " here," ); System.out.println( "and a " + animalTwo.sound() + " " + animalTwo.sound() + " there." ); System.out.println( "Here a " + animalOne.sound()); System.out.println( "there a " + animalTwo.sound()); System.out.println( "everywhere a " + animalTwo.sound() + " " + animalTwo.sound() + "." ); } }

6. Interfaces • Interfaces are used by Java in several ways. • event-driven programs • They can also be used to specify class design. • They list all the methods that you must implement • You add instance variables and constructors