Interfaces

Inside Interfaces only methods can be defined.

The methods don't have body and must be declared in the classes that implement the interface.

Syntax

Defining interfaces:

interface InterfaceName { // code }

Implementing interface:

class ClassName implements InterfaceName

Example

interface Vehicle { // Function that must be defined in // each class that extends the interface double getInsuranceCost(int months); } class Car implements Vehicle { // Implementation of interface. public double getInsuranceCost(int months) { return 25 * months; } } class Bicycle implements Vehicle { // Implementation of interface. public double getInsuranceCost(int months) { return 0; } } public class Main { public static void main(String[] args) { Car car = new Car(); System.out.println( "Car's Year Insurance: " + car.getInsuranceCost(12) + " $"); Bicycle bicycle = new Bicycle(); System.out.println( "Bike's Year Insurance: " + bicycle.getInsuranceCost(12) + " $"); } }

Output:

Car's Year Insurance: 300.0 $ Bike's Year Insurance: 0.0 $

Explanation:

• An interface is defined at line 1 - 6.
• At line 5 a method is declared. This method must be declared in each class that implements the interface.
• At line 8 and 17 the classes Car and Bicycle implement the interface. For this reason they must define the function getInsuranceCost. If this is not done then the code won't compile.