Before Java 8, Java interfaces could only have static constants and abstract methods. The implementation of those abstract methods has to be provided by the class that implements an interface. This means that when you modify an interface, it breaks all the implementation classes.
Therefore, to overcome this issue, Java 8 has introduced the concept of default methods (Also known as Extension Methods) which allow the interfaces to have methods with implementation without affecting the classes that implement the interface.
You can do this by using the default keyword.
Default Method Java8 Example
interface Vehicle {
String speedUp();
String slowDown();
default String alarmOn() {
return "Turning the alarm on.";
}
default String alarmOff() {
return "Turning the alarm off.";
}
}
class Car implements Vehicle {
private String brand;
public Car(String brand){
this.brand = brand;
}
@Override
public String speedUp(){
return "Speed up!";
}
@Override
public String slowDown(){
return "Slow down!";
}
public String getBrand(){
return brand;
}
}
As you can notice above, we do not have to provide the implementation for the alarmOn and alarmOff methods in our Car implementation.
Lastly, let’s define a typical main class and creates an instance of Car to see how the default methods are called:
public class VehicleExmaple {
public static void main(String[] args) {
Car car = new Car("Land Rover");
System.out.println(car.getBrand());
System.out.println(car.speedUp());
System.out.println(car.slowDown());
//Calling default methods
System.out.println(car.alarmOn());
System.out.println(car.alarmOff());
}
}
Default Methods Inheritance & Diamond Problem
If your class implements multiple interfaces that define the same default methods, then the code simply won’t compile, as there’s ambiguity caused by multiple interface inheritance (a.k.a the Diamond Problem)
This can be solved by explicitly overriding the default methods in your implementation class.
class Car implements Vehicle1, Vehicle2 {
// Your code goes here..
@Override
public String alarmOn() {
// Your code goes here..
//...
return Vehicle2.super.alarmOn();
}
@Override
public String alarmOff() {
// Your code goes here..
//...
return Vehicle2.super.alarmOff();
}
}