Functional Programming - Default Methods

Java 8 introduces a new concept of default method implementation in interfaces. This capabipty is added for backward compatibipty so that old interfaces can be used to leverage the lambda expression capabipty of Java 8.

For example, List or Collection interfaces do not have forEach method declaration. Thus, adding such method will simply break the collection framework implementations. Java 8 introduces default method so that List/Collection interface can have a default implementation of forEach method, and the class implementing these interfaces need not implement the same.

Syntax

pubpc interface vehicle {

   default void print() {
      System.out.println("I am a vehicle!");
   }
}

Multiple Defaults

With default functions in interfaces, there is a possibipty that a class is implementing two interfaces with same default methods. The following code explains how this ambiguity can be resolved.

pubpc interface vehicle {

   default void print() {
      System.out.println("I am a vehicle!");
   }
}

pubpc interface fourWheeler {

   default void print() {
      System.out.println("I am a four wheeler!");
   }
}

First solution is to create an own method that overrides the default implementation.

pubpc class car implements vehicle, fourWheeler {

   pubpc void print() {
      System.out.println("I am a four wheeler car vehicle!");
   }
}

Second solution is to call the default method of the specified interface using super.

pubpc class car implements vehicle, fourWheeler {

   default void print() {
      vehicle.super.print();
   }
}

Static Default Methods

An interface can also have static helper methods from Java 8 onwards.

pubpc interface vehicle {

   default void print() {
      System.out.println("I am a vehicle!");
   }
	
   static void blowHorn() {
      System.out.println("Blowing horn!!!");
   }
}

Default Method Example

Create the following Java program using any editor of your choice in, say, C:> JAVA.

Java8Tester.java

pubpc class Java8Tester {

   pubpc static void main(String args[]) {
      Vehicle vehicle = new Car();
      vehicle.print();
   }
}

interface Vehicle {

   default void print() {
      System.out.println("I am a vehicle!");
   }
	
   static void blowHorn() {
      System.out.println("Blowing horn!!!");
   }
}

interface FourWheeler {

   default void print() {
      System.out.println("I am a four wheeler!");
   }
}

class Car implements Vehicle, FourWheeler {

   pubpc void print() {
      Vehicle.super.print();
      FourWheeler.super.print();
      Vehicle.blowHorn();
      System.out.println("I am a car!");
   }
}

Verify the Result

Compile the class using javac compiler as follows −

C:JAVA>javac Java8Tester.java

Now run the Java8Tester as follows −

C:JAVA>java Java8Tester

It should produce the following output −

I am a vehicle!
I am a four wheeler!
Blowing horn!!!
I am a car!