Functional Programming - Default Methods

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

package com.tutorialspoint;

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

Multiple Defaults

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

package com.tutorialspoint;

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

public 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.

package com.tutorialspoint;

public class Car implements Vehicle, FourWheeler {

   public 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.

package com.tutorialspoint;

public 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.

package com.tutorialspoint;

public interface Vehicle {

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

Example - Usage of Default Methods of Interfaces

FunctionTester.java

package com.tutorialspoint;

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

Output

Run the FunctionTester and verify the output.

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