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Interfaces in Java always contained method declaration not their definitions (method body). There was no way of defining method body / definition in interfaces. This is because historically Java didn’t allow multiple inheritance of classes. It allowed multiple inheritance of interfaces as interface were nothing but method declaration. This solves the problem of ambiguity in multiple inheritance. Since Java 8 it is now possible to add method bodies in interfaces.

Java 8 has a new feature called Default Methods. It is now possible to add method bodies into interfaces!

public interface Math {

 int add(int a, int b);

 default int multiply(int a, int b) {
  return a * b;
 }
}
Code language: Java (java)

In above Math interface we added a method multiply with actual method body.

Why we need Default Methods?

Why would one want to add methods into Interfaces? We’ll it is because interfaces are too tightly coupled with their implementation classes. i.e. it is not possible to add a method in interface without breaking the implementor class. Once you add a method in interface, all its implemented classes must declare method body of this new method.

Since Java 8, things started getting ugly. A new feature Lambda was introduce which is cool. However it is not possible to use this feature in existing Java libraries such as java.util package. If you add a single method in interface List, it breaks everything. You need to add its implementation in every class that implements List interface. Imagine in real world how many custom classes would change.

So for backward compatibility, Java 8 cleverly added Default Methods.

Virtual Extension Methods

It added a new concept Virtual extension methods, or as they are often called defender methods, can now be added to interfaces providing a default implementation of the declared behaviour. So existing interfaces can be augmented without compromising backward compatibility by adding extension methods to the interface, whose declaration would contain instructions for finding the default implementation in the event that implementors do not provide a method body. A key characteristic of extension methods is that they are virtual methods just like other interface methods, but provide a default implementation in the event that the implementing class does not provide a method body.

Consider following example:

interface Person {
 //adds a java 8 default method
 default void sayHello() {
  System.out.println("Hello there!");
 }
}

class Sam implements Person {

}

public class Main {
 
 public static void main(String [] args) {
  
  Sam sam = new Sam();
  
  //calling sayHello method calls the method
  //defined in interface
  sam.sayHello();
 }
}
Code language: Java (java)

Output:

Hello there!

In above code we added a defender method sayHello() in Person interface. So it was ok for class Sam to avoid declaring this methods body.

What about Multiple Inheritance?

Adding method definitions in interfaces can add ambiguity in multiple inheritance. isn’t it? Well, it does. However Java 8 handle this issue at Compile type. Consider below example:

interface Person {
 default void sayHello() {
  System.out.println("Hello");
 }
}

interface Male {
 default void sayHello() {
  System.out.println("Hi");
 }
}

class Sam implements Person, Male {

}
Code language: Java (java)

In this example we have same defender method sayHello in both interfaces Person and Male. Class Sam implements these interfaces. So which version of sayHello will be inherited? We’ll if you try to compile this code in Java 8, it will give following error.

class Sam inherits unrelated defaults for sayHello() from types Person and Male class Sam implements Person, Male { ^ 1 error

So that solves multiple inheritance problem. You cannot implement multiple interfaces having same signature of Java 8 default methods (without overriding explicitly in child class).

We can solve the above problem by overriding sayHello method in class Sam.

interface Person {
 default void sayHello() {
  System.out.println("Hello");
 }
}

interface Male {
 default void sayHello() {
  System.out.println("Hi");
 }
}

class Sam implements Person, Male {
 
 //override the sayHello to resolve ambiguity
 void sayHello() {
 
 }
}
Code language: Java (java)

It is also possible to explicitly call method from child class to parent interface. Consider in above example you want to call sayHello method from Male interface when Sam.sayHello is called. You can use super keyword to explicitly call the appropriate method.

class Sam implements Person, Male {
 
 //override the sayHello to resolve ambiguity
 void sayHello() {
  Male.super.sayHello();
 }
}
Code language: Java (java)

Difference between default methods and abstract class

Ok, so far it looks good. In Java 8 we can have concrete methods within interfaces.. right.. So how it is different from Abstract classes? Remember an abstract class is a class that can not be instantiated (i.e. objects can not be created of) and which may contain method bodies. Default method in Java 8 looks similar to Abstract class isn’t it?

We’ll its different actually. Abstract class can hold state of object. It can have constructors and member variables. Whereas interfaces with Java 8 default methods cannot hold state. It cannot have constructors and member variables. You should still use Abstract class whenever you think your class can have state or you need to do something in constructor. Default method should be used for backward compatibility. Whenever you want to add additional functionality in an existing legacy interface you can use default methods without breaking any existing implementor classes.

Also abstract classes cannot be root classes in Lambda expression. What?… I know that’s confusing, but Lambda expressions are the reason why virtual extension methods were introduced in Java 8. When a lambda expression is evaluated, the compiler can infers it into the interface where default method is added.