Muhammad Khojaye's Blog | Java, Cloud, Big Data: March 2014

Java 8 introduces “Default Method” or (Defender methods) new feature, which allows a developer to add new methods to the Interfaces without breaking the existing implementation of these Interface. It provides flexibility to allow Interface define implementation which will use as default in the situation where a concrete Class fails to provide an implementation for that method.

Let consider small example to understand how it works:

public interface OldInterface {
    public void existingMethod();

    default public void newDefaultMethod() {
        System.out.println("New default method"
               + " is added in interface");
    }
}

The following Class will compile successfully in Java JDK 8:

public class OldInterfaceImpl implements OldInterface {
    public void existingMethod() {
     // existing implementation is here…
    }
}

If we create an instance of OldInterfaceImpl:

OldInterfaceImpl obj = new OldInterfaceImpl ();
// print “New default method add in interface”
obj.newDefaultMethod();

Why Default Method?

Reengineering an existing JDK framework is always very complex. Modify one Interface in JDK framework breaks all Classes that extends the Interface which means that adding any new method could break millions of lines of code. Therefore, default methods have introduced as a mechanism to extending Interfaces in a backward compatible way.

Default methods can be provided to an Interface without affecting implementing Classes as it includes an implementation. If each added method within Interface defined with implementation then no implementing Class is affected. An implementing Class can override the default implementation provided by the Interface.

For Java 8, the JDK collections have been extended and forEach method is added to the entire collection (which work in conjunction with lambdas). With conventional way, the code looks like below:

public interface Iterable<T> {
    public void forEach(Consumer<? super T> consumer);
}

Since this result each implementing Class with compile errors, as a result, a default method added with a required implementation in order that the existing implementation should not be changed. The Iterable Interface with the Default method is below:

public interface Iterable<T> {
    public default void forEach(Consumer
                   <? super T> consumer) {
        for (T t : this) {
            consumer.accept(t);
        }
    }
}

The same mechanism has been used to add Stream in JDK Interface without breaking the implementing Classes.

When to use Default Method over Abstract Classes

Abstract Classes versus Interfaces in Java 8

After being introduced Default Method, the Interfaces and abstract Classes seems similar, however, they still different concept in Java 8. Abstract Class can define constructor. They are more structured and can have a state associated with them. While in contrast, default method can be implemented only in the terms of invoking other Interface methods, with no reference to a particular implementation's state. Hence, both use for different purposes and choosing between two really depends on the scenario context.

Default Method and Multiple Inheritance Ambiguity Problems

Since java Class can implement multiple Interfaces and each Interface can define default method with same method signature, therefore, the inherited methods can conflict with each other.

Consider below example:

public interface InterfaceA {  
       default void defaultMethod(){  
           System.out.println("Interface A default method");  
    }  
}

public interface InterfaceB {
   default void defaultMethod(){
       System.out.println("Interface B default method");
   }
}

public class Impl implements InterfaceA, InterfaceB  {
}

The above code will fail to compile with the following error, java: class Impl inherits unrelated defaults for defaultMethod() from types InterfaceA and InterfaceB

In order to fix this class, we need to provide default method implementation:

public class Impl implements InterfaceA, InterfaceB {
    public void defaultMethod(){
    }
}

Further, if we want to invoke default implementation provided by any of super Interface rather than our own implementation, we can do so as follows:

public class Impl implements InterfaceA, InterfaceB {
    public void defaultMethod(){
        // existing code here.
        InterfaceA.super.defaultMethod();
    }
}

We can choose any default implementation or both as part of our new method.

Difference between Default Method and Regular Method

Default Method is different from the regular method in the sense that default method comes with default modifier. Additionally, methods in Classes can use and modify method arguments as well as the fields of their Class but default method, on the other hand, can only access its arguments as Interfaces do not have any state.

In summary, Default methods enable to add new functionality to existing Interfaces without breaking older implementation of these Interfaces.
When we extend an interface that contains a default method, we can perform the following action:

Not override the default method and will inherit the default method.
Override the default method similar to other methods we have overridden subclasses.
Redeclare default method as abstract, which will force subclasses to override it.