Java 8 Default Methods Explained in 5 minutes

In my previous articles, we have looked at Java 8 Lambda Expressions and Streams. In this article will be looking at Defaults Methods which is another cool feature of Java 8.

Default methods enable us to add new functionalities to interfaces without breaking the classes that implements that interface. Lets take a look at the example below.

public class MyClass implements InterfaceA {
 
    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
    }
 
    @Override
    public void saySomething() {
        System.out.println("Hello World");
    }
 
}
 
interface InterfaceA {
  public void saySomething(); 
 
}

The code above shows class MyClass implementing InterfaceA’s method saySomething(). Now lets add a new method called sayHi() to InterfaceA. By doing so, we have introduce a problem to class MyClass as it will not compile until we provide implementation for method sayHi().
This is when Defaults methods becomes useful. By Adding the keyword default before the method’s access modifier, we do not have to provide implementation for the method sayHi() in class MyClass.

In ‘the strictest sense’, Default methods are a step backwards because they allow you to ‘pollute’ your interfaces with code. But they provide the most elegant and practical way to allow backwards compatibility. It made it much easier for Oracle to update all the Collections classes and for you to retrofit your existing code for Lambda.

public class MyClass implements InterfaceA {
 
    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
    }
 
    @Override
    public void saySomething() {
        System.out.println("Hello World");
    }
 
}
 
interface InterfaceA {
 
    public void saySomething();
 
    default public void sayHi() {
      System.out.println("Hi");
    }
 
}

Note that we have to provide implementation for all default methods. So default methods provides us the flexibility to allow methods to be implemented in interfaces. The implementation will be used as default if a concrete class does not provide implementation for that method.

Conflicts with Multiple Interface.

Since classes in java can implement multiple interfaces, there could be a situation where 2 or more interfaces has a default method with the same signature hence causing conflicts as java will not know what methods to use at a time. This will then result in a compilation error with the message MyClass inherits unrelated defaults for sayHi() from types InterfaceA and InterfaceB
Lets take a look at the example below.

public class MyClass implements InterfaceA, InterfaceB {
 
    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
    }
 
    @Override
    public void saySomething() {
        System.out.println("Hello World");
    }
 
}
 
interface InterfaceA {
 
    public void saySomething();
 
    default public void sayHi() {
        System.out.println("Hi from InterfaceA");
    }
 
}
 
interface InterfaceB {
     default public void sayHi() {
        System.out.println("Hi from InterfaceB");
    }
}

In order to work around situations like this,We will have to provide implementation for sayHi() method in the class MyClass therefore overriding both methods in InterfaceA and InterfaceB.

public class MyClass implements InterfaceA, InterfaceB {
 
    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
    }
 
    @Override
    public void saySomething() {
        System.out.println("Hello World");
    }
 
    @Override
    public void sayHi() {
        System.out.println("implemetation of sayHi() in MyClass");
    }
 
}
 
interface InterfaceA {
 
    public void saySomething();
 
    default public void sayHi() {
        System.out.println("Hi from InterfaceA");
    }
 
}
 
interface InterfaceB {
     default public void sayHi() {
        System.out.println("Hi from InterfaceB");
    }
}

If we want to specifically invoke one of the sayHi() methods in either InterfaceA or InterfaceB, we can also do as follows:

public class MyClass implements InterfaceA, InterfaceB {
 
    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
    }
 
    @Override
    public void saySomething() {
        System.out.println("Hello World");
    }
 
    @Override
    public void sayHi() {
       InterfaceA.super.sayHi();
    }
 
}
 
interface InterfaceA {
 
    public void saySomething();
 
    default public void sayHi() {
        System.out.println("Hi from InterfaceA");
    }
 
}
 
interface InterfaceB {
     default public void sayHi() {
        System.out.println("Hi from InterfaceB");
    }
}

Hopefully you have found this quick guide useful. Next Time I’ll be looking at Java 8 Method References.

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Ernest Duodu

Developer at IDRSolutions
Ernest is a developer at IDRSolutions. He focuses mainly on our JavaFX PDF Viewer implementation. He was a speaker at JavaOne 2014, where he did a joint session titled "Lessons learnt developing a NetBeans PDF viewer plugin in JavaFX". Aside programming, he also enjoys a wide variety of hobbies which includes sky-diving, photography, exercising and listening to music.
Ernest

About Ernest Duodu

Ernest is a developer at IDRSolutions. He focuses mainly on our JavaFX PDF Viewer implementation. He was a speaker at JavaOne 2014, where he did a joint session titled "Lessons learnt developing a NetBeans PDF viewer plugin in JavaFX". Aside programming, he also enjoys a wide variety of hobbies which includes sky-diving, photography, exercising and listening to music.

17 thoughts on “Java 8 Default Methods Explained in 5 minutes

  1. Thanks! This is a pretty good, digestible runthrough of default methods. I’m not too crazy about them but at least now I understand how they work 😀

  2. Wow this is exciting stuff. I think public updates for Java 7 are ending so we might have to migrate to Java 8. Learning features a little at a time. First lambda, then streams, now this. Exciting times!

  3. Nicolas

    Is there any advantage on use this approach instead abstract classes?

    • zhouzhou

      well,a class can only extends one abscract class,but a class can implements more than one interface.That’s the difficult,help this can solve your problem

  4. Himanshu

    With Java 8 new feature , “Default Methods ” They give this feature for ensuring backward compatibility . But now with is ,what is the main difference between interface and abstract class? Now both are same , am i right?

    • Ernest Duodu

      Himanshu, they are not the same, default methods are there to add external functionality to existing classes without changing their state and also you can have a constructor in an abstract class but not in an interface.

  5. Leandro Oliveira

    Very nice and clear. Tks.

  6. sam rubin

    Seriously it took less than 5 mins , really a great explanation

  7. Big

    Thxs for this article

  8. Henry Kim

    thanks for the java 8 quick pointers, useful.

  9. Nalini

    Thanks for the article.

  10. Ruchika

    Great Article. Thanks a lot.

  11. Milan

    Thanks a lot!

    This is what I needed a clean, precise, to the point and no non sense explanation!

  12. Mike Rodent

    Thanks, very clear … I just found out about this feature… but I find it a bit crazy that they haven’t gone the whole way and simply allowed multiple inheritance of classes: I’ve always felt that the “Diamond problem” is not a massive problem, since the compiler can do what it does here: insist that the subclass resolve the ambiguity. Now you can have a “default method” but you can’t have a “default field”. Unsatisfactory.

  13. Alessandro

    Nice article, clear and precise!

    IMHO default methods introduce in Java something that is like what they tried to avoid in the first place: multiple abstract classes inheritance!
    I wouldn’t use default methods if not really necessary.

  14. Govind

    Its Really very helpful. its easily understandable. Thank You so much.

  15. day

    Easy to understand, thank you very much I’ve learned something new today very fast

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