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.