Default Methods In Java 8

Before Java 8, interfaces could have only abstract methods. The implementation of these methods has to be provided in a separate class. So, if a new method is to be added in an interface, then its implementation code has to be provided in the class implementing the same interface. To overcome this issue, Java 8 has introduced the concept of default methods which allow the interfaces to have methods with implementation without affecting the classes that implement the interface.

// A simple program to Test Interface default
// methods in java
interface TestInterface
{
    // abstract method
    public void square(int a);

    // default method
    default void show()
    {
      System.out.println("Default Method Executed");
    }
}

class TestClass implements TestInterface
{
    // implementation of square abstract method
    public void square(int a)
    {
        System.out.println(a*a);
    }

    public static void main(String args[])
    {
        TestClass d = new TestClass();
        d.square(4);

        // default method executed
        d.show();
    }
}

Output:

 16
 Default Method Executed

The default methods were introduced to provide backward compatibility so that existing interfaces can use the lambda expressions without implementing the methods in the implementation class. Default methods are also known as defender methods or virtual extension methods.

Static Methods:
The interfaces can have static methods as well which is similar to static method of classes.

// A simple Java program to TestClassnstrate static
// methods in java
interface TestInterface
{
    // abstract method
    public void square (int a);

    // static method
    static void show()
    {
        System.out.println("Static Method Executed");
    }
}

class TestClass implements TestInterface
{
    // Implementation of square abstract method
    public void square (int a)
    {
        System.out.println(a*a);
    }

    public static void main(String args[])
    {
        TestClass d = new TestClass();
        d.square(4);

        // Static method executed
        TestInterface.show();
    }
}

Output:

 16
 Static Method Executed

Default Methods and Multiple Inheritance
In case both the implemented interfaces contain default methods with same method signature, the implementing class should explicitly specify which default method is to be used or it should override the default method.

// A simple Java program to demonstrate multiple
// inheritance through default methods.
interface TestInterface1
{
    // default method
    default void show()
    {
        System.out.println("Default TestInterface1");
    }
}

interface TestInterface2
{
    // Default method
    default void show()
    {
        System.out.println("Default TestInterface2");
    }
}

// Implementation class code
class TestClass implements TestInterface1, TestInterface2
{
    // Overriding default show method
    public void show()
    {
        // use super keyword to call the show
        // method of TestInterface1 interface
        TestInterface1.super.show();

        // use super keyword to call the show
        // method of TestInterface2 interface
        TestInterface2.super.show();
    }

    public static void main(String args[])
    {
        TestClass d = new TestClass();
        d.show();
    }
}

Output:

Default TestInterface1
Default TestInterface2

Important Points:

1. Interfaces can have default methods with implementation in Java 8 on later.
2. Interfaces can have static methods as well, similar to static methods in classes.
3. Default methods were introduced to provide backward compatibility for old interfaces so that they can have new methods without affecting existing code.

This article is contributed by Akash Ojha. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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