Default Methods in Java

Last Updated: 12 October, 2023

Before JDK 1.8, Inteface can only have abstract methods, and all the abstract methods must be overridden in the implementing class. These interface methods are by default public and abstract.

Java 8 introduced a feature called default method, which creates a default method inside an interface with a body. We can have one or more default methods inside the interface.

Default methods are non-abstract methods in Java.

We can create a default method in Java by simply adding the default keyword to the method signature and providing the default implementation that can be used, overridden, or ignored without causing problems for existing classes that have implemented an interface.

The default method is one of the strong features of Java 8. It can be used to improve the flexibility and extensibility of interfaces.

Let's see the syntax of the default method implementation inside an interface.

Syntax:

public interface interface_name { void method1(); // By default, public and abstract default void method2() { // default method with method body // code } }

Here is an example of a default method:

Java default method example

interface Printer { // abstract method void print(); // default method default void display() { System.out.println("Hello Guys... This is default first method."); } // default method default void show() { System.out.println("Hello Guys... This is default second method."); } } class OutputPrinter implements Printer { @Override public void print() { System.out.println("Print method."); } @Override public void show() { System.out.println("show method overridden in OutputPrinter class"); } } public class DefaultMethodExample { public static void main(String[] args) { OutputPrinter printer = new OutputPrinter(); printer.display(); printer.show(); printer.print(); } }

Here, the Printer class has two default methods with their own default implementation. The OutputPrinter class implements the Printer interface, overrides both the default methods, and provides its own implementation.

Output

Hello Guys... This is default first method.
show method overridden in OutputPrinter class
Print method.

Here are some of the use cases for default methods:

• Adding new functionality to existing interfaces: Default methods can be used to add new functionality to existing interfaces without breaking existing implementations. This is useful for adding new features to libraries and frameworks.
• Implementing common functionality: Default methods can be used to implement common functionality that is useful for all implementations of an interface. This can help reduce code duplication and make the code more maintainable.
• Providing a default implementation: Default methods can be used to provide a default implementation for a method. This can be useful for methods that are not essential for all implementations of an interface but are still useful for some implementations.