When to use: Java 8+ interface default method, vs. abstract method

Question

Java 8 allows for default implementation of methods in interfaces called Default Methods.

I am confused between when would I use that sort of interface default

Answer 1

Default methods in Java interface enables interface evolution.

Given an existing interface, if you wish to add a method to it without breaking the binary compatibility with older versions of the interface, you have two options at hands: add a default or a static method. Indeed, any abstract method added to the interface would have to be impleted by the classes or interfaces implementing this interface.

A static method is unique to a class. A default method is unique to an instance of the class.

If you add a default method to an existing interface, classes and interfaces which implement this interface do not need to implement it. They can

• implement the default method, and it overrides the implementation in implemented interface.
• re-declare the method (without implementation) which makes it abstract.
• do nothing (then the default method from implemented interface is simply inherited).

More on the topic here.