Java™ How To Program (Early Objects), Tenth Edition

10.10.1 default Interface Methods

Prior to Java SE 8, interface methods could be only public abstract methods. This meant that an interface specified what operations an implementing class must perform but not how the class should perform them.

In Java SE 8, interfaces also may contain public default methods with concrete default implementations that specify how operations are performed when an implementing class does not override the methods. If a class implements such an interface, the class also receives the interface’s default implementations (if any). To declare a default method, place the keyword default before the method’s return type and provide a concrete method implementation.

Adding Methods to Existing Interfaces

Prior to Java SE ...