Java 17 Recipes

 How It Works
Before Java 8, it was impossible to include code implementation within a Java interface. 
An interface is a reference type in Java, similar to a class. However, its original intent 
only allowed abstract methods, constants, static methods, and nested types. Therefore, 
classes that implemented an interface must implement each abstract method. In Java 8, 
that restriction was lifted, and it became possible to include method implementations 
Chapter 2 enhanCements from Java 9 through Java 17 

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in the form of default methods. A default method can contain an implementation in the 
interface, or an implementing class could override its implementation to do a complete 
override or add an extension. Hence, the term default method, meaning that the default 
method implementation resides in the interface if the implementation class does not 
provide one. Private methods were not allowed in interfaces.
Situations have arisen by which multiple default methods within an interface may 
contain similar code. This code can now be encapsulated within a private method 
implementation within the interface. The private method implementation cannot be 
used outside of the interface. In the solution to this recipe, the volumeCalc() method 
returns the calculated volume of a square or rectangular swimming pool using a 
standard formula. Each of the default methods within the interface can utilize the 
volumeCalc() method to find the volume. However, the volumeCalc() method is not 
available outside the interface.
This seems controversial, as interfaces were originally intended for field and method 
declarations only. It can also be argued that copying the same code throughout several 
default method implementations would be a bad practice. Take it as you will, this feature 
makes it easier to reuse code within an interface, thereby reducing the chance for errors 
and making maintenance much easier.