Packages and Interfaces - Java: A Beginner's Guide - page 406

Now, when getAdminID( ) is called, a value other than its default is returned.
A More Practical Example of a Default Method
Although the preceding shows the mechanics of using default methods, it doesn't illustrate
their usefulness in a more practical setting. To do this, let's return to the Series interface
shown earlier in this chapter. For the sake of discussion, assume that Series is widely used
and many programs rely on it. Further assume that through an analysis of usage patterns, it
was discovered that many implementations of Series were adding a method that returned
an array that contained the next n elements in the series. Given this situation, you decide to
enhance Series so that it includes such a method, calling the new method getNextArray( )
and declaring it as shown here:
Here, n specifies the number of elements to retrieve. Prior to default methods, adding this
method to Series would have broken preexisting code because existing implementations
would not have defined the method. However, by providing a default for this new method,
it can be added to Series without causing harm. Let's work through the process.
In some cases, when a default method is added to an existing interface, its implementa-
tion simply reports an error if an attempt is made to use the default. This approach is neces-
sary in the case of default methods for which no implementation can be provided that will
work in all cases. These types of default methods define what is, essentially, optional code.
However, in some cases, you can define a default method that will work in all cases. This is
the situation for getNextArray( ) . Because Series already requires that a class implement
getNext( ) , the default version of getNextArray( ) can use it. Thus, here is one way to im-
plement the new version of Series that includes the default getNextArray( ) method: