Challenge 1: An Interface With a Default Method — Possible Solution
====================================================================

Printable.java:

public interface Printable {
    void print();

    default void printTwice() {
        print();
        print();
    }
}

Document.java:

public class Document implements Printable {
    @Override
    public void print() {
        System.out.println("Printing document...");
    }

    public static void main(String[] args) {
        Document d = new Document();
        d.printTwice();
    }
}

Output:

    Printing document...
    Printing document...

Explanation:
Document only implements print() -- the single abstract method
Printable actually requires. printTwice() is never written in
Document at all; it's inherited automatically from Printable's own
default body, which calls print() twice. Because print() is
dynamically dispatched, printTwice() ends up calling Document's own
print() implementation both times.

WHY THIS WORKS AS AN ANSWER
------------------------------
This demonstrates a default method being inherited and used without
the implementing class writing any code for it, exactly matching the
chapter's own claim that default methods are supplied by the contract
itself rather than repeated by every implementer.