Listing 2 imports the java.util.Comparator functional interface, which describes a function that can perform a comparison on two objects of arbitrary but identical type.

A comparator is an object of a class that implements the Comparator interface, which defines methods that allow you to compare two objects to sort them.

Java's interface language feature often puzzles newcomers to this language. Jeff eliminates this mystery by presenting the six roles that interfaces play in the Java language.