ArrayLists - Building Java Programs: A Back to Basics Approach - page 646

Not all types have natural orderings because not all types have comparison func-
tions. For example, in this chapter we have been exploring how to construct a variety
of ArrayList objects. How would you compare two ArrayList objects to deter-
mine whether one is less than another? What would it mean for one ArrayList to be
less than another? You might decide to use the lengths of the lists to determine which
one is less, but what would you do with two ArrayList objects of equal length that
store different values? You wouldn't want to describe them as “equal.” There is no
agreed-upon way of ordering ArrayList s, and therefore there is no comparison
function for this type. As a result, we say that the ArrayList type does not have a
natural ordering.
Java has a convention for indicating the natural ordering of a type. Any type that
has such an ordering should implement the Comparable interface:

public interface Comparable<T> {
public int compareTo(T other);

}

This interface provides a second example of a generic type in Java. In the case of
ArrayList , Sun decided to use the letter “E,” which is short for “Element.” In the
case of Comparable , Sun used the letter “T,” which is short for “Type.”
The compareTo method is the comparison function for the type. A boolean return
type can't be used because there are three possible answers: less than, equal to, or
greater than. The convention for compareTo is that an object should return one of the
following results:

• A negative number to indicate a less-than relationship

• 0 to indicate equality

• A positive number to indicate a greater-than relationship

Let's look at a few examples to help you understand this concept. We have seen
that Java has Integer objects that serve as wrappers for individual int values. We
know how to compare int values to determine their relative order, so it is not surpris-
ing that the Integer class implements the Comparable interface. Consider the fol-
lowing code:

Integer x = 7;
Integer y = 42;
Integer z = 7;
System.out.println(x.compareTo(y));
System.out.println(x.compareTo(z));
System.out.println(y.compareTo(x));

This code begins by constructing three Integer objects: