Classes Revisited - Learning Java Through Games - page 173

Suppose that we create a new methodology for computing the strength of a fictional
character. Then, if the user has saved past results, this value would be meaningless and
can result in the program behaving erroneously. Therefore, we will rewrite our program and
only provide a public interface that allows us to compare two fictional characters.
Remember that we used the compareTo method to compare two strings.

⎧
⎨

positive integer if s1 > s2

s1 . compareTo ( s2 )=

negative integer if s1 < s2 (8.1)

⎩

0 if s1=s2

For strings, < and > refer to the lexicographical ordering of strings. In general, we can
use the compareTo method to compare two objects and define the meaning of an object to
be smaller, equal to, or greater than another object.
In Java, there is a class that includes the compareTo method. It is a simple class; it
only includes the method and defines it as an abstract method. We can tell the world that
our class supports the compareTo method by inheriting from this class. This class is special
because it only contains the abstract compareTo method. It does not contain any methods
with bodies, constructors, or variables. In Java, such classes are referred to as interfaces .

An interface is a special class that contains only methods without bodies (i.e.,
abstract methods). The abstract keyword is not used when defining a method inside
an interface because all the methods are abstract. An interface cannot contain regular
variables, but it can contain constants (i.e., final static variables). An interface
cannot contain constructors.

Note that an interface is similar to an abstract class because one cannot directly create an
object that belongs to an interface or an abstract class. However, there is a subtle difference.
Since all the methods in an interface are abstract and there are no instance variables inside,
we will never need to use the super keyword to refer to an interface object. In other words,
an object for an interface is never created. Conversely, an object for an abstract class is
created as part of the super object of a subclass. Since we cannot use the super keyword
when we inherit from an interface and a super object is not created, Java allows us to inherit
from multiple interfaces.

In Java, inheriting from an interface is different than inheriting from a class. When
inheriting from an interface, a super object is not created. In order to emphasize the
difference, Java uses the following terminology. A class extends from another class,
but it implements an interface. A class implementing an interface means that the
class overrides and implements (i.e., includes the bodies of) all the methods of the
interface. In Java, one can extend a single class, but implement multiple interfaces.

The interface that contains the compareTo method is called Comparable . Therefore,
the FictionalCharacter class can implement this interface and override the compareTo
method. Here is the new implementation.

public abstract class FictionalCharacter implements Comparable <
FictionalCharacter >{
private String name;
public FictionalCharacter () {
}