In our department directory example, an object of type Entry only has one form, namely an instance of class Entry. If we were designing the data for a more comprehensive directory such as a city phone directory, we would need more than one form of entry. At a minimum, we would need entry formats suitable for business listings, government listings, and residential listings. For such a phone directory, we might define an entry as follows.
A CityEntry is either:
The BusinessEntry and GovernmentEntry forms include city and state information because businesses and government agencies that serve clients in cities outside their local calling area often elect to have their phone numbers included in the directories of other cities (in addition to the cities where they are located). In addition, government listings include a string specifying the government entity to which they belong. For example, a listing for the Federal Bureau of Investigation would specify the "U.S. Government" as the gov field.
In Scheme, we would represent such an entry data type by defining three different structures:
;; a CityEntry is either: ;; a BusinessEntry ;; (make-BusinessEntry name addr phone city state), ;; or a GovernmentEntry ;; (make-GovernmentEntry name addr phone city state gov), ;; or a ResidentialEntry ;; (make-ResidentialEntry name addr phone). (define-struct BusinessEntry (name addr phone city state)) (define-struct GovernmentEntry (name addr phone city state gov)) (define-struct ResidentialEntry (name addr phone))Note that the type CityEntry consisting of the union of the types BusinessEntry, GovernmentEntry, and ResidentialEntry is not defined in program text because all union types in Scheme are implicit.
In Java, we can define the CityEntry type by introducing a ``dummy'' CityEntry class that we extend by ``concrete'' classes1.3 for each different form of Entry data. This technique, which is widely used in object-oriented programming, is called the union pattern. In this pattern, an abstract class serves as the root of a hierarchy of subclasses called variants, which are the component types of the union. In this example, there are three variant classes: BusinessEntry, GovernmentEntry, ResidentialEntry. The following Java code defines the city-entry type:
/** a CityEntry is either:
* (i) a BusinessEntry
* new BusinessEntry(name,addr,phone,city,state),
* (ii) a GovernmentEntry
* new GovernmentEntry(name,addr,phone,city,state,gov), or
* (iii) a ResidentialEntry
* new ResidentialEntry(name,addr,phone).
*/
abstract class CityEntry {
}
class BusinessEntry extends CityEntry {
/* fields */
String name;
String address;
String phone;
String city;
String state;
/* constructor */
BusinessEntry(String n, String a, String p, String c, String s) {
this.name = n;
this.address = a;
this.phone = p;
this.city = c;
this.state = s;
}
/* accessors */
String getName() { return this.name; }
String getAddress() { return this.address; }
String getPhone() { return this.phone; }
String getCity() { return this.city; }
String getState() { return this.state; }
}
class GovernmentEntry extends CityEntry {
/* fields */
String name;
String address;
String phone;
String city;
String state;
String government;
/* constructor */
GovernmentEntry(String n, String a, String p, String c, String s, String g) {
this.name = n;
this.address = a;
this.phone = p;
this.city = c;
this.state = s;
this.government = g;
}
/* accessors */
String getName() { return this.name; }
String getAddress() { return this.address; }
String getPhone() { return this.phone; }
String getCity() { return this.city; }
String getState() { return this.state; }
String getGovernment() { return this.government; }
}
class ResidentialEntry extends CityEntry {
String name;
String address;
String phone;
/* constructor */
ResidentialEntry(String n, String a, String p) {
this.name = n;
this.address = a;
this.phone = p;
}
/* accessors */
String getName() { return this.name; }
String getAddress() { return this.address; }
String getPhone() { return this.phone; }
}
Figure 3: The City Directory union class hierarchy
Note that each concrete class includes exactly the same fields as the corresponding Scheme structure definition. The pivotal differences between the Java code and the corresponding Scheme code are (i) the presence of the abstract class CityEntry in the Java Code identifying the union type which is left implicit in Scheme and (ii) the explicit definitions of constructors and accessors (selectors) in the Java concrete classes which are automatically generated by the Scheme structure definitions.
The Java code in the CityEntry example above involves several concepts that we have not discussed before.
The following expression creates a BusinessEntry for Rice University
new BusinessEntry("Rice University", "6100 Main Street", "713-348-8101",
"Houston", "Texas")
This syntax is wordy but straightforward. Don't forget to include the
keyword new at the front on each constructor invocation!
Finger Exercise 1.6.1 Enter the preceding class definitions into the
Definitions pane of DrJava. Compile this program and evaluate the
following expressions in the Interactions pane:
BusinessEntry e1 =
new BusinessEntry("Rice University", "6100 Main St.", "713-527-8101",
"Houston", "TX");
ResidentialEntry e2 =
new ResidentialEntry("Robert Cartwright", "3310 Underwood St.",
"713-660-0967");
e1.getName()
e2.getName()
Did you get the results that you expected?