Java How to Program Fourth Edition - DCC

414 Object-Based Programming Chapter 8
8.11 Composition: Objects as Instance Variables of Other
Classes
An AlarmClock class object needs to know when it is supposed to sound its alarm, so
why not include a reference to a Time object as a member of the AlarmClock object?
Such a capability is called composition. A class can have references to objects of other
classes as members.
Software Engineering Observation 8.18
One form of software reuse is composition, in which a class has references to objects of other
classes as members. 8.18
The next program contains three classes—Date (Fig. 8.13), Employee (Fig. 8.14) and
EmployeeTest (Fig. 8.15). Class Employee contains instance variables firstName,
lastName, birthDate and hireDate. Members birthDate and hireDate are references
to Dates that contain instance variables month, day and year. This demonstrates
that a class can contain references to objects of other classes. Class EmployeeTest instantiates
an Employee and initializes and displays its instance variables. The Employee constructor
(Fig. 8.14, lines 12–20) takes eight arguments—first, last, birthMonth,
birthDay, birthYear, hireMonth, hireDay and hireYear. Arguments birth-
Month, birthDay and birthYear are passed to the Date constructor (Fig. 8.13, lines
13–28) to initialize the birthDate object and hireMonth, hireDay and hireYear
are passed to the Date constructor to initialize the hireDate object.
A member object does not need to be initialized immediately with constructor arguments.
If an empty argument list is provided when a member object is created, the object’s
default constructor (or no-argument constructor if one is available) will be called automatically.
Values, if any, established by the default constructor (or no-argument constructor)
can then be replaced by set methods.
Performance Tip 8.2
Initialize member objects explicitly at construction time by passing appropriate arguments
to the constructors of the member objects. This eliminates the overhead of initializing member
objects twice—once when the member object’s default constructor is called and again
when set methods are used to provide initial values for the member object. 8.2
Note that both class Date (Fig. 8.13) and class Employee (Fig. 8.14) are defined as
part of the package com.deitel.jhtp4.ch08 as specified on line 3 of each file.
Because they are in the same package (i.e., the same directory), class Employee does not
need to import class Date to use it. When the compiler searches for the file Date.class,
the compiler knows to search the directory where Employee.class is located. Classes
in a package never need to import other classes from the same package.
1 // Fig. 8.13: Date.java
2 // Declaration of the Date class.
3 package com.deitel.jhtp4.ch08;
4
5 public class Date extends Object {
6 private int month; // 1-12
Fig. Fig. 8.13 8.13 Date class (part 1 of 2).
© Copyright 1992–2002 by Deitel & Associates, Inc. All Rights Reserved. 7/3/01