Java Reference
In-Depth Information
Predicate Methods
Another common use for accessor methods is to test whether a condition is
true
or
false
—
such methods are often called
predicate methods.
An example would be class
ArrayList
's
isEmpty
method, which returns
true
if the
ArrayList
is empty and
false
otherwise. A
program might test
isEmpty
before attempting to read another item from an
ArrayList
.
A class can have references to objects of other classes as members. This is called
composi-
tion
and is sometimes referred to as a
has-a
relationship
. For example, an
AlarmClock
ob-
ject needs to know the current time
and
the time when it's supposed to sound its alarm,
so it's reasonable to include
two
references to
Time
objects in an
AlarmClock
object. A car
has-a
steering wheel, a break pedal and an accelerator pedal.
Class
Date
This composition example contains classes
Date
(Fig. 8.7),
Employee
(Fig. 8.8) and
Em-
ployeeTest
(Fig. 8.9). Class
Date
(Fig. 8.7) declares instance variables
month
,
day
and
year
(lines 6-8) to represent a date. The constructor receives three
int
parameters. Lines
17-19 validate the
month
—if it's out-of-range, lines 18-19 throw an exception. Lines 22-
25 validate the
day
. If the day is incorrect based on the number of days in the particular
month
(except February 29th which requires special testing for leap years), lines 24-25
throw an exception. Lines 28-31 perform the leap year testing for February. If the month
is February and the day is 29 and the
year
is not a leap year, lines 30-31 throw an excep-
tion. If no exceptions are thrown, then lines 33-35 initialize the Date's instance variables
and lines 38-38 output the
this
reference as a
String
. Since
this
is a reference to the
current
Date
object, the object's
toString
method (lines 42-45) is called
implicitly
to ob-
tain the object's
String
representation. In this example, we assume that the value for
year
is correct—an industrial-strength
Date
class should also validate the year.
1
// Fig. 8.7: Date.java
2
// Date class declaration.
3
4
public class
Date
5
{
6
private int
month;
// 1-12
7
private int
day;
// 1-31 based on month
8
private int
year;
// any year
9
10
private static final int
[] daysPerMonth =
11
{
0
,
31
,
28
,
31
,
30
,
31
,
30
,
31
,
31
,
30
,
31
,
30
,
31
};
12
13
// constructor: confirm proper value for month and day given the year
14
public
Date(
int
month,
int
day,
int
year)
15
{
16
// check if month in range
17
if
(month <=
0
|| month >
12
)
18
throw new
IllegalArgumentException(
19
"month ("
+ month +
") must be 1-12"
);
Fig. 8.7
|
Date
class declaration. (Part 1 of 2.)
