Java Reference
In-Depth Information
14 uses the random value to determine which
frequency
element to increment during
each iteration of the loop. The calculation in line 14 produces random numbers from 1 to
6, so the array
frequency
must be large enough to store six counters. However, we use a
seven-element array in which we ignore
frequency[0]
—it's more logical to have the face
value 1 increment
frequency[1]
than
frequency[0]
. Thus, each face value is used as an
index for array
frequency
. In line 14, the calculation inside the square brackets evaluates
first to determine which element of the array to increment, then the
++
operator adds one
to that element. We also replaced lines 49-51 from Fig. 6.7 by looping through array
fre-
quency
to output the results (lines 19-20). When we study Java SE 8's new functional pro-
gramming capabilities in Chapter 17, we'll show how to replace lines 13-14 and 19-20
with a
single
statement!
Our next example uses arrays to summarize data collected in a survey. Consider the fol-
lowing problem statement:
Twenty students were asked to rate on a scale of 1 to 5 the quality of the food in the
student cafeteria, with 1 being “awful” and 5 being “excellent.” Place the 20 responses
in an integer array and determine the frequency of each rating.
This is a typical array-processing application (Fig. 7.8). We wish to summarize the num-
ber of responses of each type (that is, 1-5). Array
responses
(lines 9-10) is a 20-element
integer array containing the students' survey responses. The last value in the array
is inten-
tionally
an incorrect response (
14
). When a Java program executes, array element indices
are checked for validity—all indices must be greater than or equal to 0 and less than the
length of the array. Any attempt to access an element outside that range of indices results
in a runtime error that's known as an
ArrayIndexOutOfBoundsException
. At the end of
this section, we'll discuss the invalid response value, demonstrate array
bounds checking
and introduce Java's
exception-handling
mechanism, which can be used to detect and
handle an
ArrayIndexOutOfBoundsException
.
1
// Fig. 7.8: StudentPoll.java
2
// Poll analysis program.
3
4
public class
StudentPoll
5
{
6
public static void
main(String[] args)
7
{
8
// student response array (more typically, input at runtime)
9
int
[] responses = {
1
,
2
,
5
,
4
,
3
,
5
,
2
,
1
,
3
,
3
,
1
,
4
,
3
,
3
,
3
,
10
2
,
3
,
3
,
2
,
14
};
11
int
[] frequency =
new int
[
6
];
// array of frequency counters
12
13
// for each answer, select responses element and use that value
// as frequency index to determine element to increment
for
(
int
answer =
0
; answer < responses.length; answer++)
{
14
15
16
Fig. 7.8
|
Poll analysis program. (Part 1 of 2.)
