Java Reference
In-Depth Information
inserting the numbers in the collection. Because generic classes can be used only with class
or interface types, the numbers would be
autoboxed
as objects of the type-wrapper classes.
For example, any
int
value would be
autoboxed
as an
Integer
object, and any
double
val-
ue would be
autoboxed
as a
Double
object. We'd like to be able to total all the numbers in
the
ArrayList
regardless of their type. For this reason, we'll declare the
ArrayList
with
the type argument
Number
, which is the superclass of both
Integer
and
Double
. In addi-
tion, method
sum
will receive a parameter of type
ArrayList<Number>
and total its ele-
ments. Figure 20.13 demonstrates totaling the elements of an
ArrayList
of
Number
s.
1
// Fig. 20.13: TotalNumbers.java
2
// Totaling the numbers in an ArrayList<Number>.
3
import
java.util.ArrayList;
4
5
public class
TotalNumbers
6
{
7
public static void
main(String[] args)
8
{
9
// create, initialize and output ArrayList of Numbers containing
10
// both Integers and Doubles, then display total of the elements
11
Number[] numbers = {
1
,
2.4
,
3
,
4.1
};
// Integers and Doubles
ArrayList<Number> numberList =
new
ArrayList<>();
12
13
14
for
(Number element : numbers)
15
numberList.add(element);
// place each number in numberList
16
17
System.out.printf(
"numberList contains: %s%n"
,
numberList
);
18
System.out.printf(
"Total of the elements in numberList: %.1f%n"
,
19
sum(numberList)
);
20
}
21
22
// calculate total of ArrayList elements
23
public static double
sum(
ArrayList<Number>
list)
24
{
25
double
total =
0
;
// initialize total
26
27
// calculate sum
28
for
(Number element : list)
total += element.doubleValue();
29
30
31
return
total;
32
}
33
}
// end class TotalNumbers
numberList contains: [1, 2.4, 3, 4.1]
Total of the elements in numberList: 10.5
Fig. 20.13
|
Totaling the numbers in an
ArrayList<Number>
.
Line 11 declares and initializes an array of
Number
s. Because the initializers are prim-
itive values, Java
autoboxes
each primitive value as an object of its corresponding wrapper
type. The
int
values
1
and
3
are
autoboxed
as
Integer
objects, and the
double
values
2.4
