Java Reference
In-Depth Information
While assertions can be stated as comments to guide you during program develop-
ment, Java includes two versions of the
assert
statement for validating assertions progra-
matically. The
assert
statement evaluates a
boolean
expression and, if
false
, throws an
AssertionError
(a subclass of
Error
). The first form of the
assert
statement is
assert
expression
;
which throws an
AssertionError
if
expression
is
false
. The second form is
assert
expression1
:
expression2
;
which evaluates
expression1
and throws an
AssertionError
with
expression2
as the error
message if
expression1
is
false
.
You can use assertions to implement
preconditions
and
postconditions
programmati-
cally or to verify any other
intermediate
states that help you ensure that your code is
working correctly. Figure 11.8 demonstrates the
assert
statement. Line 11 prompts the
user to enter a number between 0 and 10, then line 12 reads the number. Line 15 deter-
mines whether the user entered a number within the valid range. If the number is out of
range, the
assert
statement reports an error; otherwise, the program proceeds normally.
1
// Fig. 11.8: AssertTest.java
2
// Checking with assert that a value is within range
3
import
java.util.Scanner;
4
5
public
class
AssertTest
6
{
7
public
static
void
main(String[] args)
8
{
9
Scanner input =
new
Scanner(System.in);
10
11
System.out.print(
"Enter a number between 0 and 10: "
);
12
int
number = input.nextInt();
13
14
// assert that the value is >= 0 and <= 10
15
assert
(number >=
0
&& number <=
10
) :
"bad number: "
+ number;
16
17
System.out.printf(
"You entered %d%n"
, number);
18
}
19
}
// end class AssertTest
Enter a number between 0 and 10:
5
You entered 5
Enter a number between 0 and 10:
50
Exception in thread "main" java.lang.AssertionError: bad number: 50
at AssertTest.main(AssertTest.java:15)
Fig. 11.8
|
Checking with
assert
that a value is within range.
You use assertions primarily for debugging and identifying logic errors in an applica-
tion. You must explicitly enable assertions when executing a program, because they reduce
