Java Reference
In-Depth Information
EXAMPLE:
(continued)
while
((c != 'a') || (c != 'b'));
This forces the loop to exit if
'a'
or
'b'
is entered, and it will make the program work.
Unfortunately, the result is a coding atrocity that should be avoided at all costs. This
“quick fix” does not address the root cause of the error—only the symptoms. Moreover,
such patches usually will not work for new cases. This particular fix also results in
inconsistent code because the expression
((c! = 'a') || (c! = 'b'))
becomes
meaningless when we already handle the
'a'
and
'b'
with the
if
and
break
statements.
To really find the bug, we can turn again to tracing, this time focusing on the
Boolean values that control the
do-while
loop:
do
{
System.out.println("Enter 'A' for option A " +
"or 'B' for option B.");
s = keyboard.next( );
s = s.toLowerCase( );
c = s.charAt(0);
System.out.println("c != 'a' is " + (c!= 'a'));
System.out.println("c != 'b' is " + (c!= 'b'));
System.out.println("(c != 'a') || (c != 'b')) is "
+ ((c != 'a') || (c != 'b')));
}
while
((c != 'a') || (c != 'b'));
The sample output is now as follows:
Enter 'A' for option A or 'B' for option B.
A
c != 'a' is false
c != 'b' is true
(c != 'a') || (c != 'b')) is true
Since
c
equals
'a'
, the statement
(c != 'a')
evaluates to
false
and the statement
(c !='b')
evaluates to
true
. When combined,
(false || true)
is
true
, which
makes the loop repeat. In spoken English, it sounds like “c not equal to
'a'
” or “c
not equal to
'b'
” is a correct condition to repeat the loop. After all, if the character
typed in is not
'a'
or the character typed in is not
'b'
, then the user should
be prompted to try again. Logically however, if
(c != 'a')
is
false
(i.e., the
character is
'a'
), then
(c != 'b')
must be true. A character cannot make both
expressions
false
, so the final Boolean condition will always be
true
. The solution
is to replace the “or” with an “and” so that the loop repeats only if
(c != 'a') &&
(c != 'b'))
. This makes the loop repeat as long as the input character is not
'a'
and it is not
'b'
.
(continued)
