Java Reference
In-Depth Information
boolean
madeIt = (time < limit) && (limit < max);
A Boolean expression can be evaluated in the same way that an arithmetic expres-
sion is evaluated. The only difference is that an arithmetic expression uses operations
such as
+
,
*
, and
/
and produces a number as the final result, whereas a Boolean
expression uses relational operations such as
==
and
<
and Boolean operations such as
&&
,
||
, and
!
, and produces one of the two values
true
and
false
as the final result.
First, let's review evaluating an arithmetic expression. The same technique will work
in the same way to evaluate Boolean expressions. Consider the following arithmetic
expression:
(number + 1) * (number + 3)
Assume that the variable
number
has the value 2. To evaluate this arithmetic expres-
sion, you evaluate the two sums to obtain the numbers 3 and 5, and then you combine
these two numbers 3 and 5 using the
*
operator to obtain 15 as the final value. Notice
that in performing this evaluation, you do not multiply the expressions
(number + 1)
and
(number + 3)
.
Instead, you multiply the values of these expressions. You use 3;
you do not use
(number + 1)
.
You use 5; you do not use
(number + 3)
.
The computer evaluates Boolean expressions the same way. Subexpressions are evalu-
ated to obtain values, each of which is either
true
or
false
. In particular,
==
,
!=
,
<
,
<=
, and
so forth operate on pairs of any primitive type to produce a Boolean value of
true
or
false
. These individual values of
true
or
false
are then combined according to the rules
in the
truth tables
shown in Display 3.5. For example, consider the Boolean expression
truth tables
!( ( count < 3) || (count > 7) )
which might be the controlling expression for an
if-else
statement. Suppose the
value of
count
is 8. In this case,
(count < 3)
evaluates to
false
and
(count > 7)
eval-
uates to
true
, so the preceding Boolean expression is equivalent to
!(
false
||
true
)
Consulting the tables for
||
(which is labeled “OR”), the computer sees that the
expression inside the parentheses evaluates to
true
. Thus, the computer sees that the
entire expression is equivalent to
!(
true
)
Consulting the tables again, the computer sees that
!(true)
evaluates to
false
, and so
it concludes that
false
is the value of the original Boolean expression.
The
boolean
Values Are
true
and
false
true
and
false
are predefined constants of type
boolean
. (They must be written in lower-
case.) In Java, a Boolean expression evaluates to the
boolean
value
true
when it is satisfied
and evaluates to the
boolean
value
false
when it is not satisfied.
