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1
Boolean Expressions
• Boolean expression is just a
test
for a condition
• Essentially, evaluates to
true
or
false
• Value comparisons:
==
“equals”
(note: not single
=
)
!=
“not equals”
(cannot say
<>
)
>
“greater than”
<
“less than”
>=
“greater than or equal to”
<=
“less than or equal to”
More Boolean Expressions
• Boolean comparisons (in order of precedence):
!
“not”
!p
if
p
is true, then
!p
is false, and vice versa
&&
“and”
p && q
only true if
p
and
q
are both true

“or”
p  q
true if
p
or
q
(or both) are true
boolean p = (x != 1)  (x != 2);
p
is always
true
, you really want:
boolean p = (x != 1) && (x != 2);
Short Circuit Evaluation
•
Stop evaluating boolean expression as soon as we know the answer
•
Consider:
p = (5 > 3)  (4 <= 2);
The test
(4 <= 2)
is not performed!
•
Example of useful case:
p = (x != 0) && ((y / x) == 0);
Avoid division by 0, since
((y / x) == 0)
is not performed
Statement Blocks
• A Compound statement (or block) is a set of
statements enclosed in braces
{
int x = 5;
double y = readDouble("y: ");
println("y = " + y);
}
• Variable’s
scope
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 Fall '08
 SAHAMI,M

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