SELECTIONS - Introduction to Java Programming - page 104

is correct. However, it is incorrect in Java, because 1 <= numberOfDaysInAMonth is
evaluated to a boolean value, which cannot be compared with 31 . Here, two operands
(a boolean value and a numeric value) are incompatible . The correct expression in Java is

incompatible operands

( 1 <= numberOfDaysInAMonth) && (numberOfDaysInAMonth <= 31 )

Note
As shown in the preceding chapter, a char value can be cast into an int value, and
vice versa. A boolean value, however, cannot be cast into a value of another type, nor
can a value of another type be cast into a boolean value.

cannot cast boolean

Note
De Morgan's law, named after Indian-born British mathematician and logician Augustus
De Morgan (1806-1871), can be used to simplify Boolean expressions. The law states:

De Morgan's law

!(condition1 && condition2) is the same as
!condition1 || !condition2
!(condition1 || condition2) is the same as
!condition1 && !condition2

For example,

! (number % 2 == 0 && number % 3 == 0 )

can be simplified using an equivalent expression:

(number % 2 != 0 || number % 3 != 0 )

As another example,

!(number == 2 || number == 3 )

is better written as

number != 2 && number != 3

If one of the operands of an && operator is false , the expression is false ; if one of the
operands of an || operator is true , the expression is true . Java uses these properties to improve
the performance of these operators. When evaluating p1 && p2 , Java first evaluates p1 and then,
if p1 is true , evaluates p2 ; if p1 is false , it does not evaluate p2 . When evaluating p1 || p2 ,
Java first evaluates p1 and then, if p1 is false , evaluates p2 ; if p1 is true , it does not evaluate
p2 . Therefore, && is referred to as the conditional or short-circuit AND operator, and is
referred to as the conditional or short-circuit OR operator. Java also provides the conditional
AND ( & ) and OR ( | ) operators, which are covered in Supplement III.C and III.D for advanced
readers.

conditional operator
short-circuit operator

3.18

✓ ✓

Assuming that x is 1 , show the result of the following Boolean expressions.

Check

Point

( true ) && ( 3 > 4 )
!(x > 0 ) && (x > 0 )
(x > 0 ) || (x < 0 )

(x != 0 ) || (x == 0 )
(x >= 0 ) || (x < 0 )
(x != 1 ) == !(x == 1 )

3.19

Write a Boolean expression that evaluates to true if a number stored in variable num
is between 1 and 100 .

3.20

Write a Boolean expression that evaluates to true if a number stored in variable num
is between 1 and 100 or the number is negative.