Java Reference

In-Depth Information

• Conjunction, or
and
:
b1 && b2

Expression
b1 && b2
evaluates to
true
if both
b1
and
b2
are
true
and

evaluates to
false
otherwise. Operands
b1
and
b2
are called
conjuncts
.

• Disjunction, or
or
:
b1 || b2

Expression
b1 || b2
evaluates to
true
if either
b1
or
b2
(or both) is

true
and to
false
otherwise. Operands
b1
and
b2
are called
disjuncts
.

For example, we evaluate the expression
true
&& !(
false
||
false
)
:

1.
false
||
false
evaluates to
false
,

so the expression becomes
true
&& !
false
.

2.
!false
evaluates to
true
,

so the expression becomes
true
&&
true
.

3.
true
&&
true
evaluates to
true
.

You are not likely to use operations
!
,
&&
, and
||
very much in the begin-

ning. However, we need type
boolean
in order to talk about arithmetic
relations
,

which yield
boolean
values. Assuming that
e1
and
e2
are both
int
or
double

expressions, we can use the following arithmetic relations:

e1 < e2
true
if
e1
is less than
e2
and
false
otherwise.

e1 > e2
true
if
e1
is greater than
e2
and
false
otherwise.

e1 <= e2
true
if
e1
is at most
e2
and
false
otherwise.

e1 >= e2
true
if
e1
is at least
e2
and
false
otherwise.

e1 == e2
true
if
e1
and
e2
are equal and
false
otherwise.

e1 != e2
true
if
e1
and
e2
are different and
false
otherwise.

For example, the expression
5+3<9
evaluates to
true
, and
5+3<8
eval-

uates to
false
. The operators
<
,
>
,
<=
,
>=
,
==
, and
!=
are called
relational oper-

ators
.

Be careful with equality: The phrase
5=6
is not a Java expression! Java uses

the sign
==
for an equality test, rather than
=
, and uses
=
in another way, which

we describe in Sec. 1.2. For example,
6==6
evaluates to
true
and
5==6
eval-

uates to
false
. This breaking of mathematical convention is unfortunate. For

hundreds of years, ever since Robert Recorde introduced the sign = for equality

in the 1600s, that convention has been in use. To have programming languages

break with that tradition is a travesty.

Operator

Operation

Example

Result

not
, or
negation

!

!
true

false

and
, or
conjunction

&&

true
&&
false

false

or
, or
disjunction

||

true
||
false

true

Figure 1.2:
Basic boolean operators in Java

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