Java Reference
In-Depth Information
for the
switch
expression. And finally, it is not legal to have two or more
case
labels
with the same value or more than one
default
label.
The while Statement
The
while
statement is a basic statement that allows Java to perform repetitive
actions—or, to put it another way, it is one of Java's primary
looping constructs
. It has
the following syntax:
while
(
expression
)
statement
The
while
statement works by first evaluating the
expression
, which must result in
a
boolean
or
Boolean
value. If the value is
false
, the interpreter skips the
state
ment
associated with the loop and moves to the next statement in the program. If it
is
true
, however, the
statement
that forms the body of the loop is executed, and the
expression
is reevaluated. Again, if the value of
expression
is
false
, the inter‐
preter moves on to the next statement in the program; otherwise, it executes the
statement
again. This cycle continues while the
expression
remains
true
(i.e.,
until it evaluates to
false
), at which point the
while
statement ends, and the inter‐
preter moves on to the next statement. You can create an infinite loop with the syn‐
tax
while(true)
.
Here is an example
while
loop that prints the numbers 0 to 9:
int
count
=
0
;
while
(
count
<
10
)
{
System
.
out
.
println
(
count
);
count
++;
}
As you can see, the variable
count
starts off at 0 in this example and is incremented
each time the body of the loop runs. Once the loop has executed 10 times, the
expression becomes
false
(i.e.,
count
is no longer less than 10), the
while
state‐
ment finishes, and the Java interpreter can move to the next statement in the pro‐
gram. Most loops have a counter variable like
count
. The variable names
i
,
j
, and
k
are commonly used as loop counters, although you should use more descriptive
names if it makes your code easier to understand.
The do Statement
A
do
loop is much like a
while
loop, except that the loop expression is tested at the
bottom of the loop rather than at the top. This means that the body of the loop is
always executed at least once. The syntax is:
do
statement
while
(
expression
);
Notice a couple of differences between the
do
loop and the more ordinary
while
loop. First, the
do
loop requires both the
do
keyword to mark the beginning of the
