Java Reference
In-Depth Information
A loop can be used to tell a program to execute statements repeatedly.
Key
Point
Suppose that you need to display a string (e.g.,
Welcome to Java!
) a hundred times. It
would be tedious to have to write the following statement a hundred times:
problem
System.out.println(
"Welcome to Java!"
);
System.out.println(
"Welcome to Java!"
);
...
System.out.println(
"Welcome to Java!"
);
100 times
So, how do you solve this problem?
Java provides a powerful construct called a
loop
that controls how many times an operation
or a sequence of operations is performed in succession. Using a loop statement, you simply
tell the computer to display a string a hundred times without having to code the print state-
ment a hundred times, as follows:
loop
int
count =
0
;
while
(count <
100
) {
System.out.println(
"Welcome to Java!"
);
count++;
}
The variable
count
is initially
0
. The loop checks whether
count < 100
is
true
. If so, it
executes the loop body to display the message
Welcome to Java!
and increments
count
by
1
. It repeatedly executes the loop body until
count < 100
becomes
false
. When
count
< 100
is
false
(i.e., when
count
reaches
100
), the loop terminates and the next statement
after the loop statement is executed.
Loops
are constructs that control repeated executions of a block of statements. The concept
of looping is fundamental to programming. Java provides three types of loop statements:
while
loops,
do
-
while
loops, and
for
loops.
Key
Point
The syntax for the
while
loop is:
while
(loop-continuation-condition) {
// Loop body
Statement(s);
while
loop
}
Figure 4.1a shows the
while
-loop flowchart. The part of the loop that contains the state-
ments to be repeated is called the
loop body.
A one-time execution of a loop body is referred
to as an
iteration (
or
repetition) of the loop.
Each loop contains a
loop-continuation-
condition,
a Boolean expression that controls the execution of the body. It is evaluated each
time to determine if the loop body is executed. If its evaluation is
true
, the loop body is exe-
cuted; if its evaluation is
false
, the entire loop terminates and the program control turns to
the statement that follows the
while
loop.
The loop for displaying
Welcome to Java!
a hundred times introduced in the pre-
ceding section is an example of a
while
loop. Its flowchart is shown in Figure 4.1b. The
loop body
iteration
loop-continuation-
condition