Java Reference
In-Depth Information
Listing 5-15. A more complete while loop
int counter = 0;
while (counter < 10) {
System.out.println(counter);
counter++;
}
That (still simple) example shows the typical structure of a
while
loop: declare a variable, test the
variable at the top of the
while
loop, do some work in the
while
loop, and increment the variable in the
while
loop. After the test fails, the
while
loop is done, and the code below the
while
loop runs.
You can create a
while
loop that runs forever (or, more likely, until some condition that gets checked
by an
if
statement in the body of the loop triggers a
break
statement) by using
while(true)
. That
construct is useful in some cases, but it's not a safe thing to do. Savvy programmers try to ensure that
loops can't run forever (that's called an infinite loop). What if, due to unexpected input data, your break
condition in the body of the loop never gets hit? Then your program is stuck (and that can be loads of fun
if it happens to be writing into a file—you can quickly cause problems for your operating system this
way). If you feel that you must use
while(true)
, test thoroughly.
Another use for a
while
loop is to try something for a set amount of time when you don't know how
long each try will take. For example, suppose a mail client tries for one minute to connect to a mail
server and then gives up (that is, it times out). Listing 5-14 shows one way to wait for a connection.
Listing 5-14. While loop that waits for an event
boolean connected = false;
long now = System.currentTimeMillis();
long oneMinuteFromStart = System.currentTimeMillis() + 60000;
while (!connected && now < oneMinuteFromStart) {
System.out.println("Trying to connect....");
if (MailServer.connect()) {
connected = true;
break;
}
// pause for two seconds
// to let other processes work, too
try {
System.out.println("(letting other processes run)");
Thread.sleep(2000);
} catch (Exception e) {
e.printStackTrace();
}
// reset now for the next comparison
now = System.currentTimeMillis();
}
if (connected) {
System.out.println("Connected to the mail server");
} else {
System.out.println("Timed out while trying to connect to the mail server");
}
