Program Logic and Indefinite Loops - Building Java Programs: A Back to Basics Approach - page 312

As we have seen, the while loop tests at the “top” of the loop, before it executes
its controlled statement. Java has an alternative known as the do/while loop that
tests at the “bottom” of the loop. The do/while loop has the following syntax:

do {

<statement>;
...
<statement>;
} while (<test>);

Here is some sample code using a do/while loop:

int number = 1;
do {

number *= 2;
} while (number <= 200);

This loop produces the same result as the corresponding while loop, doubling the
variable number until its value reaches 256 , which is the first power of 2 greater than
200 . But unlike the while loop, the do/while loop always executes its controlled
statements at least once. The diagram in Figure 5.2 shows the flow of control in a
do/while loop.

Execute the
controlled statement(s)

Ye s

Is the test true?

No

Execute the statement
after the do/while loop

Figure 5.2 Flow of do/while loop

The do/while loop is most useful in situations in which you know you have to
execute the loop at least once. For example, in the last section we wrote the following
code that simulates the rolling of two dice until you get a sum of 7:

Random r = new Random();
int sum = 0; // set to 0 to make sure we enter the loop
while (sum != 7) {
// roll the dice once
int roll1 = r.nextInt(6) + 1;