Program Logic and Indefinite Loops - Building Java Programs: A Back to Basics Approach - page 305

Common Programming Error
Infinite Loop
It is relatively easy to write a while loop that never terminates. One reason it's
so easy to make this mistake is that a while loop doesn't have an update step
in its header like a for loop does. It's crucial for the programmer to include a
correct update step because this step is needed to eventually cause the loop's
test to fail.
Consider the following code, which is intended to prompt the user for a num-
ber and repeatedly print that number divided in half until 0 is reached. This first
attempt doesn't compile:
Scanner console = new Scanner(System.in);
System.out.print("Type a number: ");
// this code does not compile
while (number > 0) {
int number = console.nextInt();
System.out.println(number / 2);
}
The problem with the preceding code is that the variable number needs to be in
scope during the loop's test, so it cannot be declared inside the loop. An incorrect
attempt to fix this compiler error would be to cut and paste the line initializing
number outside the loop:
// this code has an infinite loop
int number = console.nextInt(); // moved out of loop
while (number > 0) {
System.out.println(number / 2);
}
This version of the code has an infinite loop; if the loop is entered, it will
never be exited. This problem arises because there is no update inside the while
loop's body to change the value of number . If number is greater than 0 , the loop
will keep printing its value and checking the loop test, and the test will evaluate
to true every time.
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