There are a number of approaches to repeat a fragment of code while a certain condition is true. In this lesson, we will learn how to do it using two kinds of loops. The difference between them is the order of the repetitive fragment execution and the condition evaluation.
The while loop
The while loop consists of a block of code and a condition (a Boolean expression). If the condition is true, the code within the block is executed. This code repeats until the condition becomes false. Because this loop checks the condition before the block is executed, the control structure is often also known as a pre-test loop. You can think of the while loop as a repeating conditional statement.
The basic syntax of the while loop is the following:
while (condition) {
// body: do something repetitive
}A loop's body can contain any correct Java statements including conditional statements and even other loops (nested loops).
It is also possible to write an infinite loop if the condition is invariably true
while (true) {
// body: do something indefinitely
}The use of infinite loops will be considered in the following topics.
Example 1. The following loop prints integer numbers while a variable is less than 5.
int i = 0;
while (i < 5) {
System.out.println(i);
i++;
}
// a next statementLet's explain how this
loop works. First, the value 0 is assigned to the variable
i. Before the first
execution of the loop's body, the program checks if
i < 5. That is true (because
i is 0), so the body of the loop
starts executing. The body has two statements: displaying the current value of i and incrementing it
by 1. After this, the expression
i < 5
is evaluated again. Now
i equals 1, so
the conditional is
true
again, and the loop's body is repeated again. This is
repeated until
i has taken the value 5, after which the expression i < 5
ceases be
true, and the execution of this loop terminates. The program proceeds
to the next statement after the loop.
0
1
2
3
4Example 2. The following program displays English letters in a single line.
public class WhileDemo {
public static void main(String[] args) {
char letter = 'A';
while (letter <= 'Z') {
System.out.print(letter);
letter++;
}
}
}The program takes the first letter 'A' and then repeats:
- if the letter is less or equal to
'Z'the program go to the loop's body - inside the body, it prints the current character and gets the next letter.
The program prints:
ABCDEFGHIJKLMNOPQRSTUVWXYZRemember that it is possible to get the next character (according to the Unicode table) using the increment operator.
The do-while loop
In the do-while loop,
the body is executed first and the condition is tested afterwards. If the
condition is
true,
statements within the block are executed again. This repeats
until the condition becomes
false.
Because do-while loops check the condition
after the block is executed, the control structure is often also known as a
post-test loop. In contrast to the while loop, which tests the condition before
the code within the block is executed, the do-while loop is an exit-condition
loop. So, the code within the block is always executed at least once.
do keyword, a body, and while(condition):do {
// body: do something
} while (condition);The following program reads an integer number from the standard input and displays the number. If the number 0 is entered, the program prints it and then stops. It demonstrates the do-while loop.
public class DoWhileDemo {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
int value;
do {
value = scanner.nextInt();
System.out.println(value);
} while (value > 0);
}
}Input numbers:
1 2 4 0 3The program prints:
1
2
4
0Note that, like the while loop, the do-while loop can be infinite.
In practice, the do-while loop is used less than the while loop. A good example of using it is a program that reads data from the standard input until a user enters a certain number or string. It is assumed that the program will be executed at least once, and repeated execution is optional.