Java (The do-while Loop)

The do-while Loop

The do-while loop is a variation of the while loop. Its syntax is given below:

do {
  // Loop body;
  Statement(s);
} while (loop-continuation-condition);

Its execution flow chart is shown in Figure 4.4.

Figure 4.4. The do-while loop executes the loop body first, and then checks the loop-continuation-condition to determine whether to continue or terminate the loop.

The loop body is executed first. Then the loop-continuation-condition is evaluated. If the evaluation is true, the loop body is executed again; if it is false, the do-while loop terminates. The major difference between a while loop and a do-while loop is the order in which the loop-continuation-condition is evaluated and the loop body executed.

The while loop and the do-while loop have equal expressive power. Sometimes one is a more convenient choice than the other. For example, you can rewrite the while loop in Listing 4.2 using a do-while loop, as shown in Listing 4.3.

Listing 4.3. TestDo.java

1 import javax.swing.JOptionPane; 2 3 public class TestDoWhile { 4 /** Main method */ 5 public static void main(String[] args) { 6 int data; 7 int sum = 0; 8 9 // Keep reading data until the input is 0 10 do { 11 // Read the next data 12 String dataString = JOptionPane.showInputDialog(null, 13 "Enter an int value:\n(the program exits if the input is 0)", 14 "TestDo", JOptionPane.QUESTION_MESSAGE); 15 16 data = Integer.parseInt(dataString); 17 18 sum += data; 19 } while (data != 0); 20 21 JOptionPane.showMessageDialog(null, "The sum is " + sum, 22 "TestDo", JOptionPane.INFORMATION_MESSAGE); 23 } 24 }