How to Use Java’s do Statement

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import java.io.File;

import java.util.Scanner;

class IHopeYouKnowWhatYoureDoing {

public static void main(String args[]) {

Scanner keyboard = new Scanner(System.in);

char reply;

<strong> </strong>

<strong>do {</strong>

<strong> </strong>

<strong>System.out.print("Reply with Y or N…");</strong>

<strong>System.out.print(" Delete the importantData file? ");</strong>

<strong>reply = keyboard.findWithinHorizon(".", 0).charAt(0);</strong>

<strong> </strong>

<strong>} while (reply != 'Y' && reply != 'N');</strong>

if (reply == 'Y') {

new File("importantData.txt").delete();

System.out.println("Deleted!");

} else {

System.out.println("No harm in asking!");

}

keyboard.close();

}

}

The loop tests its condition at the end of each iteration, after each of the user’s responses.

Here we go loop, do loop.

That’s why the program has a do loop (also known as a do … while loop).

With a do loop, the program jumps right in, executes some statements, and then checks a condition. If the condition is true, the program goes back to the top of the loop for another go-around. If the condition is false, the computer leaves the loop (and jumps to whatever code comes immediately after the loop).

A closer look at the do statement

do {

<em> Statements</em>

} while (<em>Condition</em>)

Writing the Condition at the end of the loop reminds the programmer that the computer executes the Statements inside the loop first. After the computer executes the Statements, the computer goes on to check the Condition. If the Condition is true, the computer goes back for another iteration of the Statements.

With a do loop, the computer always executes the statements inside the loop at least once:

<strong>//This code prints something:</strong>

int twoPlusTwo = 2 + 2;

do {

System.out.println("Are you kidding?");

System.out.println("2+2 doesn’t equal 5.");

System.out.print ("Everyone knows that");

System.out.println(" 2+2 equals 3.");

} while (twoPlusTwo == 5);

This code displays Are you kidding? 2+2 doesn't equal 5 …and so on and then tests the condition twoPlusTwo == 5. Because twoPlusTwo == 5 is false, the computer doesn’t go back for another iteration. Instead, the computer jumps to whatever code comes immediately after the loop.

Do I hear an echo?

Here's a sample run of the program:

Enter a number: 5

5

Continue? (y/n) y

Enter a number: 81

81

Continue? (y/n) y

Enter a number: 29

29

Continue? (y/n) n

Done!

Tally ho!