In the previous sections you have learnt while and for statements. You can program most situtations using these loop statements. Additionally, Java provides another loop statement known as the do-while statement.
As you have seen, if the conditional expression controlling a loop statement
evaluates to false initially, then the body of the loop will not be executed
at all.
However, sometimes you might want to execute the body of a loop at least once, even if the conditional expression is false to begin with. In such cases, you can use the do-while statement provided by Java.
The do-while loop always executes its body at least once, because its conditional expression is at the bottom of the loop.
The general form of the do-while statement is shown here.
do {
// body
}
while (condition);
Each iteration of the do-while loop first executes the body of the loop and
then evaluates the conditional expression. If this expression is true, the
loop will repeat. Otherwise, the loop is terminated.
Here’s an example of using a do-while statement in a menu driven program.
public class MenuUsingDoWhile {
public static void main(String ... arguments) throws Exception {
char choice;
do {
System.out.println(
"\nThe valid choices are as follows.\n" +
"1 - Pizza\n" +
"2 - Biryani\n" +
"3 - Fried Rice\n" +
"4 - Burger\n" +
"Any other number causes the program to exit.\n");
System.out.print("What's on your mind, master Bruce? ");
choice = (char)System.in.read();
System.in.skip(10);
switch(choice) {
case '1':
System.out.println("Who is mood for some Joey's special?");
break;
case '2':
System.out.println("A moment on the lips. Not sure about other items here, but Biryani surely stays forever on your hips!");
break;
case '3':
System.out.println("A fine choice, sire!");
break;
case '4':
System.out.println("Sorry, we are out of burgers. Come back later.");
break;
default:
System.out.println("Bye! See you again soon.");
}
}
while ((choice >= '1') && (choice <= '4'));
}
}
Here is a sample run produced by this program.
The valid choices are as follows. 1 - Pizza 2 - Biryani 3 - Fried Rice 4 - Burger Any other number causes the program to exit.
What’s on your mind, master Bruce? 4 Sorry, we are out of burgers. Come back later.
The valid choices are as follows. 1 - Pizza 2 - Biryani 3 - Fried Rice 4 - Burger Any other number causes the program to exit.
What’s on your mind, master Bruce? 3 A fine choice, sire!
The valid choices are as follows. 1 - Pizza 2 - Biryani 3 - Fried Rice 4 - Burger Any other number causes the program to exit.
What’s on your mind, master Bruce? ```
In the previous example, the do-while statement is used to display a menu and serve accordingly. When the user enters an unknown choice, the program terminates.
Notice that characters are read from the keyboard using the System.in.read()
method. Whenever the user enters a choice, some extra characters are read by
your program. We call the System.in.skip(10) method to discard these characters.
Further, to deal with input errors we add the throws clause at the beginning of the main method.
You will learn more about input/output and exceptions later in this book. Don’t worry, you don’t have to understand these concepts right now.