Do while loop in Java

 Do while loop in Java

1. Syntax and flow of the while loop

do {
// body of a loop
} 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 terminates. As with all of Java’s loops, a condition must be a Boolean expression.

do-while loop example

public class DoWhileLoopExample {
    public static void main(String args[]) {
        int n = 10;
        do {
            System.out.println("tick " + n);
            n--;
        } while (n > 0);
    }
}

Output:

tick 10
tick 9
tick 8
tick 7
tick 6
tick 5
tick 4
tick 3
tick 2
tick 1

do-while loop with Menu Selection example

The do-while loop is especially useful when you process a menu selection because you will usually want the body of a menu loop to execute at least once.

Consider the following program, which implements a very simple help system for Java’s selection and iteration statements:

public class DoWhileMenuExample {
    public static void main(String args[]) throws java.io.IOException {
        char choice;
        do {
            System.out.println("Help on: ");
            System.out.println(" 1. if");
            System.out.println(" 2. switch");
            System.out.println(" 3. while");
            System.out.println(" 4. do-while");
            System.out.println(" 5. for\n");
            System.out.println("Choose one:");
            choice = (char) System.in.read();
        } while (choice < '1' || choice > '5');
        System.out.println("\n");
        switch (choice) {
            case '1':
                System.out.println("The if:\n");
                System.out.println("if(condition) statement;");
                System.out.println("else statement;");
                break;
            case '2':
                System.out.println("The switch:\n");
                System.out.println("switch(expression) {");
                System.out.println(" case constant:");
                System.out.println(" statement sequence");
                System.out.println(" break;");
                System.out.println(" //...");
                System.out.println("}");
                break;
            case '3':
                System.out.println("The while:\n");
                System.out.println("while(condition) statement;");
                break;
            case '4':
                System.out.println("The do-while:\n");
                System.out.println("do {");
                System.out.println(" statement;");
                System.out.println("} while (condition);");
                break;
            case '5':
                System.out.println("The for:\n");
                System.out.print("for(init; condition; iteration)");
                System.out.println(" statement;");
                break;
        }
    }
}

Result:

Help on:
1. if
2. switch
3. while
4. do-while
5. for
Choose one:
4
The do-while:
do {
statement;
} while (condition);