do while loop in Java

The do-while loop is similar to the while loop but with a variation. The condition is checked at the end of the loop.

This loop is used less than the while loop. When there is a demand that the statements (body of loop) will be executed at least once then we go with a do-while loop.

In this article, we will discuss do-while loop, syntax, flowchart, and some programming examples.

What is do while loop in Java?

do-while loop is also called an exit-controlled looping statement because the condition is checked at the end of the loop instead of at the beginning of the loop as in the case of a while loop.

First, the statement will be executed whether the condition is true or false.

If the condition is false in the case of do-while then statements (body of loop) will be executed only one time and control is transferred to the next executable statements.

If the condition is true in the case of do-while then statements (body of loop) will be executed until the condition becomes false.

Unlike the while loop, the do-while loop guarantees that the code inside the loop will execute at least once, regardless of the condition. This is because the condition is checked after the loop’s body is executed.

Syntax

do {
    // Code to be executed
} while (condition);

do {
    // Code to be executed
} while (condition);

Explanation

• do: The keyword do begins the loop.
• Loop Body: This contains the code that will execute with each iteration.
• while (condition): The loop will continue as long as this condition is true. The condition is evaluated after each iteration.
• Valid Test Expression: The condition can be any valid Java expression that results in a boolean value. This can include relational expressions (e.g., x < 10), logical expressions (e.g., x < 10 && y > 5), or even a method call that returns a boolean value.
• Always add (;) after the (condition).

do while loop Flowchart

Explanation

1. Start: The program starts, and the do-while loop is ready to run.
2. Execute Loop Body: The code inside the do { ... } block is executed first. This is the main action of the loop, and it runs at least once, no matter what.
3. Check Condition: After running the loop body, the condition is checked (while (condition);). This is the decision point:
   • If the condition is true, the loop goes back and runs the code inside the loop body again.
   • If the condition is false, the loop stops, and the program moves to the next part of the code.
4. Loop Again or Exit: The loop repeats the cycle of running the body and checking the condition until the condition becomes false.
5. End: Once the condition is false, the loop ends, and the program continues with the rest of the code.

Program using do while loop in Java

public class SimpleDoWhileLoop {
    public static void main(String[] args) {
        int counter = 1; // Initialize a counter variable

        // Do-while loop to print fruit names
        do {
            if (counter == 1) {
                System.out.println("Fruit: Apple");
            } else if (counter == 2) {
                System.out.println("Fruit: Banana");
            } else if (counter == 3) {
                System.out.println("Fruit: Cherry");
            } else if (counter == 4) {
                System.out.println("Fruit: Date");
            } else if (counter == 5) {
                System.out.println("Fruit: Elderberry");
            }
            counter++; // Increment the counter by 1
        } while (counter <= 5); // Check if the counter is less than or equal to 5
    }
}

public class SimpleDoWhileLoop {
    public static void main(String[] args) {
        int counter = 1; // Initialize a counter variable

        // Do-while loop to print fruit names
        do {
            if (counter == 1) {
                System.out.println("Fruit: Apple");
            } else if (counter == 2) {
                System.out.println("Fruit: Banana");
            } else if (counter == 3) {
                System.out.println("Fruit: Cherry");
            } else if (counter == 4) {
                System.out.println("Fruit: Date");
            } else if (counter == 5) {
                System.out.println("Fruit: Elderberry");
            }
            counter++; // Increment the counter by 1
        } while (counter <= 5); // Check if the counter is less than or equal to 5
    }
}

Explanation

1. Initialize a Counter Variable: The program starts by setting the counter variable to 1. This variable is used to control the loop and determine which fruit name to print.
2. Do-While Loop:
3. do { ... } Block: The code inside this block executes first. Based on the value of the counter, it checks which fruit to print using a series of if-else statements.
4. Print Statements: Depending on the value of counter, different fruit names are printed:
   • When counter is 1, it prints “Apple.”
   • When counter is 2, it prints “Banana.”
   • When counter is 3, it prints “Cherry.”
   • When counter is 4, it prints “Date.”
   • When counter is 5, it prints “Elderberry.”
5. Increment: counter++ increases the value of counter by 1 after printing each fruit.
6. Condition Check: After the code inside the do { ... } block runs, the condition while (counter <= 5); is checked.
7. If counter is less than or equal to 5, the loop runs again.
8. If counter is greater than 5, the loop stops.

Output

Fruit: Apple
Fruit: Banana
Fruit: Cherry
Fruit: Date
Fruit: Elderberry

Java Program Using do-while Loop with False Condition

public class SimpleDoWhileLoopFalse {
    public static void main(String[] args) {
        int counter = 6; // Initialize the counter variable to a value greater than 5

        // Do-while loop with a false condition
        do {
            System.out.println("Fruit: Apple"); // This statement will execute at least once
            counter++; // Increment the counter by 1
        } while (counter <= 5); // Condition is false (counter starts at 6)
    }
}

public class SimpleDoWhileLoopFalse {
    public static void main(String[] args) {
        int counter = 6; // Initialize the counter variable to a value greater than 5

        // Do-while loop with a false condition
        do {
            System.out.println("Fruit: Apple"); // This statement will execute at least once
            counter++; // Increment the counter by 1
        } while (counter <= 5); // Condition is false (counter starts at 6)
    }
}

Explanation

1. Initialize a Counter Variable: The program sets the counter variable to 6, which is greater than 5. This means the condition in the while statement (counter <= 5) is false from the start.
2. Do-While Loop:
   • do { ... } Block: Despite the condition being false, the code inside this block will execute at least once because that’s how the do-while loop works.
   • Print Statement: System.out.println("Fruit: Apple"); prints “Fruit: Apple” to the console.
   • Increment: counter++ increases the value of counter by 1, making it 7 after the first iteration.
3. Condition Check: The condition while (counter <= 5); is evaluated after executing the loop body:
   • Since counter is 6 initially, the condition counter <= 5 is false.
   • However, the print statement inside the loop executes once before the condition is checked.

Fruit: Apple

Conclusion

The do-while loop in Java is unique because it guarantees the loop body executes at least once before the condition is checked. This is useful for scenarios where you need to perform an action initially, regardless of the condition. By understanding this loop, you can better control the flow of your programs and ensure that critical operations are performed even if the condition is not met from the start.