Java Do While Loop With Examples

iterations do not have a fixed value, it is recommended to use a do-while loop as it is guaranteed to be executed once. This is possible because the condition is checked post the body of the loop is executed. That is why it is an Exit Control Loop. 

Thus, do while Java is a variant of while loop that runs the code block before the condition is evaluated as true and then repeat it as long as the condition is true, just like while loop. 

The do while loop in java is identical to the while loop. Both of them execute a given set of statements. But the difference is that the do while loop runs entirely although the condition is not fulfilled. It will execute until the mentioned condition is true and will exit as soon as the condition is not met.

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If the condition is true, the control reaches the start of the loop. But if the condition is false, the control will break out of the loop. It implies that the statements within the loop are executed before the condition is checked. Hence, the do while loop must be employed in all cases where the loop body has to be executed at least once.  The reason is that the condition is checked after executing the loop body. 

Typically, in a menu-driven program, the actions are supposed to be taken iteratively depending on the user input. In such programs, a do while loop helps understand the action the user wants to implement. In those cases, the control breaks out of the loop if the user input equals the exit command. You can better understand it’s working with a do while loop example java.

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The do while loop in java is known as an exit control loop. Thus, unlike “for loop” and “while loop”, the “do-while loop” tests the condition at the loop body’s termination. The do while loop program in java comes in handy when you want to execute a block of statements recurrently.

Do While Java Syntax

do{  

//body of the code  

}while(condition);  

Here the condition is a Boolean expression that appears at the end of the loop. If the expressions are evaluated to true, the control jumps back to the do statement, and the loop is executed again. The process repeats till the Boolean expression is evaluated as false. 

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Components of do-while Loop: 

If you want to test a sample do while loop program in java, you should first understand its components. Its two components are Test Expression and Update Expression:

 Test Expression: You need to test the condition in this expression.  If that condition is true, the body of the loop is executed, and the program flow goes to update expression. Else, it will exit from the while loop. You need to use a Test Expression when you start learning the execution of do while loop example java, for example, i <= 5.

Update Expression: After the loop body executes, the Update Expression increments/decrements the loop variable by a certain value. For example, you can use  i– to decrement the i’s value in a do while program in java.

Example

public class Example {  

public static void main(String[] args) {  

    int i=1;  

do{  

        System.out.println(i);  

    i++;  

    }while(i<=5);  

}  

}  

Explanation: The given loop runs six times. Hence, the value of i is 6, but it is not printed as the condition evaluates to false. 

How Does a Do-While Loop Execute?

1. The control falls into the do while the Java loop as ‘do’ is encountered.
2. The statements in the body of the loop (code) are executed.
3. The variable is updated.
4. The flow now comes to 
5. the condition.
6. If it is true, then step 6 is executed; otherwise, the flow goes out of the loop.
7. Flow moves back to step 2

Applications

Do While Java Infinite

An infinite loop is created when the Boolean expression is passed as true in the do-while java loop. 

Here is a do-while Java infinite loop example.

public class Example {

public static void main(String[] args)  {

do {

System.out.println(“Start Processing inside do while loop”);

// Any other statements can be added

System.out.println(“End Processing of do while loop”);

Thread.sleep(2 * 1000);

} while (true);

}

}

Explanation: The statements keep on being executed till the program is terminated using the IDE. 

Sum of Natural Numbers up to a Given Number

public class Example { 

public static void main(String args[]) 

{ 

int x = 7, sum = 0;

do { // The line will be printed even 

// if the condition is false 

sum += x; 

x–;

} while (x > 0); 

System.out.println(“Sum: ” + sum); 

} 

}

Iteration of Array using Do While Java Loop.

Here’s an example of iteration of an integer array using do-while loop in Java:

class Example{

    public static void main(String args[]){

         int arr[]={0,1,45,9};

         int i=0;

do{

              System.out.println(arr[i]);

              i++;

         }while(i<4);

    }

}

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Nesting of Do-While Loops

It is possible to have a do-while in a do-while Java loop. This is known as nesting of do-while construction. There is no upper bound to the depth of nesting. A do-while can have any construct like if, while, switch, etc., inside it. For example:

class Example{

        public static void main(String args[])

    {

         int a=10;

do   // 1st do while

{

System.out.println(10);

         do// 2nd do while

{ 

          System.out.println(20);

           }

          while(false); 

        }

         while(false); 

    }

}

Do While vs While Loop

• While loop is an entry control loop, whereas the do-while is an exit controlled loop
• Java while looks cleaner than do while Java loop. 
• The Do while loop executes at least once irrespective of the condition. 

Why use Do-While?

When you require your program to be executed a minimum once, use do-while. For example, you have to take input from the user until the user enters a negative number. In such a case, a do-while loop is used as the initial input can be positive or negative, but we require input. In all other cases, it is easier to use a while loop. 

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An Application of the do-while loop in Java:

This example menu helps you to show some menus to the users. Suppose you are developing a game program for demonstrating some options to the user. For example, press ‘1’ for running, press ‘2’ for stopping, and press ‘Q’ to quit the game. In this case, you want to show your game menu to users at least once. Hence, you can write this game code within the do-while loop in Java.

Things to Remember

1. The body of the do-while loop is required to have a minimum of one statement 
2. The condition (boolean expression) at the end of the loop must always result in a Boolean value.
3. Without the condition, the loop cannot be executed.
4. An error would be raised if a do statement does not correspond with a while statement. 
5. The do while program in java needs an initial statement. Using this initial statement, the loop begins execution before repeating.
6. Repeating ensures all the statements in the loop are executed in sequence.
7. There must be a statement that would cause repetition after the first pass or any pass.
8. If you want the loop to stop repeating, there must be a condition that will avoid repeating.

Conclusion

The do-while Java loop is used for iterating a set of statements until a given condition is met. In this blog, you learned about the loop, its syntax, uses, nesting, and comparison with the while loop. 

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