title: Java while & do...while loop

description: In this tutorial, we will learn how to use while and do while loop in Java with the help of examples.

In computer programming, loops are used to repeat a block of code. For example, if you want to show a message 100 times, then you can use a loop. It's just a simple example; you can achieve much more with loops.

In the previous tutorial, you learned about [Java for loop](./for-loop). Here, you are going to learn about `while` and `do...while` loops.

import { List, ListItemGood } from '@/components/List'

import { TipInfo } from '@/components/Tip'

Java while loop is used to run a specific code until a certain condition is met. The syntax of the while loop is:

while (testExpression) {

// body of loop

}

Here,

1. A `while` loop evaluates the **textExpression** inside the parenthesis `()`.

2. If the **textExpression** evaluates to `true`, the code inside the `while` loop is executed.

3. The **textExpression** is evaluated again.

4. This process continues until the **textExpression** is `false`.

5. When the **textExpression** evaluates to `false`, the loop stops.

To learn more about the conditions, visit [Java relational](./operators#3-java-relational-operators) and [logical operators](./operators#4-java-logical-operators).


class Main {

public static void main(String[] args) {

// declare variables

int i = 1, n = 5;

// while loop from 1 to 5

while(i <= n) {

System.out.println(i);

i++;

}

}

}

Here is how this program works.

| Iteration | Variable | Condition: i <= n | Action |

|-----------|-------------------|-------------------|--------------------------------------------|

| 1st | `i = 1` `n = 5` | `true` | `1` is printed. `i` is increased to **2**. |

| 2nd | `i = 2` `n = 5 ` | `true` | `2` is printed. `i` is increased to **3**. |

| 3rd | `i = 3` ` n = 5 ` | `true` | `3` is printed.`i` is increased to **4**. |

| 4th | `i = 4` ` n = 5 ` | `true` | `4` is printed. `i` is increased to **5**. |

| 5th | `i = 5` ` n = 5` | `true` | `5` is printed. `i` is increased to **6**. |

| 6th | `i = 6` `n = 5` | `false` | The loop is terminated |

import java.util.Scanner;

class Main {

public static void main(String[] args) {

int sum = 0;

// create an object of Scanner class

Scanner input = new Scanner(System.in);

// take integer input from the user

System.out.println("Enter a number");

int number = input.nextInt();

// while loop continues

// until entered number is positive

while (number >= 0) {

// add only positive numbers

sum += number;

System.out.println("Enter a number");

number = input.nextInt();

}

System.out.println("Sum = " + sum);

input.close();

}

}

In the above program, we have used the [Scanner class](./scanner) to take input from the user. Here, `nextInt()` takes integer input from the user.

The `while` loop continues until the user enters a negative number. During each iteration, the number entered by the user is added to the `sum` variable.

When the user enters a negative number, the loop terminates. Finally, the total sum is displayed.

The `do...while` loop is similar to while loop. However, the body of `do...while` loop is executed once before the test expression is checked. For example,

do {

// body of loop

} while(textExpression);

Here,

1. The body of the loop is executed at first. Then the **textExpression** is evaluated.

2. If the **textExpression** evaluates to `true`, the body of the loop inside the `do` statement is executed again.

3. The **textExpression** is evaluated once again.

4. If the **textExpression** evaluates to `true`, the body of the loop inside the `do` statement is executed again.

5. This process continues until the **textExpression** evaluates to false. Then the loop stops.


Let's see the working of `do...while` loop.

import java.util.Scanner;

class Main {

public static void main(String[] args) {

int i = 1, n = 5;

// do...while loop from 1 to 5

do {

System.out.println(i);

i++;

} while(i <= n);

}

}

Here is how this program works.

| Iteration | Variable | Condition: i <= n | Action |

|-----------|-------------------|-------------------|--------------------------------------------|

| | `i = 1` `n = 5` | not checked | `1` is printed. `i` is increased to **2**. |

| 1st | `i = 2` `n = 5 ` | `true` | `2` is printed. `i` is increased to **3**. |

| 2nd | `i = 3` ` n = 5 ` | `true` | `3` is printed.`i` is increased to **4**. |

| 3rd | `i = 4` ` n = 5 ` | `true` | `4` is printed. `i` is increased to **5**. |

| 4th | `i = 5` ` n = 5` | `true` | `5` is printed. `i` is increased to **6**. |

| 5th | `i = 6` `n = 5` | `false` | The loop is terminated |

import java.util.Scanner;

class Main {

public static void main(String[] args) {

int sum = 0;

int number = 0;

// create an object of Scanner class

Scanner input = new Scanner(System.in);

// do...while loop continues

// until entered number is positive

do {

// add only positive numbers

sum += number;

System.out.println("Enter a number");

number = input.nextInt();

} while(number >= 0);

System.out.println("Sum = " + sum);

input.close();

}

}

Here, the user enters a positive number, that number is added to the `sum` variable. And this process continues until the number is negative. When the number is negative, the loop terminates and displays the sum without adding the negative number.

Here, the user enters a negative number. The test condition will be `false` but the code inside of the loop executes once.

If **the condition** of a loop is always `true`, the loop runs for infinite times (until the memory is full). For example,

while(true){

// body of loop

}

Here is an example of an infinite `do...while` loop.

int count = 1;

do {

// body of loop

} while(count == 1)

In the above programs, the **textExpression** is always `true`. Hence, the loop body will run for infinite times.

The `for` loop is used when the number of iterations is known. For example,

for (let i = 1; i <=5; ++i) {

// body of loop

}

And `while` and `do...while` loops are generally used when the number of iterations is unknown. For example,

while (condition) {

// body of loop

}