Java While Loop Method Example
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. Here, you are going to learn about while
and do...while
loops.
Java while loop
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,
- A
while
loop evaluates the textExpression inside the parenthesis()
. - If the textExpression evaluates to
true
, the code inside thewhile
loop is executed. - The textExpression is evaluated again.
- This process continues until the textExpression is
false
. - When the textExpression evaluates to
false
, the loop stops.
To learn more about the conditions, visit Java relational and logical operators.
Flowchart of while loop
![Flowchart of while loop in Java Flowchart of while loop in Java](https://cdn.programiz.com/sites/tutorial2program/files/java-while-loop.png)
Example 1: Display Numbers from 1 to 5
// Program to display numbers from 1 to 5 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++; } } }
Output
1 2 3 4 5
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 |
Example 2: Sum of Positive Numbers Only
// Java program to find the sum of positive numbers 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(); } }
Output
Enter a number 25 Enter a number 9 Enter a number 5 Enter a number -3 Sum = 39
In the above program, we have used the Scanner class 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.
Java do...while loop
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,
- The body of the loop is executed at first. Then the textExpression is evaluated.
- If the textExpression evaluates to
true
, the body of the loop inside thedo
statement is executed again. - The textExpression is evaluated once again.
- If the textExpression evaluates to
true
, the body of the loop inside thedo
statement is executed again. - This process continues until the textExpression evaluates to
false
. Then the loop stops.
Flowchart of do...while loop
![Flowchart of do...while loop in Java Flowchart of do...while loop in Java](https://cdn.programiz.com/sites/tutorial2program/files/java-do-while-loop.png)
Let's see the working of do...while
loop.
Example 3: Display Numbers from 1 to 5
// Java Program to display numbers from 1 to 5 import java.util.Scanner; // Program to find the sum of natural numbers from 1 to 100. 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); } }
Output
1 2 3 4 5
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 | 6 is printed. i is increased to 6. |
5th | i = 6 n = 5 | false | The loop is terminated |
Example 4: Sum of Positive Numbers
// Java program to find the sum of positive numbers 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(); } }
Output 1
Enter a number 25 Enter a number 9 Enter a number 5 Enter a number -3 Sum = 39
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.
Output 2
Enter a number -8 Sum is 0
Here, the user enters a negative number. The test condition will be false
but the code inside of the loop executes once.
Infinite while loop
If the condition of a loop is always true
, the loop runs for infinite times (until the memory is full). For example,
// infinite while loop while(true){ // body of loop }
Here is an example of an infinite do...while
loop.
// 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.
for and while loops
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 }
Source: https://www.programiz.com/java-programming/do-while-loop
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