A while loop in Java repeatedly executes a block of code as long as a specified condition is true. It is useful when the number of repetitions is not known in advance.
Suppose you want to print numbers from 1 to 5. You can use a while loop instead of writing five separate print statements.
int i = 1;
while (i <= 5) {
System.out.println(i);
i++;
}Output:
1
2
3
4
5i <= 5 is true.
When i becomes 6, the loop stops.
while (condition) {
// Code to execute repeatedly
}A while loop has two important parts:
Example:
int number = 1;
while (number <= 3) {
System.out.println("Hello Java");
number++;
}This program prints "Hello Java" three times.
Consider this example:
int i = 1;
while (i <= 3) {
System.out.println(i);
i++;
}| Iteration | Value of i | Condition | Action |
|---|---|---|---|
| 1 | 1 | 1 <= 3 (true) | Print 1 |
| 2 | 2 | 2 <= 3 (true) | Print 2 |
| 3 | 3 | 3 <= 3 (true) | Print 3 |
| 4 | 4 | 4 <= 3 (false) | Stop the loop |
public class Main {
public static void main(String[] args) {
int i = 1;
while (i <= 10) {
System.out.println(i);
i++;
}
}
}Output:
1
2
3
4
5
6
7
8
9
10Use the decrement operator -- to count backward.
int i = 10;
while (i >= 1) {
System.out.println(i);
i--;
}Output:
10
9
8
7
6
5
4
3
2
1The variable starts at 10 and decreases by 1 after each iteration.
Print even numbers from 2 to 20.
int i = 2;
while (i <= 20) {
System.out.println(i);
i += 2;
}Output:
2
4
6
8
10
12
14
16
18
20The statement i += 2 increases i by 2.
Calculate the sum of numbers from 1 to 10.
public class Main {
public static void main(String[] args) {
int i = 1;
int sum = 0;
while (i <= 10) {
sum = sum + i;
i++;
}
System.out.println("Sum = " + sum);
}
}Output:
Sum = 55
The variable sum stores the running total,
while i controls the loop.
Print the multiplication table of 5.
public class Main {
public static void main(String[] args) {
int number = 5;
int i = 1;
while (i <= 10) {
System.out.println(number + " x " + i
+ " = " + (number * i));
i++;
}
}
}Output:
5 x 1 = 5
5 x 2 = 10
5 x 3 = 15
5 x 4 = 20
5 x 5 = 25
5 x 6 = 30
5 x 7 = 35
5 x 8 = 40
5 x 9 = 45
5 x 10 = 50Use a while loop to print all elements of an array.
public class Main {
public static void main(String[] args) {
String[] names = {"Amit", "Rahul", "Priya", "Neha"};
int i = 0;
while (i < names.length) {
System.out.println(names[i]);
i++;
}
}
}Output:
Amit
Rahul
Priya
Nehai < names.length
to avoid accessing an index outside the array.
The following program asks the user for a number and prints numbers from 1 up to that number.
import java.util.Scanner;
public class Main {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
System.out.print("Enter a positive number: ");
int limit = sc.nextInt();
int i = 1;
while (i <= limit) {
System.out.println(i);
i++;
}
sc.close();
}
}Example input: 5
Output:
1
2
3
4
5If the user enters zero or a negative number, the loop does not execute.
An infinite loop occurs when the condition remains true and the loop has no effective way to stop.
int i = 1;
while (i <= 5) {
System.out.println(i);
// i is never increased
}
The value of i remains 1, so the condition
i <= 5 remains true forever.
Correct version:
int i = 1;
while (i <= 5) {
System.out.println(i);
i++;
}| For Loop | While Loop |
|---|---|
| Initialization, condition, and update appear in the loop header. | Initialization and update are usually written separately. |
| Useful when the number of repetitions is known. | Useful when repetition depends on a condition. |
for (int i=1; i<=5; i++) |
while (i<=5) |
Both loops can perform many of the same tasks.
For loop:
for (int i = 1; i <= 5; i++) {
System.out.println(i);
}Equivalent while loop:
int i = 1;
while (i <= 5) {
System.out.println(i);
i++;
}Write Java programs to solve these beginner-level assignments.
1. When does a while loop execute?
2. What is the purpose of i++ in a loop?
3. What happens if the while condition is false at the beginning?