Java While Loop
In Java, loops allow you to execute a block of code repeatedly while a
specified condition remains true. They are essential for automating
repetitive tasks, processing collections, and building efficient
programs.
Among Java’s loop constructs, the while loop is ideal when the number of
iterations is not known in advance and depends on a runtime
condition.
What Is a While Loop in Java?
The while loop repeatedly executes a block of code as long as its
condition evaluates to true.
Syntax
while (condition) {
// code to execute repeatedly
}
Key idea:
- Condition checked before each iteration
- Loop stops when condition becomes false
Example 1: Printing Even Numbers
This program prints all even numbers from 2 to 10 using a while
loop.
public class EvenNumberWhileLoop {
public static void main(String[] args) {
int number = 2;
while (number <= 10) {
System.out.println(number);
number += 2; //
increment by 2
}
}
}
Output:
2
4
6
8
10
Important: Update the Loop Variable
A while loop must modify the variable used in its condition.
Otherwise, the condition never changes and the loop runs forever
(infinite loop).
Example of an infinite loop (avoid this):
int x = 1;
while (x <= 5) {
System.out.println(x);
// missing x++
}
Example 2: Reverse Countdown Timer
This program counts down from 5 to 1 and then displays a
message.
public class CountdownWhileLoop {
public static void main(String[] args) {
int seconds = 5;
while (seconds > 0) {
System.out.println("T-minus " + seconds);
seconds--;
}
System.out.println("Launch!");
}
}
Output:
T-minus 5
T-minus 4
T-minus 3
T-minus 2
T-minus 1
Launch!
While Loop with Initially False Condition
If the condition is false the first time it’s checked, the loop
body will not execute even once.
Example 3: Condition False at Start
public class WhileFalseConditionDemo {
public static void main(String[] args) {
int temperature = 35;
while (temperature < 30)
{
System.out.println("Cooling system active");
temperature--;
}
System.out.println("Temperature check
complete");
}
}
Output:
Temperature check complete
Why “i” Is Common in Loops
You’ll often see loop variables named i, j, or k. This convention
comes from mathematics and stands for index or iterator. Short names
are practical in simple counting loops, though descriptive names are
better in real applications.
When to Use a While Loop
Use a while loop when:
- The number of iterations is unknown
- Loop depends on a changing condition
- Waiting for user input or events
- Processing data until a limit is reached
Examples:
- Reading file data until EOF
- Game loops
Java do-while Loop
The do-while loop in Java is a variation of the while loop that
guarantees the loop body executes at least once, regardless of
whether the condition is initially true or false. This makes it
especially useful for scenarios such as menu-driven programs,
input validation, and retry logic.
What Is a do-while Loop in Java?
A do-while loop executes its code block first and then
evaluates the condition. If the condition
remains true, the loop continues repeating.
Syntax
do {
// code to execute
} while (condition);
Important:
The semicolon (;) after the while condition is required in Java
syntax.
Example 1: Menu Display
This program displays a simple menu at least once and repeats
until the user chooses to exit.
import java.util.Scanner;
public class MenuDoWhileExample {
public static void main(String[] args)
{
Scanner scanner = new
Scanner(System.in);
int choice;
do {
System.out.println("=== MAIN MENU ===");
System.out.println("1. View Profile");
System.out.println("2. Settings");
System.out.println("3. Exit");
System.out.print("Enter choice: ");
choice =
scanner.nextInt();
if (choice == 1)
{
System.out.println("Opening profile...");
} else if (choice ==
2) {
System.out.println("Opening settings...");
}
} while (choice !=
3);
System.out.println("Application
closed.");
scanner.close();
}
}
Why do-while here?
The menu must appear at least once before checking whether the
user selected Exit.
Example 2: Input Validation Loop
This program repeatedly asks for a positive number until the
user enters a valid value.
import java.util.Scanner;
public class PositiveNumberValidator {
public static void main(String[] args)
{
Scanner input = new
Scanner(System.in);
int number;
do {
System.out.print("Enter a positive number: ");
number =
input.nextInt();
} while (number <=
0);
System.out.println("Valid number
entered: " + number);
input.close();
}
}
do-while with Initially False Condition
Even if the condition is false at the start, the loop body
still executes once.
Example 3: Single Execution Demo
public class DoWhileSingleRun {
public static void main(String[] args)
{
int attempts = 5;
do {
System.out.println("Attempt recorded: " + attempts);
attempts++;
} while (attempts <
3);
System.out.println("Loop
finished.");
}
}
Output:
Attempt recorded: 5
Loop finished.
The condition attempts < 3 is false initially, but the loop
runs once before evaluation.
When to Use a do-while Loop
Use do-while when an action must occur at least once, such
as:
- Displaying menus
- Reading user input
- Confirmation prompts
- Retry operations
- Game loops (first frame must render)
