Javawhile Loop

Java while Loop

What is a while Loop?

A while loop repeats a block of code as long as a condition stays true. Unlike a for loop, it doesn't have a built-in initialization or update section — you set up the counter (if you need one) before the loop, and update it yourself inside the loop body. This makes while loops the natural choice when you don't know in advance exactly how many times the loop will run.

Basic Syntax

Java
while (condition) {
    // code to repeat
}

Java checks the condition BEFORE every iteration, including the very first one. If the condition is false right from the start, the loop body never runs at all.

A Simple Example

Java
int i = 1;
while (i <= 5) {
    System.out.println("Count: " + i);
    i++;
}
Count: 1
Count: 2
Count: 3
Count: 4
Count: 5
Warning
Always make sure something inside the loop body eventually makes the condition false. If you forget to update i (or update it in a way that never satisfies the condition), you get an infinite loop that never stops on its own.
Condition Checked First: Zero Runs is Possible

Java
int count = 10;
while (count < 5) {
    System.out.println("This never prints.");
    count++;
}
System.out.println("Loop skipped entirely.");
Loop skipped entirely.
Note
Because count starts at 10 and the condition is count < 5, the condition is false on the very first check — the body runs zero times. This is the key behavioral difference from do-while, covered on the next page.
Common Pattern: Sentinel-Controlled Loop

A sentinel value is a special value that signals "stop" instead of being real data. This pattern is common when reading a stream of input until a special marker shows up.

Java
import java.util.Scanner;

Scanner scanner = new Scanner(System.in);
int total = 0;
int number = scanner.nextInt();

while (number != -1) {
    total += number;
    number = scanner.nextInt();
}
System.out.println("Total: " + total);
Note
Here, -1 is the sentinel — it isn't added to the total, it just tells the loop when to stop. Notice the number is read once before the loop AND again at the end of the loop body; this "read ahead" is a very common while-loop idiom.
Common Pattern: Input Validation

while loops are also the standard tool for re-prompting a user until they provide valid input.

Java
import java.util.Scanner;

Scanner scanner = new Scanner(System.in);
int age = -1;

while (age < 0 || age > 120) {
    System.out.print("Enter a valid age (0-120): ");
    age = scanner.nextInt();
}
System.out.println("Age accepted: " + age);
while vs for

Situation

Better Choice

You know the exact number of iterations in advance

for

You're looping until a condition becomes true/false (unknown count)

while

You're validating input or waiting for a sentinel value

while

You need a counter, start, and update all visible in one line

for

Practice Exercises

  1. Use a while loop to print numbers from 10 down to 1.

  2. Write a while loop that keeps asking the user for a password until they enter the correct one.

  3. Sum numbers entered by a user until they type 0.

  4. Use a while loop to find the first power of 2 greater than 1000.