While Loop

A school program needs reliable data and clear rules. A while loop tests its condition before repeating the body. It can perform no iterations when the initial condition is already false.

In Java, a while loop is used to repeat a block of code as long as a given condition remains true. It offers flexibility and control, especially when the number of iterations isn't known beforehand.

while (condition) {
    // code block to execute
}

The loop checks the condition before every iteration. If the condition evaluates to true, the code block executes. Once the condition becomes false, the loop stops running.

public class WhileLoopExample {
    public static void main(String[] args) {
        int i = 1;
        while (i <= 5) {
            System.out.println("Count: " + i);
            i++;
        }
    }
}

Count: 1
Count: 2
Count: 3
Count: 4
Count: 5

Java · example discussed here

public class WhileLoopExample {
    public static void main(String[] args) {
        int i = 1;
        while (i <= 5) {
            System.out.println("Count: " + i);
            i++;
        }
    }
}

Here, the loop starts with i = 1. The condition i <= 5 is checked before each iteration. As long as this is true, it prints the current value of i and increments it. When i becomes 6, the loop terminates.

If the condition never turns false, the loop keeps running indefinitely. This is called an infinite loop.

while (true) {
    System.out.println("This loop runs forever!");
}

Be cautious with infinite loops. They're useful in some scenarios like real-time systems or game loops, but can also crash your program if misused.

public class ArrayWhileLoop {
    public static void main(String[] args) {
        int[] nums = {10, 20, 30, 40, 50};
        int index = 0;
        
        while (index < nums.length) {
            System.out.println("Element at index " + index + ": " + nums[index]);
            index++;
        }
    }
}

Element at index 0: 10
Element at index 1: 20
Element at index 2: 30
Element at index 3: 40
Element at index 4: 50

The while loop is ideal when waiting for a correct user input:

import java.util.Scanner;

public class InputValidation {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String input = "";

        while (!input.equals("yes")) {
            System.out.print("Type 'yes' to continue: ");
            input = scanner.nextLine();
        }

        System.out.println("Great! Moving forward...");
    }
}

Type 'yes' to continue: no
Type 'yes' to continue: maybe
Type 'yes' to continue: yes
Great! Moving forward...

• When the number of iterations is unknown
• When looping depends on dynamic conditions (user input, system state)
• To wait or retry until a condition is met

• Forgetting to update the condition inside the loop (may cause infinite loop)
• Using = instead of == for comparison
• Modifying the loop variable in an unexpected way

The update must be reached. A continue placed before the update can accidentally create an endless loop.

Before running this Java program, write down each line you expect to see. Identify the value, branch, object or exception responsible for each line. Then explain whether the following assumption is valid: The update must be reached.

Java 17 · Main.java · complete practice program

public class Main  {
  public static void main(String[] args)  {
    int roll = 1;
    while (roll <= 3)  {
      System.out.println(roll);
      roll++;
    }
  }
}

Now change one thing: Start roll at 4. Predict the new behavior before checking the worked variation.

Java 17 · Main.java · source repeated for this answer

public class Main  {
  public static void main(String[] args)  {
    int roll = 1;
    while (roll <= 3)  {
      System.out.println(roll);
      roll++;
    }
  }
}

Expected output

1
2
3

A while loop tests its condition before repeating the body. It can perform no iterations when the initial condition is already false.

The update must be reached. A continue placed before the update can accidentally create an endless loop.

Run with javac Main.java, then java Main. The declaration of Main names the class; main starts the example, and println writes a complete output line.

Worked variation: Start roll at 4. The condition is false and no roll numbers print.

A while loop tests its condition before repeating the body. It can perform no iterations when the initial condition is already false. The update must be reached. A continue placed before the update can accidentally create an endless loop.

Related explanations: Java Variables, Java If Statement - Syntax and Examples, Java Class - Blueprint of Objects, Exceptions in Java Introduction.

Supporting documentation: Official Java learning guide and Java 17 language specification.