Java while & do...while loop

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.

In the previous tutorial, you learned about Java for loop. Here, you are going to learn about while and do...while loops.

1. 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 (condition) {
  // body of loop
}

Here,

1. A while loop evaluates the condition inside the parenthesis ().
2. If the condition evaluates to true, the code inside the while loop is executed.
3. The condition is evaluated again.
4. This process continues until the condition is false.
5. When the condition evaluates to false, the loop stops.

Example 1: Display Numbers from 1 to 5

// Program to display numbers from 1 to 5
class Main {
  public static void main(String[] args) {
    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

2. 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.

do {
    // body of loop
} while(condition);

Example 2: Input Validation

The do...while loop is perfect for asking user input until they give a valid response.

import java.util.Scanner;

class Main {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        int number;

        do {
            System.out.println("Enter a positive number: ");
            number = input.nextInt();
        } while (number <= 0);

        System.out.println("You entered: " + number);
    }
}

Here, the loop will keep asking for a number at least once, and repeat if the user enters a negative number or zero.

3. Infinite Loops

If the condition of a loop is always true, the loop runs for infinite times.

// infinite while loop
while(true){
    // body of loop
}

Key Takeaways

• While: Checks condition before running. Might not run at all.
• Do-While: Checks condition after running. Always runs at least once.
• Use Case: Best when you don't know the number of iterations (e.g., reading user input until they type "exit").

Common Pitfalls

[!WARNING] Infinite Loops: It's very easy to forget to update your counter (e.g., i++) inside a while loop, causing it to run forever.

[!WARNING] Semicolon: The do...while loop MUST end with a semicolon ; after the condition. do { } while(condition);

Challenge