Lesson 11: Mastering while and do-while Loops in Java

Loops are one of the most useful tools in programming, allowing you to repeat actions multiple times. In this guide, we’ll explore two types of loops in Java: while and do-while. These loops are similar but have key differences that make them suitable for different tasks.

What Is a Loop, and Why Do We Need It?

A loop is a way to tell your program to repeat an action as long as a specific condition is met. Loops are handy in situations like:

• Asking a user to re-enter input until it’s valid.
• Processing large datasets.
• Performing tasks when the number of repetitions isn’t known beforehand.

Imagine this: you want your friend to pour water into a glass 50ml at a time until the glass is full. A loop is like giving instructions for this process.

The while Loop

The while loop checks the condition first, and only if it’s true, it performs the actions inside the loop. If the condition is false from the start, the loop won’t run at all.

Example: Filling a Glass of Water

int waterInGlass = 0; // Water in the glass (0 ml)
int fullGlass = 250;  // Full glass (250 ml)

while (waterInGlass < fullGlass) {
    waterInGlass += 50; // Add 50 ml of water
    System.out.println("The glass now contains " + waterInGlass + " ml of water.");
}

System.out.println("The glass is full!");

How It Works:

1. The loop keeps running as long as the amount of water in the glass is less than 250 ml (waterInGlass < fullGlass).
2. Each iteration adds 50 ml of water to the glass.
3. Once the glass is full, the loop stops.

The do-while Loop

The do-while loop is a bit different. It performs the actions first and then checks the condition. This means the loop will run at least once, even if the condition is false from the start.

Example: Printing a Message Once

int count = 6;

do {
    System.out.println("This message will be printed at least once.");
    count++;
} while (count < 5);

What Happens Here:

• The variable count starts at 6, so the condition count < 5 is false.
• However, because the do-while loop executes the actions before checking the condition, the message is printed once before the loop ends.

When to Use while and do-while Loops

• while: Use this loop when you want actions to run only if the condition is true.
• do-while: Use this loop when the actions should run at least once, regardless of the condition.

Handy Features for Loops

1. break — Exit the Loop Early:
   If you need to stop the loop before it naturally ends, use break.

int count = 0;

while (true) {
    System.out.println("Iteration " + count);
    count++;
    if (count == 5) {
        break; // Exit the loop when count reaches 5
    }
}

2. continue — Skip to the Next Iteration:
   Use continue to skip the rest of the code in the current loop iteration and jump to the next one.

int count = 0;

while (count < 10) {
    count++;
    if (count % 2 == 0) continue; // Skip even numbers
    System.out.println("Odd number: " + count);
}

Comparing while and do-while Loops

Real-Life Examples of Loops

1. Ask for User Input Until It’s Correct:

import java.util.Scanner;

Scanner scanner = new Scanner(System.in);
String correctPassword = "java123";
String userInput = "";

while (!userInput.equals(correctPassword)) {
    System.out.println("Enter the password:");
    userInput = scanner.nextLine();
}

System.out.println("Correct password! Access granted.");

2. Sum Numbers Until the User Quits:

import java.util.Scanner;

Scanner scanner = new Scanner(System.in);
int sum = 0;

do {
    System.out.println("Enter a number (or 'q' to quit):");
    String input = scanner.nextLine();
    if (input.equals("q")) break;
    sum += Integer.parseInt(input);
} while (true);

System.out.println("Total sum: " + sum);

Tips for Working with Loops

1. Avoid Infinite Loops:
   Always make sure the loop has a way to stop, or it will run forever.Example of an infinite loop:

while (true) {
    System.out.println("This is an infinite loop!");
}

2. Use break and continue Wisely:
   • These commands let you control the flow of the loop more precisely.

3. Pick the Right Loop:
   • Use while when the condition must be true for the actions to run.
   • Use do-while when you need the actions to run at least once.

Summing It Up

Loops like while and do-while make your programs more efficient by automating repetitive tasks.

• while is great for situations where you need to check the condition first.
• do-while works well when at least one execution of the actions is guaranteed.

Practice writing loops in Java to see how they can simplify your coding tasks. Happy coding!