Understanding Loops in Java

What Is a Loop?

A loop is just a way to repeat code. An iteration.

In Java, loops keep running as long as a condition is true.

Example: imagine you're a teacher with 25 students. You wouldn't want to type "call on student" 25 different times. With a loop, you write it once and tell the computer to repeat it 25 times. Way easier.

Types of Loops in Java

Java has three main loops.

For Loops

Use a for loop when you know how many times something should run.

for (int i = 1; i <= 5; i++) {
    System.out.println("Number: " + i);
}

This starts at 1, prints the number, adds 1 each time, and stops once it hits 5.

While Loops

A while loop keeps running as long as the condition is true. This is good when you don't know how many times the loop will run.

int attempts = 0;
while (attempts < 3) {
    System.out.println("Attempt number: " + attempts);
    attempts++;
}

Do-While Loops

A do-while loop always runs at least once, no matter what.

int number = 0;
do {
    System.out.println("This runs at least once!");
    number++;
} while (number < 0);

Even though the condition is false, the code still runs once.

Where Loops Are Used

Loops are everywhere:

• Games: movement, physics, and player input
• Apps: refreshing feeds and checking notifications
• Data: going through tons of information
• Servers: staying on 24/7 waiting for users

Common Mistakes

Infinite loops: forgetting to update the variable

int i = 0;
while (i < 10) {
    System.out.println(i);
}

• Off-by-one errors: starting or ending at the wrong number
• Using the wrong loop: sometimes a while loop is just clearer than a for loop

Mini Project: Number Guessing Game

A number guessing game is a simple way to use loops. The program picks a random number, and you keep guessing until you get it right or run out of tries.

It uses:

• While loops to keep the game running
• For loops to count attempts
• Do-while loops to make sure input actually makes sense

import java.util.Scanner;
import java.util.Random;

public class NumberGuessingGame {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        Random random = new Random();
        boolean playAgain = true;

        System.out.println(" WELCOME TO THE NUMBER GUESSING GAME ");
        System.out.println("Learn Loops While Having Fun!");
        System.out.println();

        do {
            playGame(scanner, random);

            String response;
            do {
                System.out.print("\nWould you like to play again? (yes/no): ");
                response = scanner.nextLine().trim().toLowerCase();

                if (!response.equals("yes") && !response.equals("no")) {
                    System.out.println("Please enter 'yes' or 'no'.");
                }
            } while (!response.equals("yes") && !response.equals("no"));

            playAgain = response.equals("yes");

            if (playAgain) {
                System.out.println("\n" + "=".repeat(40) + "\n");
            }

        } while (playAgain);

        System.out.println("Thanks for playing! Goodbye!");
        scanner.close();
    }

    public static void playGame(Scanner scanner, Random random) {
        int secretNumber = random.nextInt(100) + 1;
        int maxAttempts = 7;
        int attempts = 0;
        boolean hasGuessed = false;

        System.out.println("I'm thinking of a number between 1 and 100");
        System.out.println("You have " + maxAttempts + " attempts to guess it!");
        System.out.println();

        while (attempts < maxAttempts && !hasGuessed) {
            attempts++;

            System.out.print("Attempts remaining: ");
            for (int i = 0; i < (maxAttempts - attempts + 1); i++) {
                System.out.print(" ");
            }
            System.out.println("(" + (maxAttempts - attempts + 1) + " left)");

            int guess = getValidGuess(scanner, attempts);

            if (guess == secretNumber) {
                hasGuessed = true;
                displayVictory(attempts, maxAttempts);
            } else if (guess < secretNumber) {
                System.out.println("Too low! Try a higher number.");
                giveHint(secretNumber, guess);
            } else {
                System.out.println("Too high! Try a lower number.");
                giveHint(secretNumber, guess);
            }

            System.out.println();
        }

        if (!hasGuessed) {
            System.out.println("You've run out of attempts!");
            System.out.println("The number was:" + secretNumber);
        }
    }

    public static int getValidGuess(Scanner scanner, int attemptNumber) {
        int guess = -1;

        do {
            System.out.print("Attempt #" + attemptNumber + " - Enter your guess: ");

            if (scanner.hasNextInt()) {
                guess = scanner.nextInt();
                scanner.nextLine();

                if (guess < 1 || guess > 100) {
                    System.out.println("Enter a number between 1 and 100!");
                    guess = -1;
                }
            } else {
                System.out.println("That's not a valid number! Try again.");
                scanner.nextLine();
            }
        } while (guess == -1);

        return guess;
    }

    public static void giveHint(int secretNumber, int guess) {
        int difference = Math.abs(secretNumber - guess);

        if (difference <= 5) {
            System.out.println("You're very close! Almost there!");
        } else if (difference <= 15) {
            System.out.println("You're getting warm!");
        } else if (difference <= 30) {
            System.out.println("You're cold!");
        } else {
            System.out.println(" You're freezing! Way off!");
        }
    }

    public static void displayVictory(int attempts, int maxAttempts) {
        System.out.println("CONGRATULATIONS!");
        System.out.println("You guessed the number!");
        System.out.println();

        System.out.print("Your Rating: ");

        if (attempts <= 3) {
            System.out.print("AMAZING! ");
        } else if (attempts <= 5) {
            System.out.print("GREAT! ");
        } else if (attempts <= 6) {
            System.out.print("GOOD! ");
        } else {
            System.out.print("Nice! ");
        }

        System.out.println("(" + attempts + "/" + maxAttempts + " attempts)");
    }
}