09 Java Fundamentals

While Loop in Java

By Dummy for Dummies

Introduction

Remember being a kid, asking your parents for snacks, and getting refused? You didn't just walk away thinking "yeah, snacks aren't good for me, I should eat more vegetables." HELL NAH! You cried and begged for snacks until they gave up and handed them over.

You kept repeating that until you got what you wanted. You were basically stuck in a loop, and only one thing could break you out of it.

Programming works the same way. We make the program repeat the same block of code until it hits the target we set. That's a loop.

While Loop

Just as the name suggests, it will execute a part of code while a condition is true. When the condition is disturbed, the loop will stop. Let's simulate a crying baby:

import java.util.Scanner;

public class BabyCandy {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String pocket = "";

        while (!pocket.equals("candy")) {
            System.out.println("Baby is crying");
            System.out.println("Baby is asking for candy");
            System.out.print("Give something: ");
            pocket = sc.nextLine();
        }

        System.out.println("Baby got candy");
        System.out.println("Baby is happy!");
        sc.close();
    }
}

• pocket.equals("candy") checks if content in pocket is "candy" or not.
• String does not work on == like others. Therefore we use .equals()
• Here we used NOT operator i.e. !. So the condition actually checks if pocket DOES NOT contain candy, then run the loop.
• All content inside golden curly brackets {} will be repeated again and again until the user enters candy
• Kids want what they want. No matter what else you give the baby, the loop keeps running until it's candy
• Once you give candy, loop will break, and program will print: Baby got candy and Baby is happy!

Points to Remember

• Condition will be checked first, if it is true, the loop will start
• Code inside will run and will check condition again before restart
• If you change the condition, it'll stop. If you don't, it'll continue
• You must code a way to change condition inside the loop or it'll run infinitely

Infinite Loop Demonstration

public class Main {
    public static void main(String[] args) {
        int n = 0;

        while (n == 0) {
            System.out.println("HELP I AM STUCK IN LOOP!");
        }

        System.out.println("Loop exited");
    }
}

Do While Loop

Do while loop is a variant of the original while loop. There is just a small difference between them.

• While Loop: It first checks condition, if it is true then it runs. If condition is not true, it will never run the content of the loop.
• Do While Loop: It will execute the code inside the loop for once, then check condition. If it's true, it'll re-run the loop code. If it is not true, it won't re-run. But it will always run the loop code at least once.

public class Main {
    public static void main(String[] args) {
        int number = 1;

        do {
            System.out.println("Number: " + number);
            number++;  // increments the number
        } while (number < 6);  // semicolon required here!

        System.out.println("Loop exited!");
    }
}

• This code has a condition that says "While number is less than 6, go back and re-run the loop".
• When the code inside runs, it prints the number, then increases it by one, then checks the condition again.
• The first output will be given always, then every time it'll check condition and execute as it allows.

Exercise

Let's do an exercise for what we studied in this post. Study this code, break it down, analyze it and identify the concepts used here. It would be even better if you write down your observations and make a mini report on it.

import java.util.Scanner;

public class WhileLoopExercise {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String secretPin = "1234";
        String enteredPin = "";
        int attempts = 0;
        int maxAttempts = 3;
        int balance = 1000;

        // while loop to check PIN
        while (!enteredPin.equals(secretPin) && attempts < maxAttempts) {
            System.out.print("Enter your secret PIN: ");
            enteredPin = sc.nextLine();
            attempts++;

            if (!enteredPin.equals(secretPin)) {
                System.out.println("Wrong PIN! Attempts left: " + (maxAttempts - attempts) + "\n");
            }
        }

        // check why loop ended
        if (enteredPin.equals(secretPin)) {
            System.out.println("\nAccess Granted!");
            System.out.println("It took you " + attempts + " attempt(s).\n");

            String choice = "";
            while (!choice.equals("exit")) {
                System.out.println("Choose an option:");
                System.out.println("1. Check Balance");
                System.out.println("2. Withdraw Money");
                System.out.println("3. Exit");
                System.out.print("Your choice: ");
                choice = sc.nextLine();

                if (choice.equals("1")) {
                    System.out.println("Your balance is $" + balance + ".\n");
                } else if (choice.equals("2")) {
                    System.out.print("Enter withdrawal amount: ");
                    int amount = sc.nextInt();
                    sc.nextLine();
                    if (amount <= 0) {
                        System.out.println("Invalid amount.\n");
                    } else if (amount > balance) {
                        System.out.println("Not enough balance!\n");
                    } else {
                        balance -= amount;
                        System.out.println("Withdrawal successful. Remaining balance: $" + balance + ".\n");
                    }
                } else if (choice.equals("3") || choice.equalsIgnoreCase("exit")) {
                    System.out.println("Logging out... Goodbye!");
                    choice = "exit";
                } else {
                    System.out.println("Invalid choice! Try again.\n");
                }
            }
        } else {
            System.out.println("\nToo many wrong attempts! Your account is locked.");
        }

        sc.close();
    }
}

Mini Project: While Loop Simple Calculator

Your project is to create a Java program that works like a very simple calculator. The program should keep showing a menu until the user decides to exit.

Menu example:

• 1. Add two numbers
• 2. Subtract two numbers
• 3. Multiply two numbers
• 4. Divide two numbers
• 5. Exit

The program should ask the user for two numbers and perform the chosen operation. If the user enters an invalid choice, print "Invalid option". The program should continue looping until the user chooses Exit.

While writing the program, make sure you use:

• Scanner for input
• A while loop to keep the program running until exit
• If-Else statements to handle user choices
• printf or println to display results

Example Output

Closing

That's it for while loop in Java. While loop is like checking first if baby has candy before crying. Do-while is like the baby crying at least once before even checking.