Java While Loop | Hassan Bukhari

Java Notes

09 Java Fundamentals | While Loop in Java | By Dummy for Dummies

Java Beginner Guide While Loop Do-While Iteration

INTRODUCTION

What happened when you were a child and asked your parents for snacks and they refused? You just walked away saying "Yeah snacks are not good for health I should start eating more vegetables". HELL NAH! You cried and begged for snack until they gave up and provided what you asked for.

You kept on repeating until they gave you what you wanted. It was like you were on loop and only one thing could exit that loop.

In programming we will do the same, we will do some magic and the program will keep on repeating the same part of code until we achieve our target. This is called loop in programming.

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:

Java

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();
    }
}

Console Output

Baby is crying Baby is asking for candy Give something: toy Baby is crying Baby is asking for candy Give something: chocolate Baby is crying Baby is asking for candy Give something: candy Baby got candy Baby is happy!

⦿ 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, without giving candy the loop will run no matter whatever you give him

⦿ 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

Java

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");
    }
}

Console Output (infinite)

HELP I AM STUCK IN LOOP! HELP I AM STUCK IN LOOP! HELP I AM STUCK IN LOOP! HELP I AM STUCK IN LOOP! ...

Note: The code will check if n equals 0. Every time it'll find the condition true. With no change from inside, it'll spam "HELP I AM STUCK IN LOOP" until you close the console manually. It'll NEVER reach "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.

Java

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!");
    }
}

Console Output

Number: 1 Number: 2 Number: 3 Number: 4 Number: 5 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.

Note: The while part in the do-while loop ends with a semicolon: while (number < 6);

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.

Java

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();
    }
}

Console Output

Enter your secret PIN: 0000 Wrong PIN! Attempts left: 2 Enter your secret PIN: 1234 Access Granted! It took you 2 attempt(s). Choose an option: 1. Check Balance 2. Withdraw Money 3. Exit Your choice: 1 Your balance is $1000. Choose an option: 1. Check Balance 2. Withdraw Money 3. Exit Your choice: 2 Enter withdrawal amount: 300 Withdrawal successful. Remaining balance: $700. Choose an option: 1. Check Balance 2. Withdraw Money 3. Exit Your choice: 3 Logging out... Goodbye!

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

Console Output

Simple Calculator Choose an option: 1. Add two numbers 2. Subtract two numbers 3. Multiply two numbers 4. Divide two numbers 5. Exit Your choice: 1 Enter first number: 10 Enter second number: 5 Result: 10 + 5 = 15 Choose an option: 1. Add two numbers 2. Subtract two numbers 3. Multiply two numbers 4. Divide two numbers 5. Exit Your choice: 4 Enter first number: 20 Enter second number: 4 Result: 20 / 4 = 5.0 Choose an option: 1. Add two numbers 2. Subtract two numbers 3. Multiply two numbers 4. Divide two numbers 5. Exit Your choice: 4 Enter first number: 10 Enter second number: 0 Invalid: Division by zero Choose an option: 1. Add two numbers 2. Subtract two numbers 3. Multiply two numbers 4. Divide two numbers 5. Exit Your choice: 5 Exiting calculator. Goodbye!

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.

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