The while statement executes a block of code repeatedly as long as a specified boolean condition evaluates to true. Its execution flow is condition-first, meaning the body may never run if the initial state is false.

while (condition) {
    // statements to execute repeatedly
}

Example: Sentinel-Controlled Validation

Scanner scanner = new Scanner(System.in);
System.out.print("Is the submission approved (y/n)? ");
String response = scanner.nextLine();
while (!"y".equalsIgnoreCase(response)) {
    System.out.println("Reviewing documentation and practicing syntax...");
    System.out.print("Re-evaluate submission (y/n)? ");
    response = scanner.nextLine();
}

The do-while construct guarantees at least one execution because the condition is evaluated after the block. This is useful for menus or scenarios requiring initial user interaction.

do {
    // statements
} while (condition);

Example: Post-Test Iteration

int iterationCounter = 1;
do {
    System.out.println("Processing task #" + iterationCounter);
    iterationCounter++;
} while (iterationCounter <= 5);

The for loop consolidates initialization, termination condition, and increment/decrement operations into a single header. It is ideal for counter-driven iterations where the number of cycles is known beforehand.

for (initialization; condition; update) {
    // body
}

Example: Accumulating Values Over a Fixed Range

Scanner input = new Scanner(System.in);
double totalScore = 0.0;
int courseCount = 5;
for (int idx = 0; idx < courseCount; idx++) {
    System.out.print("Enter score for module " + (idx + 1) + ": ");
    totalScore += input.nextDouble();
}
System.out.printf("Average grade: %.2f%n", totalScore / courseCount);

Branching and Control Transfer

Control transfer keywords alter the standard sequential execution of loops. These are typically paired with conditional logic.

• break: Immediate terminates the innermost enclosing loop or switch statement.
• continue: Skips the remainnig statements in the current iteration and jumps to the next cycle evaluation.

Example: Authentication Attempt Limiter

Scanner kb = new Scanner(System.in);
String validUser = "sysadmin";
String validPass = "securePass99!";
for (int attempts = 1; attempts <= 3; attempts++) {
    System.out.print("Username: ");
    String enteredUser = kb.next();
    System.out.print("Password: ");
    String enteredPass = kb.next();
    
    if (enteredUser.equals(validUser) && enteredPass.equals(validPass)) {
        System.out.println("Authentication successful.");
        break;
    }
    
    int remaining = 3 - attempts;
    if (remaining > 0) {
        System.out.println("Invalid credentials. " + remaining + " attempts left.");
    } else {
        System.out.println("Account locked after maximum retries.");
    }
}

Numeric Type Conversion

Java handles numeric type promotion automatically from lower to higher precision: byte → char → short → int → long → float → double. Downcasting requires explicit syntax (targetType) value to truncate or adjust precision, which may lead to data loss if not handled carefully.

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Aplied Programming Exercises

Exercise 1: Range Summation Across Loop Variants Calculate the cumulative total of integers from 1 through 100 using each iteration type.

public class SummationExamples {
    public static void main(String[] args) {
        int boundary = 100;
        int accumulator = 0;

        // While approach
        int pointer = 1;
        while (pointer <= boundary) {
            accumulator += pointer++;
        }
        System.out.println("While total: " + accumulator);

        // Do-while approach
        accumulator = 0;
        pointer = 1;
        do {
            accumulator += pointer;
            pointer++;
        } while (pointer <= boundary);
        System.out.println("Do-while total: " + accumulator);

        // For approach
        accumulator = 0;
        for (int step = 1; step <= boundary; step++) {
            accumulator += step;
        }
        System.out.println("For total: " + accumulator);
    }
}

Exercise 2: Weekly Time Tracking Collect daily study durations for weekdays and compute the mean value.

import java.util.Scanner;
public class WeeklyTracker {
    public static void main(String[] args) {
        Scanner reader = new Scanner(System.in);
        double weeklyTotal = 0;
        String[] days = {"Mon", "Tue", "Wed", "Thu", "Fri"};
        for (String day : days) {
            System.out.printf("Hours studied on %s: ", day);
            weeklyTotal += reader.nextDouble();
        }
        System.out.println("Weekly average: " + (weeklyTotal / days.length) + " hours");
    }
}

Exercise 3: Divisibility Pattern Generator Iterate 1 to 100. Output specific tokens based on modulo arithmetic for 3 and 5.

public class PatternSequence {
    public static void main(String[] args) {
        for (int num = 1; num <= 100; num++) {
            boolean mod3 = (num % 3 == 0);
            boolean mod5 = (num % 5 == 0);
            if (mod3 && mod5) {
                System.out.println("FlipFlop");
            } else if (mod3) {
                System.out.println("Flip");
            } else if (mod5) {
                System.out.println("Flop");
            } else {
                System.out.println(num);
            }
        }
    }
}

Exercise 4: Factorial Computation with Validation Accept an integer between 1 and 10, then compute its factorial product. Reject out-of-bounds input.

import java.util.Scanner;
public class FactorialCalc {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        System.out.print("Enter integer (1-10): ");
        if (input.hasNextInt()) {
            int target = input.nextInt();
            if (target < 1 || target > 10) {
                System.out.println("Invalid range provided.");
            } else {
                long result = 1;
                System.out.print(target + "! = ");
                for (int i = 1; i <= target; i++) {
                    result *= i;
                    System.out.print(i + (i == target ? " = " : " * "));
                }
                System.out.println(result);
            }
        }
    }
}

Exercise 5: Demographic Distribution Analysis Process 10 customer ages, categorize them by a 30-year threshold, and output percentages.

import java.util.Scanner;
public class CustomerDemographics {
    public static void main(String[] args) {
        Scanner kb = new Scanner(System.in);
        int youngCount = 0, seniorCount = 0, validEntries = 0;
        for (int i = 1; i <= 10; i++) {
            System.out.printf("Age for visitor #%d: ", i);
            int age = kb.nextInt();
            if (age > 0) {
                if (age < 30) youngCount++;
                else seniorCount++;
                validEntries++;
            }
        }
        if (validEntries > 0) {
            System.out.printf("Under 30: %.1f%%%n", (youngCount * 100.0) / validEntries);
            System.out.printf("30 or older: %.1f%%%n", (seniorCount * 100.0) / validEntries);
        }
    }
}

Exercise 6: Sentinel-Based Transaction Terminal Recreate the checkout logic using a post-test loop structure.

import java.util.Scanner;
public class PointOfSale {
    public static void main(String[] args) {
        Scanner terminal = new Scanner(System.in);
        double cartTotal = 0.0;
        int itemCount = 0;
        double itemPrice;
        do {
            System.out.print("Scan item price (0 to finish): ");
            itemPrice = terminal.nextDouble();
            if (itemPrice > 0) {
                cartTotal += itemPrice;
                itemCount++;
            }
        } while (itemPrice != 0);
        System.out.printf("Items scanned: %d | Final total: $%.2f%n", itemCount, cartTotal);
    }
}

Exercise 7: Brute-Force Equation Solver Solve the classic poultry/rabbit puzzle using iteration to match head and leg constraints.

public class AlgebraSolver {
    public static void main(String[] args) {
        int targetHeads = 35;
        int targetLegs = 94;
        for (int chickens = 0; chickens <= targetHeads; chickens++) {
            int rabbits = targetHeads - chickens;
            if ((chickens * 2) + (rabbits * 4) == targetLegs) {
                System.out.println("Solution found: " + chickens + " chickens, " + rabbits + " rabbits.");
                break;
            }
        }
    }
}