Day 3: Exploring Strings, Conditionals, and Practical Java Programs

Today, I dove deeper into Java by exploring string manipulation methods, nested conditions, user input parsing, and creating small practical utilities. These exercises helped me strengthen my understanding of core Java features and build interactive console programs.

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1. String Methods and Validation — StringMethodDemo

javaCopyEdit// Program demonstrating useful String methods and validations in Java

public class StringMethodDemo {
    public static void main(String[] args) {

        // Initialize sample strings
        String name = "Penelope Eckart";
        String name2 = "REYNOLD ECKART";
        String name3 = "     Callisto Regulus    ";
        String name4 = "Berry";

        // Using String methods to manipulate and get info
        int length = name.length();
        char letter = name.charAt(7);
        int index = name.indexOf("e");
        int lastIndex = name.lastIndexOf("e");

        // Changing case, trimming spaces, replacing characters
        name = name.toUpperCase();
        name2 = name2.toLowerCase();
        name3 = name3.trim();
        name4 = name4.replace("e", "/");

        // Print results of string operations
        System.out.println("Length of original name: " + length);
        System.out.println("Character at index 7: " + letter);
        System.out.println("First index of 'e': " + index);
        System.out.println("Last index of 'e': " + lastIndex);
        System.out.println("Name in uppercase: " + name);
        System.out.println("Name2 in lowercase: " + name2);
        System.out.println("Trimmed name3: '" + name3 + "'");
        System.out.println("Modified name4: " + name4);

        // Check if the string is empty or contains spaces
        System.out.println("Is name empty? " + name.isEmpty());

        if (name.isEmpty()) {
            System.out.println("Your name is empty.");
        } else {
            System.out.println("Hello " + name);
        }

        if (name.contains(" ")) {
            System.out.println("Yes, there is whitespace in your name.");
        } else {
            System.out.println("Your name doesn't contain any whitespace.");
        }

        // Check if name equals "password"
        if (name.equals("password")) {
            System.out.println("Your name can't be 'password'.");
        } else {
            System.out.println("Hello again, " + name);
        }
    }
}

What I learned:
This program introduced me to many useful String methods like .length(), .charAt(), .indexOf(), .lastIndexOf(), .toUpperCase(), .toLowerCase(), .trim(), and .replace(). These methods help manipulate text in Java, which is crucial for tasks like validating user inputs or processing data. The program also checks for empty strings and whitespace, showing how to apply basic validations.

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2. Nested If-Else: Movie Ticket Discount Calculator — TicketDiscountCalculator

javaCopyEdit// Program using nested if-else to calculate ticket discounts

public class TicketDiscountCalculator {
    public static void main(String[] args) {

        boolean isStudent = true;
        boolean isSenior = true;
        double price = 9.99;

        System.out.println("Welcome to the Movie Ticket Discount Calculator!");

        if (isStudent) {
            if (isSenior) {
                System.out.println("Eligible for senior discount: 20%");
                System.out.println("Eligible for student discount: 10%");
                price *= 0.7; // Combined 30% discount
            } else {
                System.out.println("Eligible for student discount: 10%");
                price *= 0.9;
            }
        } else {
            if (isSenior) {
                System.out.println("Eligible for senior discount: 20%");
                price *= 0.8;
            } else {
                System.out.println("Not eligible for any discounts.");
            }
        }

        System.out.printf("Final ticket price: $%.2f%n", price);
    }
}

What I learned:
This exercise helped me understand how to use nested if-else statements to handle complex logic with multiple conditions. It also showed how to apply discounts sequentially and display formatted output using printf.

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3. Email Parsing with Substring — EmailParser

javaCopyEditimport java.util.Scanner;

// Program to extract username and domain from an email

public class EmailParser {
    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);

        System.out.print("Enter your email: ");
        String email = scanner.nextLine();

        if (email.contains("@")) {
            String username = email.substring(0, email.indexOf("@"));
            String domain = email.substring(email.indexOf("@") + 1);

            System.out.println("Username: " + username);
            System.out.println("Domain: " + domain);
        } else {
            System.out.println("Email must contain '@'");
        }

        scanner.close();
    }
}

What I learned:
This program shows how to use .substring() and .indexOf() to split a string at a specific character (@), a common need when working with email addresses or URLs.

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4. Weight Conversion Utility — WeightConverter

javaCopyEditimport java.util.Scanner;

// Program to convert weights between pounds and kilograms

public class WeightConverter {
    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);

        System.out.println("Weight Conversion Program!");
        System.out.println("1: Convert lbs to kgs");
        System.out.println("2: Convert kgs to lbs");
        System.out.print("Choose an option: ");
        int choice = scanner.nextInt();

        double weight;
        double convertedWeight;

        if (choice == 1) {
            System.out.print("Enter the weight in lbs: ");
            weight = scanner.nextDouble();
            convertedWeight = weight * 0.453592;
            System.out.printf("The weight in kilograms is: %.2f kg%n", convertedWeight);
        } else if (choice == 2) {
            System.out.print("Enter the weight in kgs: ");
            weight = scanner.nextDouble();
            convertedWeight = weight * 2.20462;
            System.out.printf("The weight in pounds is: %.2f lbs%n", convertedWeight);
        } else {
            System.out.println("Invalid choice.");
        }

        scanner.close();
    }
}

What I learned:
I practiced handling user input with Scanner and used conditional logic to perform unit conversions. I also used printf for nicely formatted output.

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5. Ternary Operator Examples — TernaryDemo

javaCopyEditimport java.util.Scanner;

// Program to demonstrate ternary operators for concise conditions

public class TernaryDemo {
    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);

        System.out.print("Enter your score: ");
        double score = scanner.nextDouble();
        String passOrFail = (score >= 60) ? "PASS" : "FAIL";
        System.out.println(passOrFail);

        System.out.print("Enter a number: ");
        int number = scanner.nextInt();
        String evenOrOdd = (number % 2 == 0) ? "EVEN" : "ODD";
        System.out.println(evenOrOdd);

        scanner.close();
    }
}

What I learned:
This program helped me understand how to use the ternary operator to replace simple if-else statements and write shorter, cleaner code.

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6. Temperature Converter with Ternary — TemperatureConverter

javaCopyEditimport java.util.Scanner;

// Program to convert temperatures between Fahrenheit and Celsius

public class TemperatureConverter {
    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);

        System.out.print("Enter the temperature: ");
        double temp = scanner.nextDouble();

        System.out.print("Convert to Celsius or Fahrenheit? (C or F): ");
        String unit = scanner.next().toUpperCase();

        double convertedTemp = (unit.equals("C")) ? (temp - 32) * 5 / 9 : (temp * 9 / 5) + 32;

        System.out.printf("Converted temperature: %.1f%n", convertedTemp);

        scanner.close();
    }
}

What I learned:
I practiced working with user input and string comparison, and applied the ternary operator to choose conversion logic dynamically.

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7. Day of the Week Switch Case — DayChecker

javaCopyEditimport java.util.Scanner;

// Program using enhanced switch to identify weekdays and weekends

public class DayChecker {
    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);

        System.out.print("Enter the day: ");
        String day = scanner.nextLine();

        switch(day) {
            case "Monday", "Tuesday", "Wednesday", "Thursday", "Friday" ->
                System.out.println("It is a weekday.");
            case "Saturday", "Sunday" ->
                System.out.println("It is the weekend!");
            default ->
                System.out.println(day + " is not a valid day.");
        }

        scanner.close();
    }
}

What I learned:
This program demonstrated the enhanced switch statement in Java, allowing cleaner syntax for multiple case matches.

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8. Simple Calculator with Switch — Calculator

javaCopyEditimport java.util.Scanner;

// Program to perform basic arithmetic operations

public class Calculator {
    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);

        System.out.print("Enter the first number: ");
        double num1 = scanner.nextDouble();

        System.out.print("Enter an operator (+, -, *, /, ^): ");
        char operator = scanner.next().charAt(0);

        System.out.print("Enter the second number: ");
        double num2 = scanner.nextDouble();

        double result = 0;
        boolean validOperation = true;

        switch(operator) {
            case '+' -> result = num1 + num2;
            case '-' -> result = num1 - num2;
            case '*' -> result = num1 * num2;
            case '/' -> {
                if (num2 == 0) {
                    System.out.println("Cannot divide by zero!");
                    validOperation = false;
                } else {
                    result = num1 / num2;
                }
            }
            case '^' -> result = Math.pow(num1, num2);
            default -> {
                System.out.println("Invalid operator.");
                validOperation = false;
            }
        }

        if (validOperation) {
            System.out.printf("Result: %.2f%n", result);
        }

        scanner.close();
    }
}

What I learned:
This calculator program helped me practice user input, switch statements, operator handling, and basic error checking for division by zero.

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Summary

Day 3 was packed with hands-on practice covering strings, conditionals, parsing, conversions, and switch statements. I learned how to:

• Manipulate and validate strings

• Use nested if-else for complex decision-making

• Parse strings using substring and indexOf

• Take user input to perform unit conversions

• Write concise conditional expressions with ternary operators

• Use enhanced switch statements

• Build a simple interactive calculator

These programs helped me combine theory with practice, improving my confidence in writing real-world Java applications.