Conditional constructs in Java

A. The Four Variants of if Constructs:

1. Simple if (Single-Alternative): Executes a code block only if the specified boolean condition evaluates to true.

   if (balance < 1000) {
       System.out.println("Low Balance Warning!");
   }

2. Dual-Alternative if-else: Executes the if block if the condition is true; otherwise executes the else block.

   if (number % 2 == 0) {
       System.out.println("Even Number");
   } else {
       System.out.println("Odd Number");
   }

3. The if-else-if Ladder (Multi-Alternative Selection): Evaluates conditions sequentially from top to bottom. As soon as a condition is satisfied, its block executes, and the remainder of the ladder is skipped. If no condition matches, the trailing else executes as a fallback.

   if (marks >= 90)
       grade = 'A';
   else if (marks >= 80)
       grade = 'B';
   else if (marks >= 70)
       grade = 'C';
   else
       grade = 'D';

4. Nested if: An if or if-else statement placed inside the body of another if or else block, used for multi-tier criteria validation.

A. Formal Syntax of switch-case:

switch (switch_expression) {
    case constant1:
        // Statements executed if switch_expression == constant1
        break; // Exits the switch block immediately
    case constant2:
        // Statements executed if switch_expression == constant2
        break;
    default:
        // Statements executed if no case constant matches
        break;
}

B. Rigid Rules for switch-case in Java:

• Allowed Data Types: The switch expression must evaluate to: byte, short, char, int, String (Java 7+), or an enum.
  STRICTLY FORBIDDEN: float, double, and boolean cannot be used in a switch expression!
• Case Labels Must Be Constants: Each case label must be an immutable literal or a final compile-time constant expression. Variables (e.g., case x:) or ranges (e.g., case 80 to 90:) are completely illegal.
• Unique Case Labels: Duplicate case labels in the same switch construct produce a compile-time error.
• Optional Default: The default label is optional and can be placed anywhere inside the switch, though it is conventionally positioned at the very end.

A. What is Fall-Through?

The break statement is a jump statement that forces immediate termination of the switch block. If a programmer omits the break statement from a matching case, execution does not stop! Instead, program control sequentially executes all subsequent case statements regardless of whether their labels match, continuing until a break is encountered or the end of the switch block is reached.

B. Tracing Fall-Through (Classic Board Problem):

int choice = 2;
switch (choice) {
    case 1:
        System.out.print("Alpha ");
    case 2:
        System.out.print("Beta "); // Matches! Executes. (No break!)
    case 3:
        System.out.print("Gamma "); // Falls through! Executes.
        break; // Breaks out here!
    case 4:
        System.out.print("Delta ");
    default:
        System.out.print("Omega ");
}

Output: Beta Gamma

Explanation: Since case 2 matches and lacks a break, execution falls through into case 3, printing "Gamma ", where it finally hits the break statement and exits.

Menu-Driven Area Calculator:

import java.util.Scanner;

public class AreaMenuDemo {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        System.out.println("====== GEOMETRIC AREA CALCULATOR ======");
        System.out.println("1. Area of Circle");
        System.out.println("2. Area of Rectangle");
        System.out.println("3. Area of Triangle");
        System.out.print("Enter your choice (1-3): ");
        int choice = in.nextInt();

        switch (choice) {
            case 1:
                System.out.print("Enter radius: ");
                double r = in.nextDouble();
                double circleArea = Math.PI * r * r;
                System.out.println("Area of Circle = " + circleArea);
                break;

            case 2:
                System.out.print("Enter length and breadth: ");
                double l = in.nextDouble();
                double b = in.nextDouble();
                double rectArea = l * b;
                System.out.println("Area of Rectangle = " + rectArea);
                break;

            case 3:
                System.out.print("Enter base and height: ");
                double base = in.nextDouble();
                double height = in.nextDouble();
                double triArea = 0.5 * base * height;
                System.out.println("Area of Triangle = " + triArea);
                break;

            default:
                System.out.println("Error: Invalid choice! Please select between 1 and 3.");
                break;
        }
    }
}

Interactive Banking Simulator:

import java.util.Scanner;

public class ATMSimulator {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        double balance = 50000.0;

        System.out.println("====== WELCOME TO NATIONAL BANK ATM ======");
        System.out.println("1. Check Balance");
        System.out.println("2. Deposit Funds");
        System.out.println("3. Withdraw Cash");
        System.out.println("4. Exit");
        System.out.print("Select an option (1-4): ");
        int option = in.nextInt();

        switch (option) {
            case 1:
                System.out.println("Your available balance is: ₹" + balance);
                break;
            case 2:
                System.out.print("Enter deposit amount: ₹");
                double deposit = in.nextDouble();
                if (deposit > 0) {
                    balance += deposit;
                    System.out.println("Deposit Successful! New Balance: ₹" + balance);
                } else {
                    System.out.println("Invalid amount!");
                }
                break;
            case 3:
                System.out.print("Enter withdrawal amount: ₹");
                double withdraw = in.nextDouble();
                if (withdraw > 0 && withdraw <= balance) {
                    balance -= withdraw;
                    System.out.println("Withdrawal Successful! Remaining Balance: ₹" + balance);
                } else {
                    System.out.println("Transaction Failed: Insufficient funds or invalid amount!");
                }
                break;
            case 4:
                System.out.println("Thank you for banking with us. Have a wonderful day!");
                break;
            default:
                System.out.println("Invalid Selection! Please choose an option between 1 and 4.");
                break;
        }
    }
}

Progressive Slab Tax Calculator:

import java.util.Scanner;

public class IncomeTaxCalculator {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        System.out.print("Enter Annual Taxable Income: ₹");
        double income = in.nextDouble();
        double tax = 0.0;

        // Progressive Tax Slabs:
        // Up to ₹2,50,000 : Nil (0%)
        // ₹2,50,001 to ₹5,00,000 : 5% of income exceeding ₹2,50,000
        // ₹5,00,001 to ₹10,00,000 : ₹12,500 + 20% of income exceeding ₹5,00,000
        // Above ₹10,00,000 : ₹1,12,500 + 30% of income exceeding ₹10,00,000
        if (income <= 250000) {
            tax = 0.0;
        } else if (income <= 500000) {
            tax = (income - 250000) * 0.05;
        } else if (income <= 1000000) {
            tax = 12500 + (income - 500000) * 0.20;
        } else {
            tax = 112500 + (income - 1000000) * 0.30;
        }

        double cess = tax * 0.04; // 4% Health and Education Cess
        double totalTax = tax + cess;

        System.out.println("Income Tax Payable   : ₹" + tax);
        System.out.println("Education Cess (4%)  : ₹" + cess);
        System.out.println("Total Tax Liability  : ₹" + totalTax);
    }
}

A. The Gregorian Leap Year Rules:

A year is a leap year if:

1. It is divisible by 4, AND
2. It is NOT divisible by 100, UNLESS
3. It is also divisible by 400 (Centurial leap years like 1600, 2000, 2400).

import java.util.Scanner;

public class LeapYearCheck {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        System.out.print("Enter a year (e.g. 2024): ");
        int year = in.nextInt();

        boolean isLeap = false;
        if ((year % 4 == 0 && year % 100 != 0) || (year % 400 == 0)) {
            isLeap = true;
        }

        if (isLeap) {
            System.out.println(year + " is a LEAP YEAR (366 days).");
        } else {
            System.out.println(year + " is a COMMON YEAR (365 days).");
        }
    }
}

A. String Matching in switch-case (Java 7+):

Starting with Java 7, programmers can use String objects directly in switch expressions. String comparison is case-sensitive and relies internally on String.equals():

String day = "Monday";
switch (day.toLowerCase()) {
    case "monday":
    case "tuesday":
    case "wednesday":
    case "thursday":
    case "friday":
        System.out.println("Weekday (School / Work)");
        break;
    case "saturday":
    case "sunday":
        System.out.println("Weekend (Holiday)");
        break;
    default:
        System.out.println("Invalid day name!");
        break;
}

A. Progressive Late Return Fine Structure:

In school libraries, overdue book fines are calculated progressively:

• First 5 days late: ₹0.50 per day
• Next 5 days (6-10 days): ₹1.00 per day
• Above 10 days: ₹2.00 per day

double fine = 0.0;
if (days <= 5) {
    fine = days * 0.50;
} else if (days <= 10) {
    fine = (5 * 0.50) + (days - 5) * 1.00;
} else {
    fine = (5 * 0.50) + (5 * 1.00) + (days - 10) * 2.00;
}
System.out.println("Total Library Fine: ₹" + fine);

Always place the most frequently matched condition at the top of an if-else-if ladder to maximize execution efficiency. In switch-case constructs, group case labels that share identical logic together (e.g., case 'a': case 'e': case 'i': case 'o': case 'u': print("Vowel"); break;) to avoid redundant code duplication.