Lecture 05 - Control Flow: Selection Statements

Java Programming

Lecture 05: Control Flow - Selection Statements

Course: 4343203 - Java Programming

GTU Semester 4 | Unit 1

Learning Objectives:

Master if-else conditional statements
Understand nested and chained conditions
Learn switch statement and its applications
Apply conditional logic in practical programs
Debug common conditional statement errors

Understanding Control Flow Statements

Control Flow Statements determine the execution path of a program by controlling which statements are executed and in what order.

Selection Statements:

if statement - Single condition
if-else statement - Two alternatives
if-else-if ladder - Multiple conditions
switch statement - Multiple values

Key Concepts:

Boolean Expression: Condition evaluation
Code Block: Statements to execute
Branch: Different execution paths
Fall-through: Continued execution

Simple if Statement

Syntax:


if (condition) {
    // statements to execute if condition is true
}

Basic Examples:


int age = 18;

if (age >= 18) {
    System.out.println("You are eligible to vote!");
}

int marks = 85;
if (marks >= 90) {
    System.out.println("Grade: A+");
}

boolean isLoggedIn = true;
if (isLoggedIn) {
    System.out.println("Welcome to your dashboard!");
}

Flowchart Logic:

Start

↓

Evaluate Condition

↓

True → Execute if block

False → Skip if block

↓

Continue

Important Rules:

Condition must be boolean expression
Braces {} are optional for single statement
Always use braces for readability
No semicolon after closing brace

if-else Statement

Syntax & Examples:


// Basic if-else syntax
if (condition) {
    // execute if condition is true
} else {
    // execute if condition is false
}

// Examples
int number = 15;
if (number % 2 == 0) {
    System.out.println(number + " is even");
} else {
    System.out.println(number + " is odd");
}

// Age category example
int age = 25;
if (age < 18) {
    System.out.println("Minor");
} else {
    System.out.println("Adult");
}

// Grade classification
double percentage = 75.5;
if (percentage >= 60) {
    System.out.println("Pass");
} else {
    System.out.println("Fail");
}

Real-world Application:


// Login validation
String username = "admin";
String password = "secret123";

if (username.equals("admin") && password.equals("secret123")) {
    System.out.println("Login successful!");
    System.out.println("Redirecting to dashboard...");
} else {
    System.out.println("Invalid credentials!");
    System.out.println("Please try again.");
}

// Temperature check
double temperature = 38.5;
if (temperature > 37.0) {
    System.out.println("You have a fever");
    System.out.println("Please consult a doctor");
} else {
    System.out.println("Temperature is normal");
    System.out.println("Stay healthy!");
}

Best Practices:

Use meaningful variable names
Keep conditions simple and readable
Group related statements in blocks
Avoid deep nesting when possible

Nested if Statements

Simple Nested Example:


int age = 25;
boolean hasLicense = true;

if (age >= 18) {
    System.out.println("Age requirement met");
    
    if (hasLicense) {
        System.out.println("You can drive!");
    } else {
        System.out.println("You need a driving license");
    }
} else {
    System.out.println("You are too young to drive");
}

Grade Calculator:


int marks = 85;

if (marks >= 0 && marks <= 100) {
    System.out.println("Valid marks");
    
    if (marks >= 90) {
        System.out.println("Grade: A+");
    } else if (marks >= 80) {
        System.out.println("Grade: A");
    } else if (marks >= 70) {
        System.out.println("Grade: B");
    } else if (marks >= 60) {
        System.out.println("Grade: C");
    } else {
        System.out.println("Grade: F");
    }
} else {
    System.out.println("Invalid marks!");
}

Complex Nested Logic:


// Banking system example
double balance = 5000.0;
double withdrawAmount = 2000.0;
boolean accountActive = true;

if (accountActive) {
    System.out.println("Account is active");
    
    if (withdrawAmount > 0) {
        System.out.println("Valid withdrawal amount");
        
        if (balance >= withdrawAmount) {
            balance = balance - withdrawAmount;
            System.out.println("Withdrawal successful!");
            System.out.println("Remaining balance: " + balance);
        } else {
            System.out.println("Insufficient funds!");
            System.out.println("Available balance: " + balance);
        }
    } else {
        System.out.println("Invalid withdrawal amount!");
    }
} else {
    System.out.println("Account is inactive");
    System.out.println("Please contact customer service");
}

Nesting Guidelines:

Limit nesting depth to 3-4 levels
Use proper indentation
Consider early returns to reduce nesting
Break complex conditions into variables

if-else-if Ladder

Syntax & Structure:


if (condition1) {
    // Block 1
} else if (condition2) {
    // Block 2
} else if (condition3) {
    // Block 3
} else {
    // Default block
}

Grade System Example:


int percentage = 78;

if (percentage >= 90) {
    System.out.println("Grade: A+ (Outstanding)");
} else if (percentage >= 80) {
    System.out.println("Grade: A (Excellent)");
} else if (percentage >= 70) {
    System.out.println("Grade: B (Good)");
} else if (percentage >= 60) {
    System.out.println("Grade: C (Satisfactory)");
} else if (percentage >= 50) {
    System.out.println("Grade: D (Pass)");
} else {
    System.out.println("Grade: F (Fail)");
}

Day of Week Example:


int day = 3;

if (day == 1) {
    System.out.println("Monday - Start of work week!");
} else if (day == 2) {
    System.out.println("Tuesday - Keep going!");
} else if (day == 3) {
    System.out.println("Wednesday - Midweek!");
} else if (day == 4) {
    System.out.println("Thursday - Almost there!");
} else if (day == 5) {
    System.out.println("Friday - TGIF!");
} else if (day == 6) {
    System.out.println("Saturday - Weekend fun!");
} else if (day == 7) {
    System.out.println("Sunday - Rest day!");
} else {
    System.out.println("Invalid day number!");
}

BMI Calculator:


double bmi = 22.5;

if (bmi < 18.5) {
    System.out.println("Underweight");
} else if (bmi < 25.0) {
    System.out.println("Normal weight");
} else if (bmi < 30.0) {
    System.out.println("Overweight");
} else {
    System.out.println("Obese");
}

switch Statement

Syntax & Basic Example:


switch (expression) {
    case value1:
        // statements
        break;
    case value2:
        // statements
        break;
    default:
        // default statements
        break;
}

// Example: Menu selection
int choice = 2;

switch (choice) {
    case 1:
        System.out.println("Option 1: Create Account");
        break;
    case 2:
        System.out.println("Option 2: Login");
        break;
    case 3:
        System.out.println("Option 3: Exit");
        break;
    default:
        System.out.println("Invalid choice!");
        break;
}

Month Name Example:


int month = 7;
String monthName;

switch (month) {
    case 1: monthName = "January"; break;
    case 2: monthName = "February"; break;
    case 3: monthName = "March"; break;
    case 4: monthName = "April"; break;
    case 5: monthName = "May"; break;
    case 6: monthName = "June"; break;
    case 7: monthName = "July"; break;
    case 8: monthName = "August"; break;
    case 9: monthName = "September"; break;
    case 10: monthName = "October"; break;
    case 11: monthName = "November"; break;
    case 12: monthName = "December"; break;
    default: monthName = "Invalid month"; break;
}

System.out.println("Month: " + monthName);

switch Statement Rules:

Expression must be int, char, String, or enum
Case values must be compile-time constants
Each case should end with break (usually)
default case is optional but recommended
No duplicate case values allowed

switch Fall-through & Advanced Usage

Fall-through Behavior:


// Without break statements
int day = 3;

switch (day) {
    case 1:
        System.out.println("Monday");
        // No break - falls through!
    case 2:
        System.out.println("Tuesday");
        // No break - falls through!
    case 3:
        System.out.println("Wednesday");
        break;  // Stops here
    default:
        System.out.println("Other day");
}

// Output for day = 3:
// Wednesday

Intentional Fall-through:


char grade = 'B';

switch (grade) {
    case 'A':
    case 'B':
    case 'C':
        System.out.println("Pass");
        break;
    case 'D':
    case 'F':
        System.out.println("Fail");
        break;
    default:
        System.out.println("Invalid grade");
}

Grouped Cases:


// Days of week grouping
int day = 6;

switch (day) {
    case 1:
    case 2:
    case 3:
    case 4:
    case 5:
        System.out.println("Weekday");
        System.out.println("Time to work!");
        break;
    case 6:
    case 7:
        System.out.println("Weekend");
        System.out.println("Time to relax!");
        break;
    default:
        System.out.println("Invalid day");
}

Calculator Example:


double num1 = 10, num2 = 5;
char operator = '+';
double result = 0;

switch (operator) {
    case '+':
        result = num1 + num2;
        System.out.println("Addition: " + result);
        break;
    case '-':
        result = num1 - num2;
        System.out.println("Subtraction: " + result);
        break;
    case '*':
        result = num1 * num2;
        System.out.println("Multiplication: " + result);
        break;
    case '/':
        if (num2 != 0) {
            result = num1 / num2;
            System.out.println("Division: " + result);
        } else {
            System.out.println("Division by zero!");
        }
        break;
    default:
        System.out.println("Invalid operator!");
}

switch vs if-else-if Comparison

When to Use switch:

Multiple discrete values to check
Single variable comparison
Integer, char, String, or enum types
Performance matters (compiler optimization)

switch Example:


// Good for switch
int statusCode = 200;

switch (statusCode) {
    case 200: System.out.println("OK"); break;
    case 404: System.out.println("Not Found"); break;
    case 500: System.out.println("Server Error"); break;
    default: System.out.println("Unknown"); break;
}

When to Use if-else-if:

Complex boolean expressions
Range comparisons
Multiple variables involved
Different data types

if-else-if Example:


// Better with if-else-if
int age = 25;
double income = 50000;

if (age >= 18 && age <= 25) {
    System.out.println("Young adult");
} else if (age >= 26 && income > 30000) {
    System.out.println("Working professional");
} else if (age >= 60) {
    System.out.println("Senior citizen");
} else {
    System.out.println("Other category");
}

Feature	switch Statement	if-else-if Ladder
Data Types	int, char, String, enum	Any boolean expression
Performance	Faster for many cases	Sequential evaluation
Flexibility	Limited to equality	Any condition possible
Readability	Clean for discrete values	Better for complex logic

Previous Year Exam Questions

Q1. (GTU Summer 2022) Write a Java program to find the largest of three numbers using nested if statements.

Solution:

public class LargestOfThree {
    public static void main(String[] args) {
        int num1 = 25, num2 = 35, num3 = 15;
        int largest;
        
        // Method 1: Using nested if statements
        if (num1 >= num2) {
            if (num1 >= num3) {
                largest = num1;
            } else {
                largest = num3;
            }
        } else {
            if (num2 >= num3) {
                largest = num2;
            } else {
                largest = num3;
            }
        }
        
        System.out.println("Numbers: " + num1 + ", " + num2 + ", " + num3);
        System.out.println("Largest number is: " + largest);
        
        // Method 2: Alternative approach with cleaner logic
        int max = num1;
        
        if (num2 > max) {
            max = num2;
        }
        
        if (num3 > max) {
            max = num3;
        }
        
        System.out.println("Alternative method result: " + max);
    }
}

Q2. (GTU Winter 2021) Explain the switch statement in Java with an example. What is the purpose of break and default statements?

Solution:

switch Statement Overview:

The switch statement is a multiway branch statement that provides an efficient way to transfer control to different parts of code based on the value of an expression.

Example Program:

public class SwitchExample {
    public static void main(String[] args) {
        int dayNumber = 3;
        String dayName;
        
        switch (dayNumber) {
            case 1:
                dayName = "Monday";
                break;
            case 2:
                dayName = "Tuesday";
                break;
            case 3:
                dayName = "Wednesday";
                break;
            case 4:
                dayName = "Thursday";
                break;
            case 5:
                dayName = "Friday";
                break;
            case 6:
                dayName = "Saturday";
                break;
            case 7:
                dayName = "Sunday";
                break;
            default:
                dayName = "Invalid day";
                break;
        }
        
        System.out.println("Day " + dayNumber + " is " + dayName);
    }
}

Purpose of break Statement:

Terminates the switch block execution
Prevents fall-through to subsequent cases
Transfers control to the statement after the switch block
Without break, execution continues to next case

Purpose of default Statement:

Executes when no case matches the switch expression
Similar to else clause in if-else statements
Optional but recommended for complete logic
Can be placed anywhere in switch block

Q3. (GTU Summer 2020) Write a menu-driven Java program using switch statement to perform basic arithmetic operations (addition, subtraction, multiplication, division).

Solution:

import java.util.Scanner;

public class Calculator {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        
        System.out.println("=== Basic Calculator ===");
        System.out.println("1. Addition");
        System.out.println("2. Subtraction");  
        System.out.println("3. Multiplication");
        System.out.println("4. Division");
        System.out.print("Enter your choice (1-4): ");
        
        int choice = scanner.nextInt();
        
        System.out.print("Enter first number: ");
        double num1 = scanner.nextDouble();
        
        System.out.print("Enter second number: ");
        double num2 = scanner.nextDouble();
        
        double result = 0;
        boolean validOperation = true;
        
        switch (choice) {
            case 1:
                result = num1 + num2;
                System.out.println(num1 + " + " + num2 + " = " + result);
                break;
                
            case 2:
                result = num1 - num2;
                System.out.println(num1 + " - " + num2 + " = " + result);
                break;
                
            case 3:
                result = num1 * num2;
                System.out.println(num1 + " * " + num2 + " = " + result);
                break;
                
            case 4:
                if (num2 != 0) {
                    result = num1 / num2;
                    System.out.println(num1 + " / " + num2 + " = " + result);
                } else {
                    System.out.println("Error: Division by zero is not allowed!");
                    validOperation = false;
                }
                break;
                
            default:
                System.out.println("Invalid choice! Please select 1-4.");
                validOperation = false;
                break;
        }
        
        if (validOperation && choice >= 1 && choice <= 4) {
            System.out.println("Operation completed successfully!");
        }
        
        scanner.close();
    }
}

Practical Programming Examples

Student Grade System:


public class StudentGradeSystem {
    public static void main(String[] args) {
        String studentName = "John Doe";
        int[] marks = {85, 92, 78, 88, 95};
        int totalMarks = 0;
        
        // Calculate total marks
        for (int mark : marks) {
            totalMarks += mark;
        }
        
        double average = totalMarks / 5.0;
        char grade;
        String description;
        
        // Determine grade using if-else-if
        if (average >= 90) {
            grade = 'A';
            description = "Outstanding";
        } else if (average >= 80) {
            grade = 'B';
            description = "Excellent";
        } else if (average >= 70) {
            grade = 'C';
            description = "Good";
        } else if (average >= 60) {
            grade = 'D';
            description = "Satisfactory";
        } else {
            grade = 'F';
            description = "Needs Improvement";
        }
        
        // Display results
        System.out.println("Student: " + studentName);
        System.out.println("Total Marks: " + totalMarks + "/500");
        System.out.println("Average: " + average + "%");
        System.out.println("Grade: " + grade);
        System.out.println("Performance: " + description);
    }
}

Traffic Light System:


public class TrafficLightSystem {
    public static void main(String[] args) {
        String lightColor = "yellow";
        
        // Using switch for traffic light logic
        switch (lightColor.toLowerCase()) {
            case "red":
                System.out.println("🔴 RED LIGHT");
                System.out.println("STOP! Do not proceed.");
                System.out.println("Wait for green light.");
                break;
                
            case "yellow":
            case "amber":
                System.out.println("🟡 YELLOW LIGHT");
                System.out.println("CAUTION! Prepare to stop.");
                System.out.println("Proceed only if safe.");
                break;
                
            case "green":
                System.out.println("🟢 GREEN LIGHT");
                System.out.println("GO! Proceed safely.");
                System.out.println("Watch for pedestrians.");
                break;
                
            default:
                System.out.println("⚪ UNKNOWN LIGHT");
                System.out.println("Traffic light malfunction!");
                System.out.println("Proceed with extreme caution.");
                break;
        }
        
        // Additional safety check
        boolean isIntersectionClear = true;
        boolean isPedestrianCrossing = false;
        
        if (lightColor.equals("green")) {
            if (!isIntersectionClear) {
                System.out.println("\n⚠️ WARNING: Intersection not clear!");
            }
            if (isPedestrianCrossing) {
                System.out.println("⚠️ WARNING: Pedestrians crossing!");
            }
        }
    }
}

Common Errors & Debugging Tips

Common if-else Errors:

1. Assignment instead of comparison

if (x = 5) {  // ❌ Wrong! Assignment
    // This causes compilation error
}

if (x == 5) {  // ✅ Correct! Comparison
    // This works properly
}

2. Missing braces

if (condition)
    statement1;
    statement2;  // ❌ Not part of if block!

if (condition) {  // ✅ Use braces
    statement1;
    statement2;
}

3. Dangling else

if (x > 0)
    if (y > 0)
        System.out.println("Both positive");
else  // ❌ Belongs to inner if!
    System.out.println("x is not positive");

Common switch Errors:

1. Missing break statements

switch (day) {
    case 1: System.out.println("Monday");
    case 2: System.out.println("Tuesday");  // ❌ Falls through!
    // Output for day=1: "Monday" and "Tuesday"
}

switch (day) {
    case 1: System.out.println("Monday"); break;  // ✅
    case 2: System.out.println("Tuesday"); break; // ✅
}

2. Non-constant case values

int x = 10;
switch (day) {
    case x: // ❌ Variable not allowed!
        break;
    case 1: // ✅ Literal constant OK
        break;
}

3. Duplicate case values

switch (grade) {
    case 'A': System.out.println("Excellent"); break;
    case 'A': // ❌ Duplicate case!
        System.out.println("Outstanding"); break;
}

Debugging Tips:

Use System.out.println() to trace execution flow
Check boolean expressions with temporary variables
Use IDE debugger to step through conditions
Test with boundary values and edge cases

Hands-on Lab Exercises

Exercise 1: Number Analysis Program

Create a program that takes an integer input
Check if the number is positive, negative, or zero
If positive, check if it's even or odd
If the number is between 1-100, categorize it as small (1-33), medium (34-66), or large (67-100)


// Template for Exercise 1
public class NumberAnalyzer {
    public static void main(String[] args) {
        int number = 42; // Test with different values
        
        // TODO: Implement the analysis logic
        // Use nested if statements and if-else-if ladder
    }
}

Exercise 2: Seasonal Activity Recommender

Create a program using switch statement
Take a month number (1-12) as input
Recommend seasonal activities based on the month
Group months by seasons and suggest appropriate activities

Exercise 3: Simple Banking System

Create a menu-driven banking program
Options: Check Balance, Deposit, Withdraw, Exit
Use switch statement for menu handling
Use if-else for validation (sufficient funds, positive amounts)

Lecture Summary

Key Concepts Covered:

if statement for single conditions
if-else for binary decisions
Nested if statements for complex logic
if-else-if ladder for multiple conditions
switch statement for discrete values
Fall-through behavior and break statements
Common errors and debugging techniques

Learning Outcomes Achieved:

✅ Master conditional statement syntax
✅ Apply appropriate selection structure
✅ Handle complex decision logic
✅ Debug conditional statement errors
✅ Write menu-driven programs
✅ Implement real-world decision systems
✅ Optimize conditional code structure

Next Lecture: Control Flow - Loop Statements

Topics: for loops, while loops, do-while loops, nested loops, loop control statements

Thank You!

Questions & Discussion

Next: Lecture 06 - Control Flow (Loop Statements)

Course: 4343203 Java Programming
Unit 1: Introduction to Java
GTU Semester 4