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Control Flow in Java

Learn how to make decisions and repeat actions in your programs

Think of control flow like a choose-your-own-adventure book! Sometimes you make decisions (if this happens, go to page 10), sometimes you repeat things (do this 5 times), and sometimes you skip parts. Control flow lets your program make choices and repeat actions just like you do in real life!

🤔 Conditional Statements (Making Decisions)

Like choosing which path to take

❓

if Statement

Executes code only if condition is true

💡 If it's raining, take an umbrella

if (condition) { /* code */ }
🔀

if-else Statement

Chooses between two options

💡 If it's hot, wear shorts, else wear pants

if (condition) { /* code1 */ } else { /* code2 */ }
🎚️

if-else if-else

Chooses from multiple options

💡 If A grade, celebrate; else if B, good job; else study more

if (c1) { } else if (c2) { } else { }
📦

Nested if

if statements inside other if statements

💡 If it's weekend, if it's sunny, go to beach

if (c1) { if (c2) { /* code */ } }

📝 Code Examples: Conditional Statements

ConditionalDemo.java
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public class ConditionalDemo {
    public static void main(String[] args) {
        // Simple if
        int temperature = 30;
        if (temperature > 25) {
            System.out.println("It's hot! Wear shorts.");
        }
        // if-else
        int age = 16;
        if (age >= 18) {
            System.out.println("You can vote!");
        } else {
            System.out.println("Too young to vote.");
        }
        // if-else if-else
        int score = 85;
        if (score >= 90) {
            System.out.println("Grade: A - Excellent!");
        } else if (score >= 80) {
            System.out.println("Grade: B - Good job!");  // This runs
        } else if (score >= 70) {
            System.out.println("Grade: C - Fair");
        } else if (score >= 60) {
            System.out.println("Grade: D - Needs improvement");
        } else {
            System.out.println("Grade: F - Study more!");
        }
        // Nested if
        boolean hasTicket = true;
        boolean hasID = true;
        if (hasTicket) {
            if (hasID) {
                System.out.println("Welcome to the concert!");  // This runs
            } else {
                System.out.println("Need ID to enter.");
            }
        } else {
            System.out.println("Need a ticket first!");
        }
        // Logical operators in conditions
        int money = 50;
        boolean isWeekend = true;
        if (money >= 30 && isWeekend) {
            System.out.println("Let's go to the movies!");  // This runs
        }
        if (money < 20 || !isWeekend) {
            System.out.println("Stay home and relax.");
        }
    }
}

🔁 Loops (Repeating Actions)

Like doing something over and over

🔢

for Loop

Repeat a specific number of times

💡 Do 10 pushups (you know exactly how many)

for (init; condition; update) { }

When you know how many times to repeat

⏰

while Loop

Repeat while condition is true

💡 Keep eating while you're hungry

while (condition) { }

When you don't know how many times

🔄

do-while Loop

Do first, then check condition

💡 Try the food first, then decide if you want more

do { } while (condition);

When you want to run at least once

📜

Enhanced for Loop

Loop through collections easily

💡 Check each item in your backpack

for (Type item : collection) { }

When iterating through arrays/collections

📝 Code Examples: Loops

LoopDemo.java
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public class LoopDemo {
    public static void main(String[] args) {
        // FOR LOOP - Know exact number of iterations
        System.out.println("Counting 1 to 5:");
        for (int i = 1; i <= 5; i++) {
            System.out.println(i);
        }
        // Loop through array
        int[] numbers = {10, 20, 30, 40, 50};
        System.out.println("\nArray elements:");
        for (int i = 0; i < numbers.length; i++) {
            System.out.println("Index " + i + ": " + numbers[i]);
        }
        // WHILE LOOP - Don't know exact iterations
        System.out.println("\nGuessing game:");
        int target = 7;
        int guess = 1;
        while (guess != target) {
            System.out.println("Guessing: " + guess);
            guess += 2;
        }
        System.out.println("Found it! " + guess);
        // Reading input (conceptual example)
        int countdown = 5;
        while (countdown > 0) {
            System.out.println("Launch in: " + countdown);
            countdown--;
        }
        System.out.println("Blast off!");
        // DO-WHILE LOOP - Run at least once
        System.out.println("\nDo-while example:");
        int password = 1234;
        int attempt;
        do {
            attempt = 5678;  // Simulating user input
            System.out.println("Trying password: " + attempt);
            if (attempt != password) {
                System.out.println("Wrong! Try again.");
            }
        } while (attempt != password && attempt > 0);
        // ENHANCED FOR LOOP (for-each) - Easy iteration
        String[] fruits = {"Apple", "Banana", "Orange", "Mango"};
        System.out.println("\nFruits in basket:");
        for (String fruit : fruits) {
            System.out.println("- " + fruit);
        }
        // Nested loops - Multiplication table
        System.out.println("\n5x5 Multiplication Table:");
        for (int row = 1; row <= 5; row++) {
            for (int col = 1; col <= 5; col++) {
                System.out.print((row * col) + "\t");
            }
            System.out.println();  // New line after each row
        }
    }
}

⏭️ Jump Statements (Skipping and Stopping)

Like taking shortcuts or saying 'I'm done!'

break

Exit the loop immediately

💡 Stop searching once you find what you need

continue

Skip to next iteration

💡 Skip this song and go to the next one

return

Exit the method immediately

💡 Leave the building and go home

📝 Code Examples: Jump Statements

JumpDemo.java
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public class JumpDemo {
    public static void main(String[] args) {
        // BREAK - Exit loop immediately
        System.out.println("Finding first even number:");
        int[] numbers = {1, 3, 5, 8, 9, 10, 11};
        for (int num : numbers) {
            if (num % 2 == 0) {
                System.out.println("Found: " + num);
                break;  // Stop searching once found
            }
            System.out.println("Checking: " + num);
        }
        // CONTINUE - Skip to next iteration
        System.out.println("\nPrint only positive numbers:");
        int[] values = {-2, 3, -5, 7, -8, 10};
        for (int val : values) {
            if (val < 0) {
                continue;  // Skip negative numbers
            }
            System.out.println(val);  // Only prints: 3, 7, 10
        }
        // Skip multiples of 3
        System.out.println("\nNumbers from 1-10 (skip multiples of 3):");
        for (int i = 1; i <= 10; i++) {
            if (i % 3 == 0) {
                continue;  // Skip 3, 6, 9
            }
            System.out.print(i + " ");  // Prints: 1 2 4 5 7 8 10
        }
        // RETURN - Exit method
        int result = findFirstNegative(values);
        System.out.println("\n\nFirst negative: " + result);
        // Labeled break (advanced)
        System.out.println("\nLabeled break example:");
        outerLoop:
        for (int i = 1; i <= 3; i++) {
            for (int j = 1; j <= 3; j++) {
                System.out.println("i=" + i + ", j=" + j);
                if (i == 2 && j == 2) {
                    break outerLoop;  // Breaks out of both loops!
                }
            }
        }
    }
    // Method demonstrating return
    public static int findFirstNegative(int[] arr) {
        for (int num : arr) {
            if (num < 0) {
                return num;  // Exit method and return value
            }
        }
        return 0;  // No negative found
    }
}

🎛️ Switch Statement (Multiple Choices)

Like a menu with many options

SwitchDemo.java
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public class SwitchDemo {
    public static void main(String[] args) {
        // Basic switch
        int day = 3;
        switch (day) {
            case 1:
                System.out.println("Monday");
                break;
            case 2:
                System.out.println("Tuesday");
                break;
            case 3:
                System.out.println("Wednesday");  // This runs
                break;
            case 4:
                System.out.println("Thursday");
                break;
            case 5:
                System.out.println("Friday");
                break;
            default:
                System.out.println("Weekend!");
        }
        // Multiple cases, same action
        char grade = 'B';
        switch (grade) {
            case 'A':
            case 'B':
                System.out.println("Excellent!");  // This runs
                break;
            case 'C':
                System.out.println("Good");
                break;
            case 'D':
                System.out.println("Passed");
                break;
            case 'F':
                System.out.println("Failed");
                break;
            default:
                System.out.println("Invalid grade");
        }
        // Switch with String
        String fruit = "Apple";
        switch (fruit) {
            case "Apple":
                System.out.println("Red and sweet");  // This runs
                break;
            case "Banana":
                System.out.println("Yellow and soft");
                break;
            case "Orange":
                System.out.println("Orange and juicy");
                break;
            default:
                System.out.println("Unknown fruit");
        }
        // Switch expression (Java 12+)
        String season = "Summer";
        String activity = switch (season) {
            case "Spring" -> "Plant flowers";
            case "Summer" -> "Go swimming";
            case "Fall" -> "Collect leaves";
            case "Winter" -> "Build snowman";
            default -> "Stay inside";
        };
        System.out.println("Activity: " + activity);
        // Calculator example
        int a = 10, b = 5;
        char operator = '+';
        int result;
        switch (operator) {
            case '+':
                result = a + b;
                System.out.println("Result: " + result);  // 15
                break;
            case '-':
                result = a - b;
                System.out.println("Result: " + result);
                break;
            case '*':
                result = a * b;
                System.out.println("Result: " + result);
                break;
            case '/':
                result = a / b;
                System.out.println("Result: " + result);
                break;
            default:
                System.out.println("Invalid operator");
        }
    }
}

🌟 Real-World Examples

RealWorldExamples.java
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public class RealWorldExamples {
    public static void main(String[] args) {
        // Example 1: Validate password strength
        String password = "Pass123!";
        boolean hasUpper = false, hasLower = false, hasDigit = false;
        for (int i = 0; i < password.length(); i++) {
            char ch = password.charAt(i);
            if (Character.isUpperCase(ch)) hasUpper = true;
            if (Character.isLowerCase(ch)) hasLower = true;
            if (Character.isDigit(ch)) hasDigit = true;
        }
        if (hasUpper && hasLower && hasDigit && password.length() >= 8) {
            System.out.println("Strong password!");
        } else {
            System.out.println("Weak password. Needs improvement.");
        }
        // Example 2: Find prime numbers
        System.out.println("\nPrime numbers from 1 to 20:");
        for (int num = 2; num <= 20; num++) {
            boolean isPrime = true;
            for (int i = 2; i <= Math.sqrt(num); i++) {
                if (num % i == 0) {
                    isPrime = false;
                    break;  // Not prime, stop checking
                }
            }
            if (isPrime) {
                System.out.print(num + " ");
            }
        }
        // Example 3: Calculate sum of array
        int[] scores = {85, 92, 78, 95, 88};
        int sum = 0;
        for (int score : scores) {
            sum += score;
        }
        double average = (double) sum / scores.length;
        System.out.println("\n\nAverage score: " + average);
        // Example 4: Pattern printing
        System.out.println("\nRight triangle:");
        for (int i = 1; i <= 5; i++) {
            for (int j = 1; j <= i; j++) {
                System.out.print("* ");
            }
            System.out.println();
        }
        // Example 5: Reverse a string
        String text = "Hello";
        String reversed = "";
        for (int i = text.length() - 1; i >= 0; i--) {
            reversed += text.charAt(i);
        }
        System.out.println("\nOriginal: " + text);
        System.out.println("Reversed: " + reversed);
    }
}

💼 Interview Tips

  • 💡Know the difference between for, while, and do-while loops
  • 💡Understand when to use break vs continue
  • 💡Remember: do-while runs at least once, while may not run at all
  • 💡Be careful with infinite loops (condition always true)
  • 💡Switch works with int, char, String, and enums
  • 💡Enhanced for loop (for-each) can't modify the collection
  • 💡Nested loops: understand time complexity (O(n²) for double loop)

⚠️ Common Mistakes

  • ✗Using = instead of == in conditions (if (x = 5) assigns, not compares)
  • ✗Forgetting break in switch statements (causes fall-through)
  • ✗Creating infinite loops accidentally (while(true) without break)
  • ✗Off-by-one errors in for loops (starting at 1 instead of 0)
  • ✗Modifying loop variable inside enhanced for loop
  • ✗Using semicolon after if/while condition: if (x > 5); { }
  • ✗Not considering edge cases (empty arrays, null values)