Day 3: Control Flow

What You'll Learn Today

• Conditional branching (if / else)
• switch statements and switch expressions
• for loops
• while / do-while loops
• break and continue

---

if Statements

int score = 85;

if (score >= 90) {
    System.out.println("A");
} else if (score >= 80) {
    System.out.println("B");
} else if (score >= 70) {
    System.out.println("C");
} else {
    System.out.println("D");
}
// Output: B

flowchart TB
    Start["score = 85"]
    C1{"score >= 90?"}
    C2{"score >= 80?"}
    C3{"score >= 70?"}
    A["A"]
    B["B"]
    C["C"]
    D["D"]
    Start --> C1
    C1 -->|"Yes"| A
    C1 -->|"No"| C2
    C2 -->|"Yes"| B
    C2 -->|"No"| C3
    C3 -->|"Yes"| C
    C3 -->|"No"| D
    style B fill:#22c55e,color:#fff

Ternary Operator

int age = 20;
String status = (age >= 18) ? "Adult" : "Minor";
System.out.println(status); // Adult

---

switch Statements

Traditional switch Statement

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");
        break;
    default:
        System.out.println("Other");
        break;
}

Caution: Forgetting break causes fall-through, where the next case executes as well.

switch Expressions (Java 14+)

int day = 3;

String dayName = switch (day) {
    case 1 -> "Monday";
    case 2 -> "Tuesday";
    case 3 -> "Wednesday";
    case 4 -> "Thursday";
    case 5 -> "Friday";
    case 6 -> "Saturday";
    case 7 -> "Sunday";
    default -> "Unknown";
};

System.out.println(dayName); // Wednesday

Recommendation: From Java 14 onward, prefer the arrow-style switch expression. It eliminates the risk of missing break and can return a value.

Matching Multiple Values

String type = switch (day) {
    case 1, 2, 3, 4, 5 -> "Weekday";
    case 6, 7 -> "Weekend";
    default -> "Unknown";
};

Types Supported by switch

Type	Example
int, byte, short, char	case 1:
String	case "hello":
enum	case MONDAY:

---

for Loops

Basic for Loop

for (int i = 0; i < 5; i++) {
    System.out.println("i = " + i);
}
// i = 0, 1, 2, 3, 4

flowchart TB
    Init["int i = 0<br>Initialization"]
    Cond{"i < 5?"}
    Body["Loop Body<br>Execute"]
    Update["i++<br>Update"]
    End["Loop Ends"]
    Init --> Cond
    Cond -->|"true"| Body --> Update --> Cond
    Cond -->|"false"| End
    style Init fill:#3b82f6,color:#fff
    style Cond fill:#f59e0b,color:#fff
    style Body fill:#22c55e,color:#fff
    style Update fill:#8b5cf6,color:#fff

Counting Down

for (int i = 5; i > 0; i--) {
    System.out.println(i);
}
// 5, 4, 3, 2, 1

Nested for Loops

// Multiplication table
for (int i = 1; i <= 9; i++) {
    for (int j = 1; j <= 9; j++) {
        System.out.printf("%3d", i * j);
    }
    System.out.println();
}

Enhanced for Loop (for-each)

Iterates through every element in an array or collection:

String[] fruits = {"Apple", "Banana", "Cherry"};

for (String fruit : fruits) {
    System.out.println(fruit);
}

Recommendation: Use the enhanced for loop when you need to process every element and don't need the index.

---

while Loops

while

int count = 0;

while (count < 5) {
    System.out.println("count = " + count);
    count++;
}

do-while

int count = 0;

do {
    System.out.println("count = " + count);
    count++;
} while (count < 5);

Loop	Condition Check	Minimum Executions
while	Before the body	0
do-while	After the body	1

---

break and continue

break

Exits the loop immediately:

for (int i = 0; i < 10; i++) {
    if (i == 5) {
        break; // Exit when i reaches 5
    }
    System.out.println(i);
}
// 0, 1, 2, 3, 4

continue

Skips the rest of the current iteration:

for (int i = 0; i < 10; i++) {
    if (i % 2 == 0) {
        continue; // Skip even numbers
    }
    System.out.println(i);
}
// 1, 3, 5, 7, 9

Labeled break

Breaks out of nested loops all at once:

outer:
for (int i = 0; i < 3; i++) {
    for (int j = 0; j < 3; j++) {
        if (i == 1 && j == 1) {
            break outer; // Exit both loops
        }
        System.out.printf("(%d, %d) ", i, j);
    }
}
// (0, 0) (0, 1) (0, 2) (1, 0)

---

Practice: Number Guessing Game

import java.util.Random;
import java.util.Scanner;

public class NumberGuessing {
    public static void main(String[] args) {
        Random random = new Random();
        int answer = random.nextInt(100) + 1; // 1 to 100
        Scanner scanner = new Scanner(System.in);
        int attempts = 0;

        System.out.println("=== Number Guessing Game ===");
        System.out.println("Guess a number between 1 and 100.");

        while (true) {
            System.out.print("Your guess: ");
            int guess = scanner.nextInt();
            attempts++;

            if (guess == answer) {
                System.out.printf("Correct! You got it in %d attempts!%n", attempts);
                break;
            } else if (guess < answer) {
                System.out.println("Too low.");
            } else {
                System.out.println("Too high.");
            }
        }

        scanner.close();
    }
}

---

Summary

Concept	Description
if / else	Conditional branching
switch	Branch by value (arrow syntax recommended)
for	Loop with a known number of iterations
for-each	Iterate through every element in a collection
while	Loop while a condition is true
do-while	Loop that executes at least once
break	Exit a loop
continue	Skip the current iteration

Key Takeaways

1. Use switch expressions (-> syntax) to avoid missing break
2. Use the enhanced for loop to process all elements
3. while(true) + break is useful for loops with complex exit conditions
4. Labeled break lets you exit nested loops in one step

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Exercises

Exercise 1: Basic

Write a FizzBuzz program: print numbers from 1 to 100, but print "Fizz" for multiples of 3, "Buzz" for multiples of 5, and "FizzBuzz" for multiples of both.

Exercise 2: Applied

Using Scanner, repeatedly read numbers from the user. When 0 is entered, display the total and average of all previously entered numbers, then exit.

Exercise 3: Challenge

Using nested for loops, write a program that prints the following triangle pattern:

*
**
***
****
*****

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References

• Oracle - Control Flow Statements
• Oracle - Switch Expressions

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Next up: On Day 4, we'll cover Arrays and Strings. You'll learn how to work with collections of data and master string manipulation.