Control Flow in Java

The if statement is the simplest decision-maker. It checks a condition (something that's either true or false), and if it's true, it runs the code inside the curly braces. If it's false, it skips that code entirely.

Here's the pattern you'll use thousands of times:

if (condition) {
    // This code ONLY runs when condition is true
}
// Code here runs no matter what

// Real example:
int temperature = 35;
if (temperature > 30) {
    System.out.println("It's hot outside! ☀️");
}
// Output: It's hot outside! ☀️

int temperature2 = 15;
if (temperature2 > 30) {
    System.out.println("It's hot outside!");
}
// Output: (nothing — the condition was false, so it was skipped)

A plain if only handles the "yes" case. But what if you want to do something different when the condition is false? That's where else comes in. It's like saying "if this is true, do A; otherwise, do B."

int age = 20;

if (age >= 18) {
    System.out.println("You are an adult. You can vote! 🗳️");
} else {
    System.out.println("You are a minor. You can't vote yet.");
}
// Output: You are an adult. You can vote! 🗳️

// Try changing age to 15 — now the else block runs instead

Exactly one block always runs. If the condition is true, the if block runs and the else block is skipped. If the condition is false, the if block is skipped and the else block runs. There's no scenario where both run, or neither runs.

What if you have more than two possibilities? For example, a grade calculator needs to check A, B, C, D, and F — that's five options, not just two. Use else if to chain conditions together:

int score = 85;
String grade;

if (score >= 90) {
    grade = "A";        // 90-100
} else if (score >= 80) {
    grade = "B";        // 80-89
} else if (score >= 70) {
    grade = "C";        // 70-79
} else if (score >= 60) {
    grade = "D";        // 60-69
} else {
    grade = "F";        // Below 60
}

System.out.println("Score: " + score + " → Grade: " + grade);
// Output: Score: 85 → Grade: B

Rule of thumb: Put the most specific (or highest) condition first, then work your way down to the most general. The final else acts as a "catch everything else" safety net.

Sometimes you need to check a condition, and then check another condition inside that. This is called nesting. Think of it like a security checkpoint: first, check if you have a ticket. If yes, then check if it's VIP or regular.

boolean hasTicket = true;
boolean isVIP = true;

if (hasTicket) {
    System.out.println("Welcome to the event!");
    
    if (isVIP) {
        System.out.println("Please proceed to the VIP area. 🥂");
    } else {
        System.out.println("Please take a seat in the general area.");
    }
} else {
    System.out.println("Sorry, you need a ticket to enter. 🚫");
}

// Instead of nesting:
if (hasTicket && isVIP) {
    System.out.println("VIP access granted");
}

When you're comparing one variable against many exact values, a switch statement is cleaner than a long if-else-if chain. It's like a vending machine — you press a button (A1, A2, B1), and the machine gives you the matching item.

String day = "Wednesday";

switch (day) {
    case "Monday":
        System.out.println("Back to work! 💼");
        break;
    case "Tuesday":
    case "Wednesday":
    case "Thursday":
        System.out.println("Midweek grind 💪");
        break;
    case "Friday":
        System.out.println("TGIF! 🎉");
        break;
    case "Saturday":
    case "Sunday":
        System.out.println("Weekend! 🏖️");
        break;
    default:
        System.out.println("Not a valid day");
}
// Output: Midweek grind 💪

How it works step by step:

1. Java evaluates the expression in switch(day)
2. It jumps to the case that matches the value
3. It runs the code until it hits a break statement
4. If no case matches, it runs the default block (optional but recommended)

The #1 bug beginners make with switch is forgetting break. Without it, Java doesn't stop at the matching case — it "falls through" and runs ALL the cases below it too!

// ❌ BUG: Missing break statements!
int num = 1;
switch (num) {
    case 1:
        System.out.println("One");
        // No break! Falls through to case 2...
    case 2:
        System.out.println("Two");
        // No break! Falls through to case 3...
    case 3:
        System.out.println("Three");
}
// Output: One  Two  Three  (ALL three print!)

// ✅ FIXED: Add break after each case
switch (num) {
    case 1:
        System.out.println("One");
        break;   // ← Stops here!
    case 2:
        System.out.println("Two");
        break;
    case 3:
        System.out.println("Three");
        break;
}
// Output: One  (only the matching case)

String result = switch (day) {
    case "Monday" -> "Start of week";
    case "Friday" -> "TGIF!";
    case "Saturday", "Sunday" -> "Weekend!";
    default -> "Midweek";
};

Sometimes you just need to pick between two values based on a simple condition. Writing a full if/else for that feels like overkill. The ternary operator is a one-line shortcut:

// Syntax: condition ? valueIfTrue : valueIfFalse

// Instead of this:
String status;
if (age >= 18) {
    status = "Adult";
} else {
    status = "Minor";
}

// You can write this:
String status = (age >= 18) ? "Adult" : "Minor";

// More examples:
int max = (a > b) ? a : b;                    // Find the bigger number
String label = (count == 1) ? "item" : "items"; // Pluralize
double price = isMember ? 9.99 : 14.99;        // Membership discount

Each control flow tool has its sweet spot. Using the right one makes your code cleaner and easier to read:

Let's combine everything you've learned into a realistic example. Here's a login system that checks multiple conditions:

String username = "admin";
String password = "secret123";
boolean isAccountLocked = false;
int failedAttempts = 0;

// Step 1: Check if account is locked
if (isAccountLocked) {
    System.out.println("🔒 Account is locked. Contact support.");

// Step 2: Check credentials
} else if (username.equals("admin") && password.equals("secret123")) {
    System.out.println("✅ Login successful! Welcome, " + username);
    failedAttempts = 0;  // Reset on success

// Step 3: Wrong password (right username)
} else if (username.equals("admin")) {
    failedAttempts++;
    System.out.println("❌ Wrong password! Attempts: " + failedAttempts);
    
    if (failedAttempts >= 3) {
        isAccountLocked = true;
        System.out.println("🔒 Too many attempts. Account locked.");
    }

// Step 4: Unknown user
} else {
    System.out.println("❌ User '" + username + "' not found.");
}

Notice how this uses nested if (the failed attempts check inside the else-if), logical AND (&&), and String comparison with .equals(). This is the kind of real code you'll write in actual applications.