Java Basics #3 — Control Flow: If, Switch & Loops

If / Else If / Else

int score = 85;

if (score >= 90) {
    System.out.println("A");
} else if (score >= 80) {
    System.out.println("B");  // this runs
} else if (score >= 70) {
    System.out.println("C");
} else {
    System.out.println("F");
}

Ternary operator — for simple one-liners:

String result = score >= 60 ? "Pass" : "Fail";

ℹDon't nest ternaries

A single ternary is readable. Nested ternaries like a ? b : c ? d : e are not. Use if/else when logic gets complex.

Switch — Old vs New Style

Old switch (statement) — still works, but error-prone:

int day = 3;
switch (day) {
    case 1:
        System.out.println("Monday");
        break;  // ⚠ forget this and it falls through to next case
    case 2:
        System.out.println("Tuesday");
        break;
    default:
        System.out.println("Other");
}

New switch expression (Java 14+) — use this instead:

String dayName = switch (day) {
    case 1 -> "Monday";
    case 2 -> "Tuesday";
    case 3 -> "Wednesday";
    case 4, 5 -> "Thu or Fri";   // multiple labels
    default -> "Weekend";
};

Switch with blocks (for multi-line logic):

int result = switch (day) {
    case 1, 2, 3, 4, 5 -> {
        System.out.println("Weekday");
        yield day * 2;  // yield returns a value from a block
    }
    default -> 0;
};

Switch on Strings (Java supports this, unlike C/C++):

String role = "ADMIN";
String access = switch (role) {
    case "ADMIN"  -> "Full access";
    case "USER"   -> "Read-only";
    case "GUEST"  -> "Public only";
    default       -> "No access";
};

For Loop

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

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

// Step by 2
for (int i = 0; i <= 10; i += 2) {
    System.out.println(i);  // 0, 2, 4, 6, 8, 10
}

For-Each Loop — Use This for Collections

int[] numbers = {10, 20, 30, 40, 50};

for (int num : numbers) {
    System.out.println(num);
}

// Works on any Iterable
List<String> names = List.of("Alice", "Bob", "Charlie");
for (String name : names) {
    System.out.println(name);
}

ℹPrefer for-each over indexed loops

When you don't need the index, for-each is cleaner and less error-prone. Use the classic for only when you actually need i.

While & Do-While

// while — checks condition BEFORE executing
int count = 0;
while (count < 3) {
    System.out.println(count);  // 0, 1, 2
    count++;
}

// do-while — executes ONCE then checks condition
int x = 10;
do {
    System.out.println(x);  // prints 10 even though condition is false immediately
    x++;
} while (x < 5);

Break & Continue

// break — exits the loop entirely
for (int i = 0; i < 10; i++) {
    if (i == 5) break;
    System.out.println(i);  // 0, 1, 2, 3, 4
}

// continue — skips 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 — for breaking out of nested loops:

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

Putting It Together — A Real Example

public class FizzBuzz {
    public static void main(String[] args) {
        for (int i = 1; i <= 20; i++) {
            String output = switch (0) {
                case 0 when i % 15 == 0 -> "FizzBuzz";
                case 0 when i % 3 == 0  -> "Fizz";
                case 0 when i % 5 == 0  -> "Buzz";
                default                  -> String.valueOf(i);
            };
            System.out.println(output);
        }
    }
}

✓Pattern matching switch (Java 21)

Java 21 adds pattern matching to switch — you can match on types and conditions together. We'll cover this in the Advanced Java section.

What's Next?

In Java Basics #4 we cover methods and arrays — how to structure reusable logic and work with fixed-size data.