Lesson 1.1: FizzBuzz

Learning goals:

• Implement a small, well-tested pure function.
• Explain the arrange-act-assert pattern.

1. Introduction

FizzBuzz is a classic beginner exercise. For each number from 1 to n:

• print Fizz if it is divisible by 3
• print Buzz if it is divisible by 5
• print FizzBuzz if it is divisible by both
• otherwise print the number itself

2. Implementation (Java)

public class FizzBuzzer {
    public String fizzBuzz(int n) {
        if (n % 15 == 0) return "FizzBuzz";
        if (n % 3 == 0) return "Fizz";
        if (n % 5 == 0) return "Buzz";
        return String.valueOf(n);
    }
}

Extra slides: FizzBuzz, step by step

3. Command-line application

public class Main {
    public static void main(String[] args) {
        int limit = args.length == 0 ? 100 : Integer.parseInt(args[0]);
        FizzBuzzer fizzBuzzer = new FizzBuzzer();

        for (int number = 1; number <= limit; number++) {
            System.out.println(fizzBuzzer.fizzBuzz(number));
        }
    }
}

4. Unit test (JUnit)

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.assertEquals;

class FizzBuzzerTest {
    private final FizzBuzzer fizzBuzzer = new FizzBuzzer();

    @Test
    void divisibleByThreeGivesFizz() {
        assertEquals("Fizz", fizzBuzzer.fizzBuzz(9));
    }

    @Test
    void divisibleByFiveGivesBuzz() {
        assertEquals("Buzz", fizzBuzzer.fizzBuzz(10));
    }

    @Test
    void divisibleByFifteenGivesFizzBuzz() {
        assertEquals("FizzBuzz", fizzBuzzer.fizzBuzz(15));
    }

    @Test
    void otherNumbersReturnThemselves() {
        assertEquals("8", fizzBuzzer.fizzBuzz(8));
    }
}

5. Class diagram

The example has a Main entry point that loops from 1 to a configurable limit and delegates each value to FizzBuzzer.fizzBuzz. The default limit is 100.

PlantUML source for "5. Class diagram"

@startuml
class FizzBuzzer {
    +fizzBuzz(n: int): String
}

class Main {
    +main(args: String[]): void
}

Main ..> FizzBuzzer : uses
@enduml

Class diagram with 2 class(es) and 1 relation(s).

Classes:

• Class FizzBuzzer with:
  • public method 'fizzBuzz', with parameter(s) 'n' of type int, return type String
  • no attributes
• Class Main with:
  • public method 'main', with parameter(s) 'args' of type String[], return type void
  • no attributes

Relations:

• Main has a dependency to FizzBuzzer