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About Rafal Borowiec

Rafal Borowiec
Software developer, Team Leader, Agile practitioner, occasional blogger, lecturer. Open Source enthusiast, quality oriented and open-minded.

Unit Testing exercise with FizzBuzz and Mockito

I sometimes use FizzBuzz to demonstrate the basics of unit testing to newbies. Although FizzBuzz is really simple problem, it can also be used to demonstrate more advanced unit testing techniques like mocking.

The FizzBuzz Kata:

Write a program that prints the numbers from 1 to 100. But for multiples of three print “Fizz” instead of the number and for the multiples of five print “Buzz”. For numbers which are multiples of both three and five print “FizzBuzz”“.

The possible solution to FizzBuzz algorithm:

public class FizzBuzz {

    private static final int FIVE = 5;
    private static final int THREE = 3;

    public String calculate(int number) {

        if (isDivisibleBy(number, THREE) && isDivisibleBy(number, FIVE)) {
            return "FizzBuzz";

        if (isDivisibleBy(number, THREE)) {
            return "Fizz";

        if (isDivisibleBy(number, FIVE)) {
            return "Buzz";

        return "" + number;

    private boolean isDivisibleBy(int dividend, int divisor) {
        return dividend % divisor == 0;

As the above code solves the FizzBuzz algorithm it does not solve the FizzBuzz problem. To finish it we need code to print the numbers from 1 to 100 using the algorithm. And this part of the code can be used to show the idea of mocking in JUnit with Mockito.

As the result of this exercise I ended up with a NumberPrinter that takes two arguments: Printer and NumberCalculator and has one public method to print numbers:

public class NumberPrinter {

    private NumberCalculator numberCalculator;
    private Printer printer;

    public NumberPrinter(NumberCalculator numberCalculator, Printer printer) {
        this.numberCalculator = numberCalculator;
        this.printer = printer;

    public void printNumbers(int limit) {
        if (limit < 1) {
            throw new RuntimeException("limit must be >= 1");
        for (int i = 1; i <= limit; i++) {
            try {
            } catch (Exception e) {
                // noop

public interface NumberCalculator {
    String calculate(int number);

public interface Printer {
    void print(String s);

With the interfaces introduced I have not only testable but more robust code. To test NumberPrinter I simply mock dependencies with the power and simplicity of Mockito. With Mockito annotations the configuration test code reads better.

Mockito features demonstrated:

  • creating and injecting mocks
  • stubbing methods also with setting different behavior for consecutive method calls.
  • stubbing the void method with an exception
  • verifications

Annotations used:

  • @RunWith(MockitoJUnitRunner.class) – initializes @Mocks before each test method
  • @Mock – marks a field as mock
  • @InjectMocks – marks a field on which injection should be performed
public class NumberPrinterTest {

    private Printer printer;

    private NumberCalculator numberCalculator;

    private NumberPrinter numberPrinter;

    public void printsCalculatorResultsHundredTimes() {
        // arrange
        int limit = 100;
                .thenReturn("0")  // first invocation returns "0"
                .thenReturn("1"); // other invocations return "1"
        // act
        // assert
        verify(numberCalculator, times(limit)).calculate(anyInt());
        verify(printer, times(1)).print("0");
        verify(printer, times(limit - 1)).print("1");
        verifyNoMoreInteractions(numberCalculator, printer);

    public void continuesOnCalculatorOrPrinterError() {
        // arrange
                .thenThrow(new RuntimeException())
        // stub the void method with an exception
        doThrow(new RuntimeException()).when(printer).print("3");
        // act
        // assert
        verify(numberCalculator, times(3)).calculate(anyInt());

        verifyNoMoreInteractions(numberCalculator, printer);

Enjoy Mockito!

Reference: Unit Testing exercise with FizzBuzz and Mockito from our JCG partner Rafal Borowiec at the Codeleak.pl blog.
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