Functional FizzBuzz Kata in Java

A while ago I solved the FizzBuzz kata using Java 8 streams and lambdas. While the end result was functional, the intermediate steps were not. Surely I can do better.

As always, let’s start with a failing test:

package remonsinnema.blog.fizzbuzz;
+
+ import static org.junit.Assert.assertEquals;
+
+ import org.junit.Test;
+
+
+ public class WhenFunctionallyFuzzingAndBuzzing {
+
+   private final FizzBuzzer fizzBuzzer = new FizzBuzzer();
+
+   @Test
+   public void shouldReplaceMultiplesOfThreeWithFizzAndMultiplesOfFiveWithBuzz() {
+     assertEquals(“1”, “1”, fizzBuzzer.apply(1));
+   }
+
+ }

 package remonsinnema.blog.fizzbuzz;
+
+ import java.util.function.Function;
+
+
+ public class FizzBuzzer implements Function<Integer, String> {
+
+   @Override
+   public String apply(Integer n) {
+     return null;
+   }
+
+ }

Note that I start off on a functional course right away, using Java’s Function.

I fake the implementation to make the test pass:

public class FizzBuzzer implements Function<Integer, String> {
    @Override
    public String apply(Integer n) {
–     return null;
+     return “1”;
    }
  }

And refactor the test to remove duplication:

public class WhenFunctionallyFuzzingAndBuzzing {
    @Test
    public void shouldReplaceMultiplesOfThreeWithFizzAndMultiplesOfFiveWithBuzz() {
–     assertEquals(“1”, “1”, fizzBuzzer.apply(1));
+     assertFizzBuzz(“1”, 1);
+   }
+
+   private void assertFizzBuzz(String expected, int value) {
+     assertEquals(Integer.toString(value), expected, fizzBuzzer.apply(value));
    }
  }

Then I add another test to generalize the implementation:

public class WhenFunctionallyFuzzingAndBuzzing {
    @Test
    public void shouldReplaceMultiplesOfThreeWithFizzAndMultiplesOfFiveWithBuzz() {
      assertFizzBuzz(“1”, 1);
+     assertFizzBuzz(“2”, 2);
    }
    private void assertFizzBuzz(String expected, int value) {
  public class FizzBuzzer implements Function<Integer, String> {
    @Override
    public String apply(Integer n) {
–     return “1”;
+     return Integer.toString(n);
    }
  }

OK, pretty standard stuff so far. Next I need to replace 3 with “Fizz”:

public class WhenFunctionallyFuzzingAndBuzzing {
    public void shouldReplaceMultiplesOfThreeWithFizzAndMultiplesOfFiveWithBuzz() {
      assertFizzBuzz(“1”, 1);
      assertFizzBuzz(“2”, 2);
+     assertFizzBuzz(“Fizz”, 3);
    }
  nbsp;
    private void assertFizzBuzz(String expected, int value) {
  public class FizzBuzzer implements Function<Integer, String> {
    @Override
    public String apply(Integer n) {
–     return Integer.toString(n);
+     return numberReplacerFor(n).apply(n);
+   }
+
+   private Function<Integer, String> numberReplacerFor(Integer n) {
+     return n == 3
+         ? i -> “Fizz”
+         : i -> Integer.toString(i);
    }
  }

Here I recognize that I need to apply one of two functions, depending on the input. This code works, but needs some cleaning up. First, as a stepping stone, I extract the lambdas into fields:

import java.util.function.Function;
  public class FizzBuzzer implements Function<Integer, String> {
+   private final Function<Integer, String> replaceNumberWithStringRepresentation
+       = n -> Integer.toString(n);
+   private final Function<Integer, String> replaceNumberWithFizz
+       = n -> “Fizz”;
+
    @Override
    public String apply(Integer n) {
      return numberReplacerFor(n).apply(n);
    private Function<Integer, String> numberReplacerFor(Integer n) {
      return n == 3
–         ? i -> “Fizz”
–         : i -> Integer.toString(i);
+         ? replaceNumberWithFizz
+         : replaceNumberWithStringRepresentation;
    }
  }

Next I emphasize that “3” and “Fizz” go together by extracting a class:

 public class FizzBuzzer implements Function<Integer, String> {
    private final Function<Integer, String> replaceNumberWithStringRepresentation
        = n -> Integer.toString(n);
–   private final Function<Integer, String> replaceNumberWithFizz
–       = n -> “Fizz”;
+   private final Fizzer replaceNumberWithFizz = new Fizzer();
    @Override
    public String apply(Integer n) {
    }
    private Function<Integer, String> numberReplacerFor(Integer n) {
–     return n == 3
+     return replaceNumberWithFizz.test(n)
          ? replaceNumberWithFizz
          : replaceNumberWithStringRepresentation;
    }
+ package remonsinnema.blog.fizzbuzz;
+
+ import java.util.function.Function;
+ import java.util.function.Predicate;
+
+
+ public class Fizzer implements Function<Integer, String>, Predicate<Integer> {
+
+   @Override
+   public boolean test(Integer n) {
+     return n == 3;
+   }
+
+   @Override
+   public String apply(Integer n) {
+     return “Fizz”;
+   }
+
+ }

Here I’m using the standard Java Predicate functional interface.

To add “Buzz”, I need to generalize the code from a single if (hidden as the ternary operator) to a loop:

public class WhenFunctionallyFuzzingAndBuzzing {
      assertFizzBuzz(“1”, 1);
      assertFizzBuzz(“2”, 2);
      assertFizzBuzz(“Fizz”, 3);
+     assertFizzBuzz(“4”, 4);
+     assertFizzBuzz(“Buzz”, 5);
    }
    private void assertFizzBuzz(String expected, int value) {
  package remonsinnema.blog.fizzbuzz;
+ import java.util.Arrays;
+ import java.util.Collection;
  import java.util.function.Function;
    private final Function<Integer, String> replaceNumberWithStringRepresentation
        = n -> Integer.toString(n);
–   private final Fizzer replaceNumberWithFizz = new Fizzer();
+   private final Collection<ReplaceNumberWithFixedText> replacers = Arrays.asList(
+       new ReplaceNumberWithFixedText(3, “Fizz”),
+       new ReplaceNumberWithFixedText(5, “Buzz”)
+   );
    @Override
    public String apply(Integer n) {
    }
    private Function<Integer, String> numberReplacerFor(Integer n) {
–     return replaceNumberWithFizz.test(n)
–         ? replaceNumberWithFizz
–         : replaceNumberWithStringRepresentation;
+     for (ReplaceNumberWithFixedText replacer : replacers) {
+       if (replacer.test(n)) {
+         return replacer;
+       }
+     }
+     return replaceNumberWithStringRepresentation;
    }
  }

package remonsinnema.blog.fizzbuzz;
–
– import java.util.function.Function;
– import java.util.function.Predicate;
–
–
– public class Fizzer implements Function<Integer, String>, Predicate<Integer> {
–
–   @Override
–   public boolean test(Integer n) {
–     return n == 3;
–   }
–
–   @Override
–   public String apply(Integer n) {
–     return “Fizz”;
–   }
–
– }

package remonsinnema.blog.fizzbuzz;
+
+ import java.util.function.Function;
+ import java.util.function.Predicate;
+
+
+ public class ReplaceNumberWithFixedText implements Function<Integer, String>,
+     Predicate<Integer> {
+
+   private final int target;
+   private final String replacement;
+
+   public ReplaceNumberWithFixedText(int target, String replacement) {
+     this.target = target;
+     this.replacement = replacement;
+   }
+
+   @Override
+   public boolean test(Integer n) {
+     return n == target;
+   }
+
+   @Override
+   public String apply(Integer n) {
+     return replacement;
+   }
+
+ }

Oops, old habits… That should be a stream rather than a loop:

import java.util.function.Function;
  public class FizzBuzzer implements Function<Integer, String> {
–   private final Function<Integer, String> replaceNumberWithStringRepresentation
+   private final Function<Integer, String> defaultReplacer
        = n -> Integer.toString(n);
    private final Collection<ReplaceNumberWithFixedText> replacers = Arrays.asList(
        new ReplaceNumberWithFixedText(3, “Fizz”),
    }
    private Function<Integer, String> numberReplacerFor(Integer n) {
–     for (ReplaceNumberWithFixedText replacer : replacers) {
–       if (replacer.test(n)) {
–         return replacer;
–       }
–     }
–     return replaceNumberWithStringRepresentation;
+     return replacers.stream()
+         .filter(replacer -> replacer.test(n))
+         .map(replacer -> (Function<Integer, String>) replacer)
+         .findFirst()
+         .orElse(defaultReplacer);
    }
  }

Much better. The next test is for multiples:

public class WhenFunctionallyFuzzingAndBuzzing {
      assertFizzBuzz(“Fizz”, 3);
      assertFizzBuzz(“4”, 4);
      assertFizzBuzz(“Buzz”, 5);
+     assertFizzBuzz(“Fizz”, 6);
    }
    private void assertFizzBuzz(String expected, int value) {
  public class FizzBuzzer implements Function<Integer, String> {
    private final Function<Integer, String> defaultReplacer
        = n -> Integer.toString(n);
–   private final Collection<ReplaceNumberWithFixedText> replacers = Arrays.asList(
–       new ReplaceNumberWithFixedText(3, “Fizz”),
–       new ReplaceNumberWithFixedText(5, “Buzz”)
+   private final Collection<ReplaceMultipleWithFixedText> replacers = Arrays.asList(
+       new ReplaceMultipleWithFixedText(3, “Fizz”),
+       new ReplaceMultipleWithFixedText(5, “Buzz”)
    );
    @Override

+ package remonsinnema.blog.fizzbuzz;
+
+ import java.util.function.Function;
+ import java.util.function.Predicate;
+
+
+ public class ReplaceNumberWithFixedText implements Function<Integer, String>,
+     Predicate<Integer> {
+
+   private final int target;
+   private final String replacement;
+
+   public ReplaceNumberWithFixedText(int target, String replacement) {
+     this.target = target;
+     this.replacement = replacement;
+   }
+
+   @Override
+   public boolean test(Integer n) {
+     return n % target == 0;
+   }
+
+   @Override
+   public String apply(Integer n) {
+     return replacement;
+   }
+
+ }

– package remonsinnema.blog.fizzbuzz;
–
– import java.util.function.Function;
– import java.util.function.Predicate;
–
–
– public class ReplaceNumberWithFixedText implements Function<Integer, String>, Predicate<Integer> {
–
–   private final int target;
–   private final String replacement;
–
–   public ReplaceNumberWithFixedText(int target, String replacement) {
–     this.target = target;
–     this.replacement = replacement;
–   }
–
–   @Override
–   public boolean test(Integer n) {
–     return n == target;
–   }
–
–   @Override
–   public String apply(Integer n) {
–     return replacement;
–   }
–
– }

The last test is to combine Fizz and Buzz:

public class WhenFunctionallyFuzzingAndBuzzing {
      assertFizzBuzz(“4”, 4);
      assertFizzBuzz(“Buzz”, 5);
      assertFizzBuzz(“Fizz”, 6);
+     assertFizzBuzz(“7”, 7);
+     assertFizzBuzz(“8”, 8);
+     assertFizzBuzz(“Fizz”, 9);
+     assertFizzBuzz(“Buzz”, 10);
+     assertFizzBuzz(“11”, 11);
+     assertFizzBuzz(“Fizz”, 12);
+     assertFizzBuzz(“13”, 13);
+     assertFizzBuzz(“14”, 14);
+     assertFizzBuzz(“FizzBuzz”, 15);
    }
    private void assertFizzBuzz(String expected, int value) {
  package remonsinnema.blog.fizzbuzz;
  import java.util.Arrays;
  import java.util.Collection;
  import java.util.function.Function;
+ import java.util.stream.Collectors;
+ import java.util.stream.Stream;
  public class FizzBuzzer implements Function<Integer, String> {
    @Override
    public String apply(Integer n) {
–     return numberReplacerFor(n).apply(n);
+     return numberReplacersFor(n)
+         .map(function -> function.apply(n))
+         .collect(Collectors.joining());
    }
–   private Function<Integer, String> numberReplacerFor(Integer n) {
–     return replacers.stream()
+   private Stream<Function<Integer, String>> numberReplacersFor(Integer n) {
+     return Stream.of(replacers.stream()
          .filter(replacer -> replacer.test(n))
          .map(replacer -> (Function<Integer, String>) replacer)
          .findFirst()
–         .orElse(defaultReplacer);
+         .orElse(defaultReplacer));
    }
  }

I generalized the single Function into a Stream of Functions, to which I apply the Map-Reduce pattern. I could have spelled out the Reduce part using something like .reduce("", (a, b) -> a + b), but I thinkCollectors.joining() is more expressive.

This doesn’t pass the test yet, since I return a stream of a single function. The fix is a little bit tricky, because I need to know whether any applicable replacer functions were found, and you can’t do that without terminating the stream. So I need to create a new stream using StreamSupport:

package remonsinnema.blog.fizzbuzz;
  import java.util.Arrays;
  import java.util.Collection;
+ import java.util.Iterator;
+ import java.util.Spliterators;
  import java.util.function.Function;
  import java.util.stream.Collectors;
  import java.util.stream.Stream;
+ import java.util.stream.StreamSupport;
  public class FizzBuzzer implements Function<Integer, String> {
    }
    private Stream<Function<Integer, String>> numberReplacersFor(Integer n) {
–     return Stream.of(replacers.stream()
+     Iterator<Function<Integer, String>> result = replacers.stream()
          .filter(replacer -> replacer.test(n))
          .map(replacer -> (Function<Integer, String>) replacer)
–         .findFirst()
–         .orElse(defaultReplacer));
+         .iterator();
+     return result.hasNext()
+         ? StreamSupport.stream(Spliterators.spliteratorUnknownSize(result, 0), false)
+         : Stream.of(defaultReplacer);
    }
  }

And that’s it. The full code is on GitHub.

I learned two lessons from this little exercise:

1. Java comes with a whole bunch of functional interfaces, like Functionand Predicate, that are easily combined with streams to solve a variety of problems.
2. The standard if → while transformation becomes if → stream in the functional world.

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