不久前，我使用Java 8流和lambda解决了FizzBu​​zz kata问题。 尽管最终结果是可行的，但中间步骤却没有。 我当然可以做得更好。

与往常一样，让我们​​从失败的测试开始：

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;
+   }
+
+ }

请注意，我马上就开始使用Java的Function进行功能课程。

我伪造实现以使测试通过：

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

并重构测试以消除重复：

public class WhenFunctionallyFuzzingAndBuzzing {@Testpublic 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));}}

然后，我添加另一个测试以概括实现：

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

好的，到目前为止，非常标准的东西。 接下来，我需要将3替换为“ 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> {@Overridepublic 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);}}

在这里，我认识到我需要根据输入应用两个函数之一。 该代码有效，但需要进行一些清理。 首先，作为垫脚石，我将lambda提取到各个字段中：

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”;
+@Overridepublic 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;}}

接下来，我强​​调通过提取一个类将“ 3”和“ Fizz”结合在一起：

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();@Overridepublic 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”;
+   }
+
+ }

在这里，我使用标准的Java Predicate功能接口。

要添加“ Buzz”，我需要将代码从单个if （隐藏为三元运算符）推广到循环：

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”)
+   );@Overridepublic 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;
+   }
+
+ }

糟糕，旧习惯……那应该是一个循环而不是循环：

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);}}

好多了。 下一个测试是用于倍数的：

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;
–   }
–
– }

最后一个测试是结合Fizz和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> {@Overridepublic 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));}}

我概括了单一Function到一个Stream的Function S，而我应用的map-reduce模式。 我本可以使用.reduce("", (a, b) -> a + b)类的东西来拼写Reduce的部分，但我认为Collectors.joining()更具表现力。

这还没有通过测试，因为我返回了单个函数的流。 该修复有些棘手，因为我需要知道是否找到了任何适用的替换器函数，并且您必须在不终止流的情况下才能做到这一点 。 所以我需要使用StreamSupport创建一个新的流：

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);}}

就是这样。 完整代码在GitHub上 。

我从这个小练习中学到了两个教训：

1. Java附带了一堆功能接口，例如Function和Predicate ，可以很容易地与流结合以解决各种问题。
2. 在功能世界中， if → while 转换成为if → stream的标准。

翻译自: https://www.javacodegeeks.com/2016/08/functional-fizzbuzz-kata-java.html