Try an If expression in Java

Preface

This article was impulsively created under the influence of this article. 4-year-old daughter "Daddy, use only const?"

Text

Do you like Java, everyone?

I didn't like it very much until recently, but nowadays I like it a lot.

There are a lot of libraries, modern syntax and syntax sugar.

However, I can't accept that there is no If expression, so I made something like that.

Finished product

public class Iff<T, R> {
  Map<Predicate<T>, Supplier<R>> evaluations = new LinkedHashMap<>();
  private Supplier<R> elsef;

  public Iff(Predicate<T> condition, Supplier<R> then) {
    evaluations.put(condition, then);
  }

  public Iff<T, R> elseIf(Predicate<T> condition, Supplier<R> then) {
    evaluations.put(condition, then);
    return this;
  }

  public Iff<T, R> elsef(Supplier<R> elsef) {
    this.elsef = elsef;
    return this;
  }

  public Optional<R> eval(T target) {
    for (Map.Entry<Predicate<T>, Supplier<R>> entry : this.evaluations.entrySet()) {
      if (entry.getKey().test(target)) {
        var value = entry.getValue().get();
        return Objects.isNull(value) ? Optional.empty() : Optional.of(value);
      }
    }
    if (this.elsef == null) {
      return Optional.empty();
    } else {
      var result = this.elsef.get();
      if (Objects.isNull(result)) {
        return Optional.empty();
      }
      return Optional.of(result);
    }
  }
}

It is a class that takes a T parameter as an argument, checks it, and returns a value of ʻOptional ` (really this is an if expression ...?)

How to use

I wrote a sample as a test.

class IffTest {

  @Test
  public void testIf() {
    //Evaluate a string and return an Integer
    var result = new Iff<String, Integer>((str) -> str.startsWith("hoge"), () -> 100).eval("hogeFuga");
    Assertions.assertEquals(result.get(), 100);
  }

  @Test
  public void testIfElseFirstMatch() {
    //You can add elseIf as many times as you like with the elseIf method.
    var result = new Iff<String, Integer>((str) -> str.startsWith("hoge"), () -> 100)
      .elseIf((str) -> str.startsWith("hogeFuga"), () -> 200)
      .elseIf((str) -> str.startsWith("hoho"), () -> 300)
      .eval("hogeFuga");
    Assertions.assertEquals(result.get(), 100);
  }

  @Test
  public void noMatch() {
    var result = new Iff<String, Integer>((str) -> str.startsWith("hoge"), () -> 100)
      .elseIf((str) -> str.startsWith("hogeFuga"), () -> 200)
      .elseIf((str) -> str.startsWith("hoho"), () -> 300)
      //You can set the behavior in the case of else with the elsef method
      .elsef(() -> 400)
      .eval(UUID.randomUUID().toString());
    Assertions.assertEquals(result.get(), 400);
  }

  @Test
  public void noMatchNotExistsElse() {
    var result = new Iff<String, Integer>((str) -> str.startsWith("hoge"), () -> 100)
      .elseIf((str) -> str.startsWith("hogeFuga"), () -> 200)
      .elseIf((str) -> str.startsWith("hoho"), () -> 300)
      .eval(UUID.randomUUID().toString());
    //If none of the expressions match, Empty will be returned
    result.ifPresentOrElse(v -> {
      Assertions.fail();
    }, () -> {
      //If it is else, the answer is correct
    });
  }

  @Test
  public void elseResultIsNull() {
    var result = new Iff<String, Integer>((str) -> str.startsWith("hoge"), () -> 100)
      .elseIf((str) -> str.startsWith("hogeFuga"), () -> 200)
      .elseIf((str) -> str.startsWith("hoho"), () -> 300)
      .elsef(() -> null)
      .eval(UUID.randomUUID().toString());
    Assertions.assertTrue(result.isEmpty());
  }

  @Test
  public void elseIfResultIsNull() {
    var result = new Iff<String, Integer>((str) -> str.startsWith("hoge"), () -> 100)
      .elseIf((str) -> str.startsWith("mogemoge"), () -> null)
      .elseIf((str) -> str.startsWith("hoho"), () -> 300)
      .elsef(() -> null)
      .eval("mogemoge");
    Assertions.assertTrue(result.isEmpty());
  }
}