How to implement the Elvis operator in Java 8?

问题

I have the classic "Elvis operator" case, where I'm calling methods that each may return null and chaining them together:

thing?:nullableMethod1(a)?:nullableMethod2(b)?:nullableMethod3()

In Java 8, the most faithful implementation I've found is something like this:

return Optional.ofNullable(thing)
    .flatMap(x -> Optional.ofNullable(x.nullableMethod1(a)))
    .flatMap(y -> Optional.ofNullable(y.nullableMethod2(b)))
    .flatMap(z -> Optional.ofNullable(z.nullableMethod3()))

I wish that Java's Optional had something akin to the elvis operator:

public<U> Optional<U> elvisOperator(Function<? super T, ? extends U> mapper) {
    return flatMap(t -> Optional.ofNullable(mapper.apply(t));
}

So that I wouldn't have to wrap each return value:

return Optional.ofNullable(thing)
    .elvisOperator(x -> x.nullableMethod1(a))
    .elvisOperator(y -> y.nullableMethod2(b))
    .elvisOperator(Z::nullableMethod3); // also nice

Is there a more efficient and idiomatic way to implement the Elvis operator pattern in Java 8?

回答1:

Maybe I'm overlooking something, but is there a reason that you can't use Optional#map?

The following example prints nothing, as Optional is short-circuiting in the sense that, if the value inside the Optional doesn't exist (it's null or the Optional is empty), it's treated as empty.

Optional.ofNullable("test")
        .map(s -> null)
        .ifPresent(System.out::println);

For that reason, I'd think you could just do the following:

return Optional.ofNullable(thing)
               .map(x -> x.nullableMethod1(a))
               .map(y -> y.nullableMethod2(b))
               .map(Z::nullableMethod3);

This would map your thing if it exists, or return an empty Optional otherwise.

回答2:

In Java 8, the Elvis operator can be simulated by chaining .map(...) calls on an Optional.ofNullable(...) and capping it with .orElse(...):

Optional.ofNullable(dataObject)
.map(DataObject::getNestedDataObject)
.map(NestedDataObject::getEvenMoreNestedDataObject)
...
.orElse(null);

A full example:

import java.util.Optional;

class Main {
  // Data classes:
  static class Animal {
    Leg leg;

    Animal(Leg leg) {
      this.leg = leg;
    }

    Leg getLeg(){return this.leg;}

    public String toString(){
      String out = "This is an animal";
      out += leg != null ? " with a leg" : "";
      return out;
    }
  }

  static class Leg {
    Toes toes;

    Leg(Toes toes) {
      this.toes = toes;
    }

    Toes getToes(){return this.toes;}

    public String toString(){
      String out = "This is a leg";
      out += toes != null ? " with a collection of toes" : "";
      return out;
    }
  }

  static class Toes {
    Integer numToes;

    Toes(Integer numToes) {
      this.numToes = numToes;
    }

    Integer getNumToes(){return this.numToes;}

    public String toString(){
      String out = "This is a collection of ";
      out += numToes != null && numToes > 0 ? numToes : "no";
      out += " toes";
      return out;
    }
  }

  // A few example Elvis operators:
  static Integer getNumToesOrNull(Animal a) {
    return Optional.ofNullable(a)
      .map(Animal::getLeg)
      .map(Leg::getToes)
      .map(Toes::getNumToes)
      .orElse(null);
  }

  static Toes getToesOrNull(Animal a) {
    return Optional.ofNullable(a)
      .map(Animal::getLeg)
      .map(Leg::getToes)
      .orElse(null);
  }

  static Leg getLegOrNull(Animal a) {
    return Optional.ofNullable(a)
      .map(Animal::getLeg)
      .orElse(null);
  }

  // Main function:
  public static void main(String[] args) {
    // Trying to access 'numToes':
    System.out.println(getNumToesOrNull(new Animal(new Leg(new Toes(4))))); // 4
    System.out.println(getNumToesOrNull(new Animal(new Leg(new Toes(null))))); // null
    System.out.println(getNumToesOrNull(new Animal(new Leg(null)))); // null
    System.out.println(getNumToesOrNull(new Animal(null))); // null
    System.out.println(getNumToesOrNull(null)); // null

    // Trying to access 'toes':
    System.out.println(getToesOrNull(new Animal(new Leg(new Toes(4))))); // This is a collection of 4 toes
    System.out.println(getToesOrNull(new Animal(new Leg(new Toes(null))))); // This is a collection of no toes
    System.out.println(getToesOrNull(new Animal(new Leg(null)))); // null
    System.out.println(getToesOrNull(new Animal(null))); // null
    System.out.println(getToesOrNull(null)); // null

    // Trying to access 'leg':
    System.out.println(getLegOrNull(new Animal(new Leg(new Toes(4))))); // This is a leg with a collection of toes
    System.out.println(getLegOrNull(new Animal(new Leg(new Toes(null))))); // This is a leg with a collection of toes
    System.out.println(getLegOrNull(new Animal(new Leg(null)))); // This is a leg
    System.out.println(getLegOrNull(new Animal(null))); // null
    System.out.println(getLegOrNull(null)); // null
  }
}

来源：https://stackoverflow.com/questions/52285813/how-to-implement-the-elvis-operator-in-java-8