Question

我有一个经典的“猫王运算符”案例，我在其中调用每个可能返回null并将它们链接在一起的方法：

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

在Java 8中，我发现的最可靠的实现是这样的：

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

我希望Java的Optional类似于猫王运算符：

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

这样我就不必包装每个返回值：

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

在Java 8中是否有更高效，更惯用的方式实现Elvis运算符模式？

Answer 1

也许我正在忽略某些内容，但是是否有不能使用Optional#map的原因？

下面的示例不打印任何内容，因为Optional是短路，即如果Optional内部的值不存在（它是{{1 }}或null为空），则将其视为空。

Optional

基于这个原因，我认为您可以执行以下操作：

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

这将映射您的return Optional.ofNullable(thing) .map(x -> x.nullableMethod1(a)) .map(y -> y.nullableMethod2(b)) .map(Z::nullableMethod3);（如果存在），否则返回一个空的thing。

Answer 2

在Java 8中，可以通过在.map(...)上链接Optional.ofNullable(...)调用并用.orElse(...)进行上限来模拟Elvis运算符：

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

完整示例：

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
  }
}

Answer 3

我写了一个 compiler plugin，它在编译时操作 AST 以生成 Optional.ofNullable 链

它几乎没有限制，但确实减少了重复代码

PS：我是插件的作者

如何在Java 8中实现Elvis运算符？

3 个答案: