Guava可选类型，当转换返回另一个Optional时

问题描述

我有一种禁止Optional<String>

但是此String必须在另一个应用程序级别解析为Integer或Long.

But this String must be parsed at another application level as Integer or Long.

我有一个Function<String, Integer>，可以在String上应用它，以产生一个Integer.这种转换可能会失败，因为String可能不是整数可解析的值.

This I have a Function<String, Integer> that can be applied on the String, to produce an Integer.This transformation can fail because the String may not be an Integer parsable value.

当转换失败时，我想返回Optional，而不是抛出解析异常.

I would like to return Optional when the transformation fails, instead of throwing a parsing exception.

我不能使STRING_TO_INTEGER_FUNCTION返回null，因为番石榴不允许这样做:

I can't make the STRING_TO_INTEGER_FUNCTION return null, because it is not allowed by Guava:

Exception in thread "main" java.lang.NullPointerException: Transformation function cannot return null.

因此，我唯一能做的就是拥有一个Function<String,Optional<Integer>>，但最终得到的结果却是一个Optional<Optional<Integer>>，它并不是很酷，因为我可能需要对其进行其他转换.

Thus the only thing I can do is having a Function<String,Optional<Integer>> but then I get as final result an Optional<Optional<Integer>> which isn't really cool because I may have another transformations to apply on it.

有人知道我该如何在番石榴中做类似的事情吗?

Does someone know how can I do something like that in Guava?

Optional.of("Toto").transform(STRING_TO_INTEGER_FUNCTION) = // Optional<Integer> ?

谢谢

推荐答案

我想您可以做到:

public static void main(final String[] args) {
  final Optional<Integer> valid = Optional.of("42")
      .transform(STR_TO_INT_FUNCTION)
      .or(Optional.<Integer>absent());
  System.out.println(valid); // Optional.of(42)
  final Optional<Integer> invalid = Optional.of("Toto")
      .transform(STR_TO_INT_FUNCTION)
      .or(Optional.<Integer>absent());
  System.out.println(invalid); // Optional.absent()
  final Optional<Integer> absent = Optional.<String>absent()
      .transform(STR_TO_INT_FUNCTION)
      .or(Optional.<Integer>absent());
  System.out.println(absent); // Optional.absent()
}

private static final Function<String, Optional<Integer>> STR_TO_INT_FUNCTION =
    new Function<String, Optional<Integer>>() {
      @Override
      public Optional<Integer> apply(final String input) {
        return Optional.fromNullable(Ints.tryParse(input));
      }
    };

当您使用Optional-> transform->或一行时，

用法不是 笨拙(分配转换后的可选整数会产生Optional<Optional<Integer>>).

Usage isn't that clumsy when you use Optional -> transform -> or in one line (assigning transformed optional integer would produce Optional<Optional<Integer>>).

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