将`Optional`转换为`OptionalInt`。

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英文:

How to convert Optional<Integer> to OptionalInt

问题

以下是您要翻译的内容:

我有一个 Optional< Integer > 变量:

// es 是一个 arraylist,rock 是 arraylist 中的一个类
var as = IntStream.range(0,es.size()).boxed()
	     .filter(i->es.get(i) instanceof Rock)
	     .max((l1,l2) -> comparator(es,(Rock)es.get(l1),(Rock)es.get(l2)));

在这种情况下,我想要为我的方法返回一个 OptionalInt:

	  if (as.isPresent()) {return Optionalint.of(as);} // 显然这行不起作用
	  return OptionalInt.empty();

在这种情况下,我如何将 Optional< Integer > 转换为 OptionalInt?
顺便说一下,我在这个问题中找到了这个链接 https://stackoverflow.com/questions/34713973/how-to-convert-an-optional-to-an-optionalint ,但对我的情况帮助不大,当我这样做时:

Stream.of(as).filter(s -> s != null && s.matches("\\d+"))
	        .mapToInt(Integer::parseInt).findAny();

它会给我一个错误:方法 matches(String) 在 Optional<Integer> 类型中未定义。
有什么解决方法吗?谢谢!

英文:

I have an Optional< Integer > variable:

//es is an arraylist, rock is a class in the arraylist
var as = IntStream.range(0,es.size()).boxed()
	     .filter(i-&gt;es.get(i) instanceof Rock)
	     .max((l1,l2) -&gt; comparator(es,(Rock)es.get(l1),(Rock)es.get(l2)));

In that case, I want to return an OptionalInt for my method:

	  if (as.isPresent()) {return Optionalint.of(as);} //Obviously it is not working
	  return OptionalInt.empty();

How can I convert Optional< Integer > to OptionalInt in that case?
BTW I found this question https://stackoverflow.com/questions/34713973/how-to-convert-an-optional-to-an-optionalint is not quite helpful in my case, when I do like:

Stream.of(as).filter(s -&gt; s != null &amp;&amp; s.matches(&quot;\\d+&quot;))
	        .mapToInt(Integer::parseInt).findAny();

It will give me an error: The method matches(String) is undefined for the type Optional<Integer>
Any idea to solve this? Thanks!

答案1

得分: 1

使用 Optional::map 将值映射为 Optional&lt;OptionalInt&gt;,然后使用 Optional::orElse 返回该值或空的 OptionalInt.empty()。我假设方法 comparator 正确工作:

OptionalInt as = IntStream.range(0, es.size()).boxed()
			.filter(i -> es.get(i) instanceof Rock)
			.max((l1, l2) -> comparator(es, (Rock)es.get(l1), (Rock)es.get(l2)))
			.map(OptionalInt::of)
			.orElse(OptionalInt.empty());

现在变量 asOptionalInt 的一个实例。

然而,在你的第二个代码片段中不能使用该实例,因为 Stream.of(as) 会得到 Stream&lt;OptionalInt&gt;,我确定这不是你想要的。这只是从链接答案中复制的片段。请记住,它所包含的值是原始的 int,而不是 String

英文:

Use Optional::map to map the value into Optional&lt;OptionalInt&gt; and then use Optional::orElse to return either it or an empty OptionalInt.empty(). I assume the method comparator works correctly:

OptionalInt as = IntStream.range(0, es.size()).boxed()
			.filter(i-&gt; es.get(i) instanceof Rock)
			.max((l1, l2) -&gt; comparator(es, (Rock)es.get(l1), (Rock)es.get(l2)))
			.map(OptionalInt::of)
			.orElse(OptionalInt.empty());

Now the variable as is an instance of OptionalInt.

However, you cannot use that instance in your second snippet because Stream.of(as) results in Stream&lt;OptionalInt&gt; and I am sure it is not what you want. This is just a copied snippet from the linked answer. Remember, the value it holds is a primitive int, not String.

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  • 本文由 发表于 2020年9月29日 12:37:31
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