java findAny orElse 过滤后
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findAny orElse after filter
提问by paul
I'm using Stream filter findAny.orElse, but it's not working as I expect, so I presume I'm not understanding how really works.
Here my code
我正在使用 Stream filter findAny.orElse，但它并没有像我预期的那样工作，所以我想我不了解真正的工作原理。这是我的代码
return Stream.of(getObjectAttributeValue(product, matchCriteria.getFieldName()))
.map(o -> isIngredientRestricted(matchCriteria, (List<String>) o))
.filter(Boolean::valueOf)
.findAny().orElse(isCommercialHierarchyInfoRestricted(product, matchCriteria));
Basically what I was expecting was that if the first map emit a Boolean false, then it will be filter so then the findAny would not find any optional, so the orElse would be invoked. But even having a true in the filter the isCommercialHierarchyInfoRestricted is invoked.
基本上我期望的是，如果第一个映射发出布尔值 false，那么它将被过滤，因此 findAny 将找不到任何可选，因此将调用 orElse。但即使在过滤器中有一个 true ，也会调用 isCommercialHierarchyInfoRestricted 。
Any idea what I'm doing wrong?
知道我做错了什么吗？
回答by Tagir Valeev
You actually need to use orElseGet:
你实际上需要使用orElseGet：
.findAny().orElseGet(() -> isCommercialHierarchyInfoRestricted(product, matchCriteria));
In Java method argument is always evaluated prior to method call even if it's unnecessary inside the method, so you cannot avoid evaluating the orElseargument. That's why orElseGetexists: its argument is the function and the function can be not executed at all when it's not necessary.
在 Java 中，方法参数总是在方法调用之前进行评估，即使它在方法内部是不必要的，因此您无法避免评估orElse参数。这就是orElseGet存在的原因：它的参数是函数，并且函数在不需要时根本不能执行。
回答by Holger
As Tagir explained, using orElse(expression)always causes the evaluation of expressionbefore invoking the method orElseand you have to use orElseGet(() -> expression)instead to defer the evaluation of the expression.
正如Tagir 解释的那样， usingorElse(expression)总是会导致expression在调用方法之前评估 ，orElse而您必须改为使用orElseGet(() -> expression)来推迟表达式的评估。
However, this is an unnecessary use of the StreamAPI. If you want to evaluate a single item, you don't need to create a single-element stream just to call findAnyafterwards. You can create an Optionalin the first place:
但是，这是对StreamAPI的不必要使用。如果要评估单个项目，则无需创建单元素流以供findAny之后调用。您可以首先创建一个Optional：
return Optional.of(getObjectAttributeValue(product, matchCriteria.getFieldName()))
.map(o -> isIngredientRestricted(matchCriteria, (List<String>)o))
.filter(b -> b)
.orElseGet(() -> isCommercialHierarchyInfoRestricted(product, matchCriteria));
However, even that is an unnecessary complication compared to the equivalent ordinary Java language construct:
然而，与等效的普通 Java 语言结构相比，即使这样也是不必要的复杂化：
return isIngredientRestricted(matchCriteria,
(List<String>)getObjectAttributeValue(product, matchCriteria.getFieldName()))
|| isCommercialHierarchyInfoRestricted(product, matchCriteria);
This does exactly the same without the need for additional APIs nor lambda expressions. The ||operator also guarantees that the second expression won't get evaluated, if the first one evaluates to true.
这完全相同，无需额外的 API 或 lambda 表达式。该||运营商还保证了第二个表达式将不会得到评估，如果第一个计算结果为true。
