使用 Stream 避免 NoSuchElementException

Question

我有以下 Stream:

Stream<T> stream = stream();

T result = stream.filter(t -> {
    double x = getX(t);
    double y = getY(t);
    return (x == tx && y == ty);
}).findFirst().get();

return result;

但是，并不总是出现以下错误的结果：

NoSuchElementException: No value present

如果没有值，我怎么能 return 一个 null？

Answer 1

Stream#findFirst() returns an Optional 专门存在，因此您无需对 null 值进行操作。

A container object which may or may not contain a non-null value. If a value is present, isPresent() will return true and get() will return the value.

否则，Optional#get() 抛出 NoSuchElementException。

If a value is present in this Optional, returns the value, otherwise throws NoSuchElementException.

如果 Optional 是 null，则它永远不会暴露其值。

如果您确实需要，请自行检查isPresent()和return null。

Stream<T> stream = stream();

Optional<T> result = stream.filter(t -> {
    double x = getX(t);
    double y = getY(t);
    return (x == tx && y == ty);
}).findFirst();

if (result.isPresent()) 
    return result.get();
return null;

Answer 2

可以用Optional.orElse，比查isPresent简单多了：

T result = stream.filter(t -> {
    double x = getX(t);
    double y = getY(t);
    return (x == tx && y == ty);
}).findFirst().orElse(null);

return result;

Answer 3

替换 Optional.get (which more likely than not fails the user's intentions with a NoSuchElementException) is with a more verbose API introduced in JDK10 termed as Optional.orElseThrow(). In author's words -

的替代方法

Optional.get() is an "attractive nuisance" and is too tempting for programmers, leading to frequent errors. People don't expect a getter to throw an exception. A replacement API for Optional.get() with equivalent semantics should be added.

注意 :- 这两个 API 的底层实现是相同的，但后者更清楚地表明 NoSuchElementException 是如果值不存在则默认抛出，该值内联到消费者用作显式替代的现有 Optional.orElseThrow(Supplier<? extends X> exceptionSupplier) 实现。

Answer 4

如果您希望继续使用该对象而不抛出任何异常，那么

Optional.isPresent(object) 是要走的路

使用 Stream 避免 NoSuchElementException

Avoid NoSuchElementException with Stream

java

optional

java-8

java-stream