如何匹配流元素但如果不存在则返回false？ [英] How to match stream elements but return false if non exists?

问题描述

我有一个流，想检查是否所有匹配过滤器。如果全部匹配，则返回 true 。

I have a stream and would like to check if all match a filter. If all match, return true.

但是，如果流为空，我想返回 false 。

But, if the stream is empty, I'd like to return false.

我该怎么做？

示例代码：

public boolean validate(Stream<Whatever> stream) {
  // Problem: returns **true** if stream empty.
  // How can **false** be returned if stream is empty?
  return stream.allMatch(Whatever::someCheck);
}

推荐答案

你可以用

public boolean validate(Stream<Whatever> stream) {
    return stream.map(Whatever::someCheck).reduce(Boolean::logicalAnd).orElse(false);
}

表示意图。我们将每个元素映射到 boolean 值，表示它是否匹配并使用逻辑和操作减少所有元素，这将产生 true iff所有这些都是 true 。如果没有元素， reduce 将返回一个空的可选，我们将其映射到 false 按预期使用 orElse（false）。

which expresses the intent. We map each element to a boolean value expressing whether it matches and reducing all of them with a logical and operation which will yield true iff all of them were true. reduce will return an empty Optional if there were no elements, which we map to false using orElse(false), as intended.

唯一的缺点是这是非短路，即不会立即停止在第一个非匹配元件上。

The only disadvantage is that this is non short-circuiting, i.e. does not stop immediately at the first non-matching element.

仍然支持短路的解决方案可能会进一步发展：

A solution still supporting short-circuiting might be a bit more evolved:

public boolean validate(Stream<Whatever> stream) {
    boolean parallel = stream.isParallel();
    Spliterator<Whatever> sp = stream.spliterator();
    if(sp.getExactSizeIfKnown() == 0) return false;
    Stream.Builder<Whatever> b = Stream.builder();
    if(!sp.tryAdvance(b)) return false;
    return Stream.concat(b.build(), StreamSupport.stream(sp, parallel))
        .allMatch(Whatever::someCheck);
}

这是 Eugene的回答，但它没有松散的特性或并行性，一般来说可能会更有效率。

This is a variant of Eugene’s answer, but it doesn’t loose characteristics or parallelism and might be a bit more efficient in general.

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