英文:
Populating an existing Set<String> via Java 8 streams
问题
I am new to Java streams and functional programming and I wonder if my question will ever even have a valid use case or it is just not meaningful.
我对Java流和函数式编程很陌生，我想知道我的问题是否有有效的用例，还是根本没有意义。
I have the following code that gives me a Set<String> from an array
我有以下的代码，它从一个数组中给我一个Set<String>。
This gives me an empty dict. If I do it the way I have in second last line, it works. Is there a way to mutate already instantiated dict that was instantiated outside stream that it gets populated in the map function?
这会给我一个空的字典。如果我按照倒数第二行的方式做，它会工作。有没有一种方法可以修改已经在流外实例化的dict，以使其在map函数中得到填充？
英文:
I am new to Java streams and functional programming and I wonder if my question will ever even have a valid use case or it is just not meaningful.
I have the following code that gives me a Set<String> from an array
public static void main(String[] args) {
String []arr = {"a","abc","b","cd"};
Set<String> dict = new HashSet<>();
Arrays.stream(arr).map(item->{
System.out.println(item);
dict.add(item);
return dict;
});
// dict = Arrays.stream(arr).collect(Collectors.toSet());
System.out.println(dict);
}
This gives me an empty dict. If I do it the way I have in second last line, it works. Is there a way to mutate already instantiated dict that was instantiated outside stream that it gets populated in the map function?
答案1
得分: 5
Stream.map() 是一个中间操作，意味着它只是流水线中的一个步骤，但当你调用它时并不会执行任何操作。你需要一个终端操作来让流开始工作。在你的情况下，你可以使用 forEachOrdered()：
Arrays.stream(arr)
.peek(System.out::println)
.forEachOrdered(dict::add);
注意：我假设你需要修改现有的集合而不是生成一个新的集合。否则，可以像其他答案中建议的那样使用 collect()。
英文:
Stream.map() is an intermediate operation, meaning it only stages a step of the pipeline, but it doesn't do anything when you call it. You need a terminal operation to put the stream to work. In your case, you can use forEachOrdered():
Arrays.stream(arr)
.peek(System.out::println)
.forEachOrdered(dict::add);
Note: I'm assuming there's a reason you need to modify an existing set rather than produce a new one. Otherwise, just use collect() as suggested in the other answer.
答案2
得分: 3
- 在问题被编辑之前，已注释部分无法编译，因为
dict必须是 final 或者 effectively final。有关更多信息，请参见此答案。 - 使用 Stream API 的少数规则之一是避免副作用。
java.util.stream包的 JavaDoc 用示例很好地描述了这一点：
> 通常不鼓励在流操作的行为参数中使用副作用，因为这往往会导致无意中违反无状态要求，以及其他线程安全风险。 - 每个 Stream 的要点是通过诸如过滤、映射、归约或高级收集等操作返回新的集合/值。它从未旨在修改现有集合。将数组
arr转换为集合的正确方式是：Set<String> dict = Arrays.stream(arr) .peek(item -> System.out.println(item)) // 用于调试的可选项 .collect(Collectors.toSet());
英文:
- Before the question got edited, the commented part didn't compile because
dictmust be either final or effectively final. See this answer for more information. - One of the few rules of using Stream API is to avoid side effects. The JavaDoc for
java.util.streampackage describes it well with examples:
> Side-effects in behavioral parameters to stream operations are, in general, discouraged, as they can often lead to unwitting violations of the statelessness requirement, as well as other thread-safety hazards. - The gist of each Stream is to return a new collection/value through operations such as filtering, mapping, reducing, or advanced collecting. It was never meant to modify an existing one. The correct way how to transform the array
arrinto a set is:Set<String> dict = Arrays.stream(arr) .peek(item -> System.out.println(item)) // optional for debugging .collect(Collectors.toSet());
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